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How to Level Up in Elixir James Edward Gray II — 6/5/2021 Designing a Chat Application

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James Edward Gray II • A longtime Rubyist • Creator of the Ruby Rogues podcast • An Elixir community regular for years now • I wrote a book about Elixir @JEG2

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I Work With Brett Who is on vacation… while I'm running his group?!

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How would you do it? The Evils of Thought Leadership

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The Communication Layer

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Socket Programming • A program has one "socket" connection with each remote system it is communicating with • Sockets are two-way, supporting both the sending and receiving of data • Data can arrive on any socket at anytime

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Multitasking

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Starting Processes Conway's Game of Life with one process per cell All socket handling in one GenServer

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Starting Processes Conway's Game of Life with one process per cell All socket handling in one GenServer

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Supervisor Supervisor ConnectionManager

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Supervisor Supervisor ConnectionManager Listener Connection Connection

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defmodule ChatApp.Application do @moduledoc false use Application @impl true def start(_type, _args) do children = [ { DynamicSupervisor , strategy: :one_for_one, name: ChatApp.ConnectionSupervisor } , {ChatApp.ConnectionManager, ui: ChatApp.GUI} , ChatApp.GUI ] opts = [strategy: :one_for_one, name: ChatApp.Supervisor] Supervisor.start_link(children, opts) end end

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defmodule ChatApp.Application do @moduledoc false use Application @impl true def start(_type, _args) do children = [ { DynamicSupervisor , strategy: :one_for_one, name: ChatApp.ConnectionSupervisor } , {ChatApp.ConnectionManager, ui: ChatApp.GUI} , ChatApp.GUI ] opts = [strategy: :one_for_one, name: ChatApp.Supervisor] Supervisor.start_link(children, opts) end end

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defmodule ChatApp.ConnectionManager do use GenServer alias ChatApp.{Connection, ConnectionSupervisor, Listener} # .. . defstruct mode: :ready, name: nil, ui: nil, me: nil def start_link(options) do GenServer.start_link( __MODULE__ , options , name: Keyword.get(options, :name, __MODULE__) ) end def init(options) do case Keyword.get(options, :ui) do ui when is_atom(ui) - > me = Keyword.get(options, :name, __MODULE__) || self() {:ok, %__MODULE__{ui: ui, me: me}} _no_ui - > {:stop, "ConnectionManager must be started with a UI module"} end end end

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defmodule ChatApp.ConnectionManager do use GenServer alias ChatApp.{Connection, ConnectionSupervisor, Listener} # .. . defstruct mode: :ready, name: nil, ui: nil, me: nil def start_link(options) do GenServer.start_link( __MODULE__ , options , name: Keyword.get(options, :name, __MODULE__) ) end def init(options) do case Keyword.get(options, :ui) do ui when is_atom(ui) - > me = Keyword.get(options, :name, __MODULE__) || self() {:ok, %__MODULE__{ui: ui, me: me}} _no_ui - > {:stop, "ConnectionManager must be started with a UI module"} end end end

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defmodule ChatApp.ConnectionManager do use GenServer alias ChatApp.{Connection, ConnectionSupervisor, Listener} # .. . defstruct mode: :ready, name: nil, ui: nil, me: nil def start_link(options) do GenServer.start_link( __MODULE__ , options , name: Keyword.get(options, :name, __MODULE__) ) end def init(options) do case Keyword.get(options, :ui) do ui when is_atom(ui) - > me = Keyword.get(options, :name, __MODULE__) || self() {:ok, %__MODULE__{ui: ui, me: me}} _no_ui - > {:stop, "ConnectionManager must be started with a UI module"} end end end

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defmodule ChatApp.ConnectionManager do use GenServer alias ChatApp.{Connection, ConnectionSupervisor, Listener} # .. . defstruct mode: :ready, name: nil, ui: nil, me: nil def start_link(options) do GenServer.start_link( __MODULE__ , options , name: Keyword.get(options, :name, __MODULE__) ) end def init(options) do case Keyword.get(options, :ui) do ui when is_atom(ui) - > me = Keyword.get(options, :name, __MODULE__) || self() {:ok, %__MODULE__{ui: ui, me: me}} _no_ui - > {:stop, "ConnectionManager must be started with a UI module"} end end end

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defmodule ChatApp.ConnectionManager do use GenServer alias ChatApp.{Connection, ConnectionSupervisor, Listener} # .. . defstruct mode: :ready, name: nil, ui: nil, me: nil def start_link(options) do GenServer.start_link( __MODULE__ , options , name: Keyword.get(options, :name, __MODULE__) ) end def init(options) do case Keyword.get(options, :ui) do ui when is_atom(ui) - > me = Keyword.get(options, :name, __MODULE__) || self() {:ok, %__MODULE__{ui: ui, me: me}} _no_ui - > {:stop, "ConnectionManager must be started with a UI module"} end end end

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defmodule ChatApp.ConnectionManager do # .. . def listen(manager \\ __MODULE__, port, name) do GenServer.call(manager, {:listen, port, name}) end def handle_call({:listen, port, name}, _from, state) do case start_listening(port, state.me) do :ok - > {:reply, :ok, %__MODULE__{state | mode: :host, name: name}} error - > {:reply, error, state} end end end

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defmodule ChatApp.ConnectionManager do # .. . def listen(manager \\ __MODULE__, port, name) do GenServer.call(manager, {:listen, port, name}) end def handle_call({:listen, port, name}, _from, state) do case start_listening(port, state.me) do :ok - > {:reply, :ok, %__MODULE__{state | mode: :host, name: name}} error - > {:reply, error, state} end end end

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defmodule ChatApp.ConnectionManager do # .. . def listen(manager \\ __MODULE__, port, name) do GenServer.call(manager, {:listen, port, name}) end def handle_call({:listen, port, name}, _from, state) do case start_listening(port, state.me) do :ok - > {:reply, :ok, %__MODULE__{state | mode: :host, name: name}} error - > {:reply, error, state} end end end

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defmodule ChatApp.ConnectionManager do # .. . def listen(manager \\ __MODULE__, port, name) do GenServer.call(manager, {:listen, port, name}) end def handle_call({:listen, port, name}, _from, state) do case start_listening(port, state.me) do :ok - > {:reply, :ok, %__MODULE__{state | mode: :host, name: name}} error - > {:reply, error, state} end end end

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Listening for Connections • A listening socket queues connections as they come in • A program can "accept" them to complete the connection • The result of accepting a connection is a read/write socket 
 used to communicate with the remote program • Erlang (now) provides a low-level `socket`, 
 but the higher-level `gen_tcp` adds several niceties

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defmodule ChatApp.ConnectionManager do # .. . @packet_size 2 defp start_listening(port, me) do case :gen_tcp.listen( port , [:binary, packet: @packet_size, active: false, reuseaddr: true] ) do {:ok, listening_socket} - > Listener.listen(listening_socket, me) error - > erro r end end end

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defmodule ChatApp.ConnectionManager do # .. . @packet_size 2 defp start_listening(port, me) do case :gen_tcp.listen( port , [:binary, packet: @packet_size, active: false, reuseaddr: true] ) do {:ok, listening_socket} - > Listener.listen(listening_socket, me) error - > erro r end end end

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defmodule ChatApp.ConnectionManager do # .. . @packet_size 2 defp start_listening(port, me) do case :gen_tcp.listen( port , [:binary, packet: @packet_size, active: false, reuseaddr: true] ) do {:ok, listening_socket} - > Listener.listen(listening_socket, me) error - > erro r end end end

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defmodule ChatApp.ConnectionManager do # .. . @packet_size 2 defp start_listening(port, me) do case :gen_tcp.listen( port , [:binary, packet: @packet_size, active: false, reuseaddr: true] ) do {:ok, listening_socket} - > Listener.listen(listening_socket, me) error - > erro r end end end

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defmodule ChatApp.Listener do use GenServer, restart: :transient alias ChatApp.{Connection, ConnectionSupervisor} # .. . def listen(listening_socket, manager) do case DynamicSupervisor.start_child( ConnectionSupervisor , {__MODULE__, [listening_socket, manager]} ) do {:ok, listener} - > transfer_control(listening_socket, listener) error - > erro r end end end

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defmodule ChatApp.Listener do use GenServer, restart: :transient alias ChatApp.{Connection, ConnectionSupervisor} # .. . def listen(listening_socket, manager) do case DynamicSupervisor.start_child( ConnectionSupervisor , {__MODULE__, [listening_socket, manager]} ) do {:ok, listener} - > transfer_control(listening_socket, listener) error - > erro r end end end

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defmodule ChatApp.Listener do use GenServer, restart: :transient alias ChatApp.{Connection, ConnectionSupervisor} # .. . def listen(listening_socket, manager) do case DynamicSupervisor.start_child( ConnectionSupervisor , {__MODULE__, [listening_socket, manager]} ) do {:ok, listener} - > transfer_control(listening_socket, listener) error - > erro r end end end

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defmodule ChatApp.Listener do # .. . defp transfer_control(listening_socket, listener) do case :gen_tcp.controlling_process(listening_socket, listener) do :ok - > accept(listener) :ok error - > close(listener) erro r end end defp accept(listener), do: GenServer.cast(listener, :accept) end

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defmodule ChatApp.Listener do # .. . defp transfer_control(listening_socket, listener) do case :gen_tcp.controlling_process(listening_socket, listener) do :ok - > accept(listener) :ok error - > close(listener) erro r end end defp accept(listener), do: GenServer.cast(listener, :accept) end

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defmodule ChatApp.Listener do # .. . defp transfer_control(listening_socket, listener) do case :gen_tcp.controlling_process(listening_socket, listener) do :ok - > accept(listener) :ok error - > close(listener) erro r end end defp accept(listener), do: GenServer.cast(listener, :accept) end

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defmodule ChatApp.Listener do # .. . defp transfer_control(listening_socket, listener) do case :gen_tcp.controlling_process(listening_socket, listener) do :ok - > accept(listener) :ok error - > close(listener) erro r end end defp accept(listener), do: GenServer.cast(listener, :accept) end

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A `GenServer` Loop • Keep triggering `handle_cast/ 2` or `handle_info/2` • Timeout long running code or execute it in a linked `Task` • `Process.send_after/2` and `:timer.send_interval/2` can repeat message sends • Process other messages between calls Danger Zone!

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defmodule ChatApp.Listener do # .. . def handle_cast(:accept, state) do case :gen_tcp.accept(state.listening_socket, 1_000) do {:ok, socket} - > Connection.listen(socket, state.manager) accept(self()) {:noreply, state} {:error, :closed} - > {:stop, :normal, %__MODULE__{state | listening_socket: nil}} _error - > accept(self()) {:noreply, state} end end end

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defmodule ChatApp.Listener do # .. . def handle_cast(:accept, state) do case :gen_tcp.accept(state.listening_socket, 1_000) do {:ok, socket} - > Connection.listen(socket, state.manager) accept(self()) {:noreply, state} {:error, :closed} - > {:stop, :normal, %__MODULE__{state | listening_socket: nil}} _error - > accept(self()) {:noreply, state} end end end

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defmodule ChatApp.Listener do # .. . def handle_cast(:accept, state) do case :gen_tcp.accept(state.listening_socket, 1_000) do {:ok, socket} - > Connection.listen(socket, state.manager) accept(self()) {:noreply, state} {:error, :closed} - > {:stop, :normal, %__MODULE__{state | listening_socket: nil}} _error - > accept(self()) {:noreply, state} end end end

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defmodule ChatApp.Listener do # .. . def handle_cast(:accept, state) do case :gen_tcp.accept(state.listening_socket, 1_000) do {:ok, socket} - > Connection.listen(socket, state.manager) accept(self()) {:noreply, state} {:error, :closed} - > {:stop, :normal, %__MODULE__{state | listening_socket: nil}} _error - > accept(self()) {:noreply, state} end end end

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defmodule ChatApp.Listener do # .. . def handle_cast(:accept, state) do case :gen_tcp.accept(state.listening_socket, 1_000) do {:ok, socket} - > Connection.listen(socket, state.manager) accept(self()) {:noreply, state} {:error, :closed} - > {:stop, :normal, %__MODULE__{state | listening_socket: nil}} _error - > accept(self()) {:noreply, state} end end end

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defmodule ChatApp.Listener do # .. . defstruct ~w[listening_socket manager] a def start_link([listening_socket, manager]) do GenServer.start_link(__MODULE__, [listening_socket, manager]) end def close(listener), do: GenServer.cast(listener, :close) def init([listening_socket, manager]) do {:ok, %__MODULE__{listening_socket: listening_socket, manager: manager}} end def handle_cast(:close, state) do :gen_tcp.close(state.listening_socket) {:stop, :normal, %__MODULE__{state | listening_socket: nil}} end end

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defmodule ChatApp.Listener do # .. . defstruct ~w[listening_socket manager] a def start_link([listening_socket, manager]) do GenServer.start_link(__MODULE__, [listening_socket, manager]) end def close(listener), do: GenServer.cast(listener, :close) def init([listening_socket, manager]) do {:ok, %__MODULE__{listening_socket: listening_socket, manager: manager}} end def handle_cast(:close, state) do :gen_tcp.close(state.listening_socket) {:stop, :normal, %__MODULE__{state | listening_socket: nil}} end end

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defmodule ChatApp.ConnectionManager do # .. . def connect(manager \\ __MODULE__, host, port, name) do GenServer.call(manager, {:connect, host, port, name}) end def handle_call({:connect, host, port, name}, _from, state) do case listen_to_connection(host, port, state.me) do :ok - > new_state = %__MODULE__{state | mode: :client, name: name} queue_all_sends(:connected, new_state) {:reply, :ok, new_state} error - > {:reply, error, state} end end end

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defmodule ChatApp.ConnectionManager do # .. . def connect(manager \\ __MODULE__, host, port, name) do GenServer.call(manager, {:connect, host, port, name}) end def handle_call({:connect, host, port, name}, _from, state) do case listen_to_connection(host, port, state.me) do :ok - > new_state = %__MODULE__{state | mode: :client, name: name} queue_all_sends(:connected, new_state) {:reply, :ok, new_state} error - > {:reply, error, state} end end end

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defmodule ChatApp.ConnectionManager do # .. . def connect(manager \\ __MODULE__, host, port, name) do GenServer.call(manager, {:connect, host, port, name}) end def handle_call({:connect, host, port, name}, _from, state) do case listen_to_connection(host, port, state.me) do :ok - > new_state = %__MODULE__{state | mode: :client, name: name} queue_all_sends(:connected, new_state) {:reply, :ok, new_state} error - > {:reply, error, state} end end end

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defmodule ChatApp.ConnectionManager do # .. . def connect(manager \\ __MODULE__, host, port, name) do GenServer.call(manager, {:connect, host, port, name}) end def handle_call({:connect, host, port, name}, _from, state) do case listen_to_connection(host, port, state.me) do :ok - > new_state = %__MODULE__{state | mode: :client, name: name} queue_all_sends(:connected, new_state) {:reply, :ok, new_state} error - > {:reply, error, state} end end end

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defmodule ChatApp.ConnectionManager do # .. . defp listen_to_connection(host, port, me) do case :gen_tcp.connect( String.to_charlist(host) , port , [:binary, packet: @packet_size, active: false] ) do {:ok, socket} - > Connection.listen(socket, me) error - > erro r end end end

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defmodule ChatApp.ConnectionManager do # .. . defp listen_to_connection(host, port, me) do case :gen_tcp.connect( String.to_charlist(host) , port , [:binary, packet: @packet_size, active: false] ) do {:ok, socket} - > Connection.listen(socket, me) error - > erro r end end end

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defmodule ChatApp.ConnectionManager do # .. . defp listen_to_connection(host, port, me) do case :gen_tcp.connect( String.to_charlist(host) , port , [:binary, packet: @packet_size, active: false] ) do {:ok, socket} - > Connection.listen(socket, me) error - > erro r end end end

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Protocols in Protocols • TCP/IP is a protocol for reliably delivering messages over an unreliable network • How do we know what a full message is though? • Delimit messages with something like newlines • Send the length of the message, then the message 
 (`:gen_tcp` does this!) • How do we know what's in a message? • Erlang's `term_to_binary/1`

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defmodule ChatApp.Connection do use GenServer, restart: :transient alias ChatApp.{ConnectionManager, ConnectionSupervisor} # .. . defstruct ~w[socket manager] a def start_link([socket, manager]) do GenServer.start_link(__MODULE__, [socket, manager]) end def close(connection), do: GenServer.cast(connection, :close) def init([socket, manager]) do {:ok, %__MODULE__{socket: socket, manager: manager}} end def handle_cast(:close, state) do :gen_tcp.close(state.socket) {:stop, :normal, %__MODULE__{state | socket: nil}} end end

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defmodule ChatApp.Connection do use GenServer, restart: :transient alias ChatApp.{ConnectionManager, ConnectionSupervisor} # .. . defstruct ~w[socket manager] a def start_link([socket, manager]) do GenServer.start_link(__MODULE__, [socket, manager]) end def close(connection), do: GenServer.cast(connection, :close) def init([socket, manager]) do {:ok, %__MODULE__{socket: socket, manager: manager}} end def handle_cast(:close, state) do :gen_tcp.close(state.socket) {:stop, :normal, %__MODULE__{state | socket: nil}} end end

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defmodule ChatApp.Connection do # .. . def listen(socket, manager) do case DynamicSupervisor.start_child( ConnectionSupervisor , {__MODULE__, [socket, manager]} ) do {:ok, connection} - > transfer_control(socket, connection) error - > erro r end end end

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defmodule ChatApp.Connection do # .. . def listen(socket, manager) do case DynamicSupervisor.start_child( ConnectionSupervisor , {__MODULE__, [socket, manager]} ) do {:ok, connection} - > transfer_control(socket, connection) error - > erro r end end end

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defmodule ChatApp.Connection do # .. . def handle_cast(:activate, state) do :inet.setopts(state.socket, active: :once) {:noreply, state} end defp transfer_control(socket, connection) do case :gen_tcp.controlling_process(socket, connection) do :ok - > activate(connection) :ok error - > close(connection) erro r end end defp activate(connection), do: GenServer.cast(connection, :activate) end

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defmodule ChatApp.Connection do # .. . def handle_cast(:activate, state) do :inet.setopts(state.socket, active: :once) {:noreply, state} end defp transfer_control(socket, connection) do case :gen_tcp.controlling_process(socket, connection) do :ok - > activate(connection) :ok error - > close(connection) erro r end end defp activate(connection), do: GenServer.cast(connection, :activate) end

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defmodule ChatApp.Connection do # .. . def handle_cast(:activate, state) do :inet.setopts(state.socket, active: :once) {:noreply, state} end defp transfer_control(socket, connection) do case :gen_tcp.controlling_process(socket, connection) do :ok - > activate(connection) :ok error - > close(connection) erro r end end defp activate(connection), do: GenServer.cast(connection, :activate) end

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TCP Event Messages • `gen_tcp` can deliver events—data arriving, a socket closing, 
 and the like—as messages to a process • Of course, a process could drown in incoming messages • `active: :once` controls the f low

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defmodule ChatApp.Connection do # .. . def handle_info({:tcp_closed, _socket}, state) do {:stop, :normal, %__MODULE__{state | socket: nil}} end def handle_info({:tcp, _socket, message}, state) do ConnectionManager.receive_message(state.manager, message, self()) activate(self()) {:noreply, state} end def handle_info(_unexpected_message, state), do: {:noreply, state} end

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defmodule ChatApp.Connection do # .. . def handle_info({:tcp_closed, _socket}, state) do {:stop, :normal, %__MODULE__{state | socket: nil}} end def handle_info({:tcp, _socket, message}, state) do ConnectionManager.receive_message(state.manager, message, self()) activate(self()) {:noreply, state} end def handle_info(_unexpected_message, state), do: {:noreply, state} end

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defmodule ChatApp.Connection do # .. . def handle_info({:tcp_closed, _socket}, state) do {:stop, :normal, %__MODULE__{state | socket: nil}} end def handle_info({:tcp, _socket, message}, state) do ConnectionManager.receive_message(state.manager, message, self()) activate(self()) {:noreply, state} end def handle_info(_unexpected_message, state), do: {:noreply, state} end

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defmodule ChatApp.Connection do # .. . def handle_info({:tcp_closed, _socket}, state) do {:stop, :normal, %__MODULE__{state | socket: nil}} end def handle_info({:tcp, _socket, message}, state) do ConnectionManager.receive_message(state.manager, message, self()) activate(self()) {:noreply, state} end def handle_info(_unexpected_message, state), do: {:noreply, state} end

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defmodule ChatApp.Connection do # .. . def handle_info({:tcp_closed, _socket}, state) do {:stop, :normal, %__MODULE__{state | socket: nil}} end def handle_info({:tcp, _socket, message}, state) do ConnectionManager.receive_message(state.manager, message, self()) activate(self()) {:noreply, state} end def handle_info(_unexpected_message, state), do: {:noreply, state} end

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defmodule ChatApp.Connection do # .. . def handle_info({:tcp_closed, _socket}, state) do {:stop, :normal, %__MODULE__{state | socket: nil}} end def handle_info({:tcp, _socket, message}, state) do ConnectionManager.receive_message(state.manager, message, self()) activate(self()) {:noreply, state} end def handle_info(_unexpected_message, state), do: {:noreply, state} end

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defmodule ChatApp.Connection do # .. . def handle_info({:tcp_closed, _socket}, state) do {:stop, :normal, %__MODULE__{state | socket: nil}} end def handle_info({:tcp, _socket, message}, state) do ConnectionManager.receive_message(state.manager, message, self()) activate(self()) {:noreply, state} end def handle_info(_unexpected_message, state), do: {:noreply, state} end

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defmodule ChatApp.ConnectionManager do # .. . def receive_message(manager \\ __MODULE__, message, from) do GenServer.cast(manager, {:receive_message, message, from}) end def handle_cast({:receive_message, message, from}, state) do if state.mode == :host do for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} when pid != from - > Connection.queue_send(pid, message) _listener_or_from - > :ok end) end {name, content} = :erlang.binary_to_term(message) state.ui.show_chat_message(name, content) {:noreply, state} end end

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defmodule ChatApp.ConnectionManager do # .. . def receive_message(manager \\ __MODULE__, message, from) do GenServer.cast(manager, {:receive_message, message, from}) end def handle_cast({:receive_message, message, from}, state) do if state.mode == :host do for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} when pid != from - > Connection.queue_send(pid, message) _listener_or_from - > :ok end) end {name, content} = :erlang.binary_to_term(message) state.ui.show_chat_message(name, content) {:noreply, state} end end

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defmodule ChatApp.ConnectionManager do # .. . def receive_message(manager \\ __MODULE__, message, from) do GenServer.cast(manager, {:receive_message, message, from}) end def handle_cast({:receive_message, message, from}, state) do if state.mode == :host do for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} when pid != from - > Connection.queue_send(pid, message) _listener_or_from - > :ok end) end {name, content} = :erlang.binary_to_term(message) state.ui.show_chat_message(name, content) {:noreply, state} end end

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defmodule ChatApp.ConnectionManager do # .. . def receive_message(manager \\ __MODULE__, message, from) do GenServer.cast(manager, {:receive_message, message, from}) end def handle_cast({:receive_message, message, from}, state) do if state.mode == :host do for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} when pid != from - > Connection.queue_send(pid, message) _listener_or_from - > :ok end) end {name, content} = :erlang.binary_to_term(message) state.ui.show_chat_message(name, content) {:noreply, state} end end

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defmodule ChatApp.ConnectionManager do # .. . def receive_message(manager \\ __MODULE__, message, from) do GenServer.cast(manager, {:receive_message, message, from}) end def handle_cast({:receive_message, message, from}, state) do if state.mode == :host do for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} when pid != from - > Connection.queue_send(pid, message) _listener_or_from - > :ok end) end {name, content} = :erlang.binary_to_term(message) state.ui.show_chat_message(name, content) {:noreply, state} end end

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defmodule ChatApp.ConnectionManager do # .. . defp for_active_connections(func) do ConnectionSupervisor |> DynamicSupervisor.which_children() |> Enum. fi lter(fn {:unde fi ned, pid_or_restarting, :worker, _modules} - > is_pid(pid_or_restarting) end) |> Enum.each(func) end end

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defmodule ChatApp.ConnectionManager do # .. . defp for_active_connections(func) do ConnectionSupervisor |> DynamicSupervisor.which_children() |> Enum. fi lter(fn {:unde fi ned, pid_or_restarting, :worker, _modules} - > is_pid(pid_or_restarting) end) |> Enum.each(func) end end

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defmodule ChatApp.ConnectionManager do # .. . defp for_active_connections(func) do ConnectionSupervisor |> DynamicSupervisor.which_children() |> Enum. fi lter(fn {:unde fi ned, pid_or_restarting, :worker, _modules} - > is_pid(pid_or_restarting) end) |> Enum.each(func) end end

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Nesting Cast and Call • You can't nest a "call" to the same `GenServer` in another call (deadlock) • When nesting calls to other processes, consider the effect on timeouts • You can always nest a "cast" or `send/2` (`handle_info/2`) • Remember to report cast failure out-of-band when needed • Consider adding backpressure to protect the receiver from drowning

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defmodule ChatApp.ConnectionManager do # .. . def send_to_all(manager \\ __MODULE__, message) do GenServer.call(manager, {:send_to_all, message}) end def handle_call({:send_to_all, message}, _from, state) do result = queue_all_sends(message, state) {:reply, result, state} end end

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defmodule ChatApp.ConnectionManager do # .. . def send_to_all(manager \\ __MODULE__, message) do GenServer.call(manager, {:send_to_all, message}) end def handle_call({:send_to_all, message}, _from, state) do result = queue_all_sends(message, state) {:reply, result, state} end end

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defmodule ChatApp.ConnectionManager do # .. . defp queue_all_sends(message, %__MODULE__{mode: mode} = state) when mode in ~w[host client]a do ref = make_ref() prepared_message = :erlang.term_to_binary({state.name, message}) for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} - > Connection.queue_send(pid, ref, prepared_message) _listener - > :ok end) {ref, state.name} end defp queue_all_sends(_message, _state), do: nil end

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defmodule ChatApp.ConnectionManager do # .. . defp queue_all_sends(message, %__MODULE__{mode: mode} = state) when mode in ~w[host client]a do ref = make_ref() prepared_message = :erlang.term_to_binary({state.name, message}) for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} - > Connection.queue_send(pid, ref, prepared_message) _listener - > :ok end) {ref, state.name} end defp queue_all_sends(_message, _state), do: nil end

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defmodule ChatApp.ConnectionManager do # .. . defp queue_all_sends(message, %__MODULE__{mode: mode} = state) when mode in ~w[host client]a do ref = make_ref() prepared_message = :erlang.term_to_binary({state.name, message}) for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} - > Connection.queue_send(pid, ref, prepared_message) _listener - > :ok end) {ref, state.name} end defp queue_all_sends(_message, _state), do: nil end

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defmodule ChatApp.ConnectionManager do # .. . defp queue_all_sends(message, %__MODULE__{mode: mode} = state) when mode in ~w[host client]a do ref = make_ref() prepared_message = :erlang.term_to_binary({state.name, message}) for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} - > Connection.queue_send(pid, ref, prepared_message) _listener - > :ok end) {ref, state.name} end defp queue_all_sends(_message, _state), do: nil end

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defmodule ChatApp.ConnectionManager do # .. . defp queue_all_sends(message, %__MODULE__{mode: mode} = state) when mode in ~w[host client]a do ref = make_ref() prepared_message = :erlang.term_to_binary({state.name, message}) for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} - > Connection.queue_send(pid, ref, prepared_message) _listener - > :ok end) {ref, state.name} end defp queue_all_sends(_message, _state), do: nil end

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defmodule ChatApp.ConnectionManager do # .. . defp queue_all_sends(message, %__MODULE__{mode: mode} = state) when mode in ~w[host client]a do ref = make_ref() prepared_message = :erlang.term_to_binary({state.name, message}) for_active_connections(fn {:unde fi ned, pid, :worker, [Connection]} - > Connection.queue_send(pid, ref, prepared_message) _listener - > :ok end) {ref, state.name} end defp queue_all_sends(_message, _state), do: nil end

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defmodule ChatApp.Connection do # .. . def queue_send(connection, message), do: queue_send(connection, nil, message) def queue_send(connection, message_id, message) do GenServer.cast(connection, {:queue_send, message_id, message}) end def handle_cast({:queue_send, message_id, message}, state) do case :gen_tcp.send(state.socket, message) do :ok - > :ok error - > if is_reference(message_id) do ConnectionManager.receive_send_error(state.manager, message_id, error) end end {:noreply, state} end end

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defmodule ChatApp.Connection do # .. . def queue_send(connection, message), do: queue_send(connection, nil, message) def queue_send(connection, message_id, message) do GenServer.cast(connection, {:queue_send, message_id, message}) end def handle_cast({:queue_send, message_id, message}, state) do case :gen_tcp.send(state.socket, message) do :ok - > :ok error - > if is_reference(message_id) do ConnectionManager.receive_send_error(state.manager, message_id, error) end end {:noreply, state} end end

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defmodule ChatApp.Connection do # .. . def queue_send(connection, message), do: queue_send(connection, nil, message) def queue_send(connection, message_id, message) do GenServer.cast(connection, {:queue_send, message_id, message}) end def handle_cast({:queue_send, message_id, message}, state) do case :gen_tcp.send(state.socket, message) do :ok - > :ok error - > if is_reference(message_id) do ConnectionManager.receive_send_error(state.manager, message_id, error) end end {:noreply, state} end end

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defmodule ChatApp.ConnectionManager do # .. . def receive_send_error(manager \\ __MODULE__, message_id, error) do GenServer.cast(manager, {:receive_send_error, message_id, error}) end def handle_cast({:receive_send_error, message_id, _error}, state) do state.ui.show_send_failure(message_id) {:noreply, state} end end

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defmodule ChatApp.ConnectionManager do # .. . def receive_send_error(manager \\ __MODULE__, message_id, error) do GenServer.cast(manager, {:receive_send_error, message_id, error}) end def handle_cast({:receive_send_error, message_id, _error}, state) do state.ui.show_send_failure(message_id) {:noreply, state} end end

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One More Thing…

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defmodule ChatApp.ConnectionManager do # .. . def reset(manager \\ __MODULE__), do: GenServer.call(manager, :reset) def handle_call(:reset, _from, state) do queue_all_sends(:disconnected, state) for_active_connections(fn {:unde fi ned, pid, :worker, [module]} - > apply(module, :close, [pid]) end) {:reply, :ok, %__MODULE__{}} end end

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defmodule ChatApp.ConnectionManager do # .. . def reset(manager \\ __MODULE__), do: GenServer.call(manager, :reset) def handle_call(:reset, _from, state) do queue_all_sends(:disconnected, state) for_active_connections(fn {:unde fi ned, pid, :worker, [module]} - > apply(module, :close, [pid]) end) {:reply, :ok, %__MODULE__{}} end end

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defmodule ChatApp.ConnectionManager do # .. . def reset(manager \\ __MODULE__), do: GenServer.call(manager, :reset) def handle_call(:reset, _from, state) do queue_all_sends(:disconnected, state) for_active_connections(fn {:unde fi ned, pid, :worker, [module]} - > apply(module, :close, [pid]) end) {:reply, :ok, %__MODULE__{}} end end

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Let's Try It! I'm sure Brett won't mind if we give him a call…

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The User Interface

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The UI Problem • Will a Terminal interface work for this challenge? • What happens when a message arrives while we're typing? • This is another side of the multitasking challenge

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Cheating? • It is possible to use a Terminal • Terminals default to operation in "cooked" mode • This reads lines of input at a time and more • It's possible to switch to "raw" mode, say by shelling out to `stty` • In raw mode, you can read a character at a time • This allows keeping track of what has been entered so you can clear the screen and rerender as messages arrive

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Plan B

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defmodule ChatApp.GUI do use GenServer use Bitwise require Record alias ChatApp.ConnectionManager # constants from wx/include/wx.hr l @default 70 @multiline 32 @rich 32768 @horizontal 4 @vertical 8 @left 16 @right 32 @up 64 @down 128 @all @left ||| @right ||| @up ||| @down @expand 8192 @return_key 13 # .. . end

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defmodule ChatApp.GUI do use GenServer use Bitwise require Record alias ChatApp.ConnectionManager # constants from wx/include/wx.hr l @default 70 @multiline 32 @rich 32768 @horizontal 4 @vertical 8 @left 16 @right 32 @up 64 @down 128 @all @left ||| @right ||| @up ||| @down @expand 8192 @return_key 13 # .. . end

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defmodule ChatApp.GUI do # .. . defstruct window: nil , chat: nil , bold: nil , italic: nil , input: nil , button: nil , active_sends: Map.new() def start_link([]), do: GenServer.start_link(__MODULE__, [], name: __MODULE__) def init([]) do :timer.send_interval(5 * 60 * 1_000, :prune_active_sends) {:ok, %__MODULE__{}, {:continue, :show_gui}} end end

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defmodule ChatApp.GUI do # .. . defstruct window: nil , chat: nil , bold: nil , italic: nil , input: nil , button: nil , active_sends: Map.new() def start_link([]), do: GenServer.start_link(__MODULE__, [], name: __MODULE__) def init([]) do :timer.send_interval(5 * 60 * 1_000, :prune_active_sends) {:ok, %__MODULE__{}, {:continue, :show_gui}} end end

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defmodule ChatApp.GUI do # .. . defstruct window: nil , chat: nil , bold: nil , italic: nil , input: nil , button: nil , active_sends: Map.new() def start_link([]), do: GenServer.start_link(__MODULE__, [], name: __MODULE__) def init([]) do :timer.send_interval(5 * 60 * 1_000, :prune_active_sends) {:ok, %__MODULE__{}, {:continue, :show_gui}} end end

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defmodule ChatApp.GUI do # .. . defstruct window: nil , chat: nil , bold: nil , italic: nil , input: nil , button: nil , active_sends: Map.new() def start_link([]), do: GenServer.start_link(__MODULE__, [], name: __MODULE__) def init([]) do :timer.send_interval(5 * 60 * 1_000, :prune_active_sends) {:ok, %__MODULE__{}, {:continue, :show_gui}} end end

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defmodule ChatApp.GUI do # .. . def handle_continue(:show_gui, state) do wx = :wx.new() gui = :wx.batch(fn -> prepare_gui(wx) end) :wxWindow.show(gui.window) { :noreply , %__MODULE__{ stat e | window: gui.window , chat: gui.chat , bold: gui.bold , italic: gui.italic , input: gui.input , button: gui.butto n } } end defp prepare_gui(wx) do w x |> build_gui() |> layout_gui() |> setup_events() end end

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defmodule ChatApp.GUI do # .. . def handle_continue(:show_gui, state) do wx = :wx.new() gui = :wx.batch(fn -> prepare_gui(wx) end) :wxWindow.show(gui.window) { :noreply , %__MODULE__{ stat e | window: gui.window , chat: gui.chat , bold: gui.bold , italic: gui.italic , input: gui.input , button: gui.butto n } } end defp prepare_gui(wx) do w x |> build_gui() |> layout_gui() |> setup_events() end end

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defmodule ChatApp.GUI do # .. . def handle_continue(:show_gui, state) do wx = :wx.new() gui = :wx.batch(fn -> prepare_gui(wx) end) :wxWindow.show(gui.window) { :noreply , %__MODULE__{ stat e | window: gui.window , chat: gui.chat , bold: gui.bold , italic: gui.italic , input: gui.input , button: gui.butto n } } end defp prepare_gui(wx) do w x |> build_gui() |> layout_gui() |> setup_events() end end

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defmodule ChatApp.GUI do # .. . defp build_gui(wx) do window = :wxFrame.new(wx, -1, "Chat", size: {800, 600}) controls = :wxPanel.new(window) chat = :wxTextCtrl.new(controls, 1, style: @multiline ||| @rich) :wxTextCtrl.setEditable(chat, false) bold = :wxNORMAL_FONT |> :wxe_util.get_const() |> :wxFont.new() :wxFont.setWeight(bold, :wxe_util.get_const(:wxFONTWEIGHT_BOLD)) italic = :wxe_util.get_const(:wxITALIC_FONT) c = "Commands:\n /listen PORT NAME\n /connect HOST PORT NAME\n /quit\n" append_text_with_font(chat, c, italic) form = :wxPanel.new(controls) input = :wxTextCtrl.new(form, 2, style: @default) button = :wxButton.new(form, 3, label: "Send") %{ window: window , controls: controls , chat: chat , bold: bold , italic: italic , form: form , input: input , button: butto n } end end

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defmodule ChatApp.GUI do # .. . defp build_gui(wx) do window = :wxFrame.new(wx, -1, "Chat", size: {800, 600}) controls = :wxPanel.new(window) chat = :wxTextCtrl.new(controls, 1, style: @multiline ||| @rich) :wxTextCtrl.setEditable(chat, false) bold = :wxNORMAL_FONT |> :wxe_util.get_const() |> :wxFont.new() :wxFont.setWeight(bold, :wxe_util.get_const(:wxFONTWEIGHT_BOLD)) italic = :wxe_util.get_const(:wxITALIC_FONT) c = "Commands:\n /listen PORT NAME\n /connect HOST PORT NAME\n /quit\n" append_text_with_font(chat, c, italic) form = :wxPanel.new(controls) input = :wxTextCtrl.new(form, 2, style: @default) button = :wxButton.new(form, 3, label: "Send") %{ window: window , controls: controls , chat: chat , bold: bold , italic: italic , form: form , input: input , button: butto n } end end

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defmodule ChatApp.GUI do # .. . defp build_gui(wx) do window = :wxFrame.new(wx, -1, "Chat", size: {800, 600}) controls = :wxPanel.new(window) chat = :wxTextCtrl.new(controls, 1, style: @multiline ||| @rich) :wxTextCtrl.setEditable(chat, false) bold = :wxNORMAL_FONT |> :wxe_util.get_const() |> :wxFont.new() :wxFont.setWeight(bold, :wxe_util.get_const(:wxFONTWEIGHT_BOLD)) italic = :wxe_util.get_const(:wxITALIC_FONT) c = "Commands:\n /listen PORT NAME\n /connect HOST PORT NAME\n /quit\n" append_text_with_font(chat, c, italic) form = :wxPanel.new(controls) input = :wxTextCtrl.new(form, 2, style: @default) button = :wxButton.new(form, 3, label: "Send") %{ window: window , controls: controls , chat: chat , bold: bold , italic: italic , form: form , input: input , button: butto n } end end

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defmodule ChatApp.GUI do # .. . defp append_text(ctrl, text) do :wxTextCtrl.appendText(ctrl, text) end defp append_text_with_font(ctrl, text, font) do style = :wxTextCtrl.getDefaultStyle(ctrl) :wxTextAttr.setFont(style, font) :wxTextCtrl.setDefaultStyle(ctrl, style) append_text(ctrl, text) :wxTextAttr.setFont(style, :wxe_util.get_const(:wxNORMAL_FONT)) :wxTextCtrl.setDefaultStyle(ctrl, style) end end

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defmodule ChatApp.GUI do # .. . defp append_text(ctrl, text) do :wxTextCtrl.appendText(ctrl, text) end defp append_text_with_font(ctrl, text, font) do style = :wxTextCtrl.getDefaultStyle(ctrl) :wxTextAttr.setFont(style, font) :wxTextCtrl.setDefaultStyle(ctrl, style) append_text(ctrl, text) :wxTextAttr.setFont(style, :wxe_util.get_const(:wxNORMAL_FONT)) :wxTextCtrl.setDefaultStyle(ctrl, style) end end

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defmodule ChatApp.GUI do # .. . defp layout_gui(gui) do window_sizer = :wxBoxSizer.new(@vertical) :wxSizer.add( window_sizer , gui.controls , border: 4 , proportion: 1 , fl ag: @expand ||| @all ) controls_sizer = :wxBoxSizer.new(@vertical) :wxSizer.add(controls_sizer, gui.chat, proportion: 1, fl ag: @expand) :wxSizer.addSpacer(controls_sizer, 4) :wxSizer.add(controls_sizer, gui.form, proportion: 0, fl ag: @expand) form_sizer = :wxBoxSizer.new(@horizontal) :wxSizer.add(form_sizer, gui.input, proportion: 1, fl ag: @expand) :wxSizer.addSpacer(controls_sizer, 4) :wxSizer.add(form_sizer, gui.button) :wxWindow.setSizer(gui.form, form_sizer) :wxWindow.setSizer(gui.controls, controls_sizer) :wxWindow.setSizer(gui.window, window_sizer) gu i end end

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defmodule ChatApp.GUI do # .. . defp layout_gui(gui) do window_sizer = :wxBoxSizer.new(@vertical) :wxSizer.add( window_sizer , gui.controls , border: 4 , proportion: 1 , fl ag: @expand ||| @all ) controls_sizer = :wxBoxSizer.new(@vertical) :wxSizer.add(controls_sizer, gui.chat, proportion: 1, fl ag: @expand) :wxSizer.addSpacer(controls_sizer, 4) :wxSizer.add(controls_sizer, gui.form, proportion: 0, fl ag: @expand) form_sizer = :wxBoxSizer.new(@horizontal) :wxSizer.add(form_sizer, gui.input, proportion: 1, fl ag: @expand) :wxSizer.addSpacer(controls_sizer, 4) :wxSizer.add(form_sizer, gui.button) :wxWindow.setSizer(gui.form, form_sizer) :wxWindow.setSizer(gui.controls, controls_sizer) :wxWindow.setSizer(gui.window, window_sizer) gu i end end

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defmodule ChatApp.GUI do # .. . defp setup_events(gui) do :wxWindow.setFocus(gui.input) :wxEvtHandler.connect(gui.window, :close_window) :wxEvtHandler.connect(gui.input, :key_down, skip: true) :wxEvtHandler.connect(gui.button, :command_button_clicked) gu i end end

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defmodule ChatApp.GUI do # .. . defp setup_events(gui) do :wxWindow.setFocus(gui.input) :wxEvtHandler.connect(gui.window, :close_window) :wxEvtHandler.connect(gui.input, :key_down, skip: true) :wxEvtHandler.connect(gui.button, :command_button_clicked) gu i end end

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defmodule ChatApp.GUI do # .. . defp setup_events(gui) do :wxWindow.setFocus(gui.input) :wxEvtHandler.connect(gui.window, :close_window) :wxEvtHandler.connect(gui.input, :key_down, skip: true) :wxEvtHandler.connect(gui.button, :command_button_clicked) gu i end end

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Records

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defmodule ChatApp.GUI do # .. . Record.extract_all(from_lib: "wx/include/wx.hrl") |> Enum.map(fn {name, fi elds} -> Record.defrecordp(name, fi elds) end) def handle_info( wx( id: _id , obj: window , userData: _userData , event: wxClose(type: :close_window) ) , %__MODULE__{window: window} = stat e ) do quit(state) {:noreply, state} end defp quit(state) do ConnectionManager.reset() :wxWindow.destroy(state.window) :wx.destroy() System.stop(0) end end

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defmodule ChatApp.GUI do # .. . Record.extract_all(from_lib: "wx/include/wx.hrl") |> Enum.map(fn {name, fi elds} -> Record.defrecordp(name, fi elds) end) def handle_info( wx( id: _id , obj: window , userData: _userData , event: wxClose(type: :close_window) ) , %__MODULE__{window: window} = stat e ) do quit(state) {:noreply, state} end defp quit(state) do ConnectionManager.reset() :wxWindow.destroy(state.window) :wx.destroy() System.stop(0) end end

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defmodule ChatApp.GUI do # .. . Record.extract_all(from_lib: "wx/include/wx.hrl") |> Enum.map(fn {name, fi elds} -> Record.defrecordp(name, fi elds) end) def handle_info( wx( id: _id , obj: window , userData: _userData , event: wxClose(type: :close_window) ) , %__MODULE__{window: window} = stat e ) do quit(state) {:noreply, state} end defp quit(state) do ConnectionManager.reset() :wxWindow.destroy(state.window) :wx.destroy() System.stop(0) end end

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defmodule ChatApp.GUI do # .. . Record.extract_all(from_lib: "wx/include/wx.hrl") |> Enum.map(fn {name, fi elds} -> Record.defrecordp(name, fi elds) end) def handle_info( wx( id: _id , obj: window , userData: _userData , event: wxClose(type: :close_window) ) , %__MODULE__{window: window} = stat e ) do quit(state) {:noreply, state} end defp quit(state) do ConnectionManager.reset() :wxWindow.destroy(state.window) :wx.destroy() System.stop(0) end end

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defmodule ChatApp.GUI do # .. . def handle_info( wx( id: _id , obj: input , userData: _userData , event: wxKey( type: :key_down , x: _x , y: _y , keyCode: @return_key , controlDown: false , shiftDown: false , altDown: false , metaDown: false , uniChar: _uniChar , rawCode: _rawCode , rawFlags: _rawFlags ) ) , %__MODULE__{input: input} = stat e ) do new_active_sends = process_input(state) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end end

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defmodule ChatApp.GUI do # .. . def handle_info( wx( id: _id , obj: input , userData: _userData , event: wxKey( type: :key_down , x: _x , y: _y , keyCode: @return_key , controlDown: false , shiftDown: false , altDown: false , metaDown: false , uniChar: _uniChar , rawCode: _rawCode , rawFlags: _rawFlags ) ) , %__MODULE__{input: input} = stat e ) do new_active_sends = process_input(state) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end end

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defmodule ChatApp.GUI do # .. . def handle_info( wx( id: _id , obj: input , userData: _userData , event: wxKey( type: :key_down , x: _x , y: _y , keyCode: @return_key , controlDown: false , shiftDown: false , altDown: false , metaDown: false , uniChar: _uniChar , rawCode: _rawCode , rawFlags: _rawFlags ) ) , %__MODULE__{input: input} = stat e ) do new_active_sends = process_input(state) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end end

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defmodule ChatApp.GUI do # .. . def handle_info( wx( id: _id , obj: button , userData: _userData , event: wxCommand( type: :command_button_clicked , cmdString: _cmdString , commandInt: _commandInt , extraLong: _extraLong ) ) , %__MODULE__{button: button} = stat e ) do new_active_sends = process_input(state) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end end

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defmodule ChatApp.GUI do # .. . def handle_info( wx( id: _id , obj: button , userData: _userData , event: wxCommand( type: :command_button_clicked , cmdString: _cmdString , commandInt: _commandInt , extraLong: _extraLong ) ) , %__MODULE__{button: button} = stat e ) do new_active_sends = process_input(state) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end end

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defmodule ChatApp.GUI do # .. . def handle_info( wx( id: _id , obj: button , userData: _userData , event: wxCommand( type: :command_button_clicked , cmdString: _cmdString , commandInt: _commandInt , extraLong: _extraLong ) ) , %__MODULE__{button: button} = stat e ) do new_active_sends = process_input(state) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end end

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defmodule ChatApp.GUI do # .. . defp process_input(state) do new_active_sends = case state.input |> :wxTextCtrl.getValue() |> to_string() do "/" <> command - > process_command(command, state) state.active_send s message when byte_size(message) > 0 - > case ConnectionManager.send_to_all(message) do {ref, name} - > append_message(name, message, state) now = System.monotonic_time(:second) Map.put(state.active_sends, ref, {message, now}) nil - > state.active_send s end "" - > state.active_send s end :wxTextCtrl.clear(state.input) new_active_send s end end

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defmodule ChatApp.GUI do # .. . defp process_input(state) do new_active_sends = case state.input |> :wxTextCtrl.getValue() |> to_string() do "/" <> command - > process_command(command, state) state.active_send s message when byte_size(message) > 0 - > case ConnectionManager.send_to_all(message) do {ref, name} - > append_message(name, message, state) now = System.monotonic_time(:second) Map.put(state.active_sends, ref, {message, now}) nil - > state.active_send s end "" - > state.active_send s end :wxTextCtrl.clear(state.input) new_active_send s end end

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defmodule ChatApp.GUI do # .. . defp process_input(state) do new_active_sends = case state.input |> :wxTextCtrl.getValue() |> to_string() do "/" <> command - > process_command(command, state) state.active_send s message when byte_size(message) > 0 - > case ConnectionManager.send_to_all(message) do {ref, name} - > append_message(name, message, state) now = System.monotonic_time(:second) Map.put(state.active_sends, ref, {message, now}) nil - > state.active_send s end "" - > state.active_send s end :wxTextCtrl.clear(state.input) new_active_send s end end

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defmodule ChatApp.GUI do # .. . defp process_input(state) do new_active_sends = case state.input |> :wxTextCtrl.getValue() |> to_string() do "/" <> command - > process_command(command, state) state.active_send s message when byte_size(message) > 0 - > case ConnectionManager.send_to_all(message) do {ref, name} - > append_message(name, message, state) now = System.monotonic_time(:second) Map.put(state.active_sends, ref, {message, now}) nil - > state.active_send s end "" - > state.active_send s end :wxTextCtrl.clear(state.input) new_active_send s end end

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defmodule ChatApp.GUI do # .. . defp process_input(state) do new_active_sends = case state.input |> :wxTextCtrl.getValue() |> to_string() do "/" <> command - > process_command(command, state) state.active_send s message when byte_size(message) > 0 - > case ConnectionManager.send_to_all(message) do {ref, name} - > append_message(name, message, state) now = System.monotonic_time(:second) Map.put(state.active_sends, ref, {message, now}) nil - > state.active_send s end "" - > state.active_send s end :wxTextCtrl.clear(state.input) new_active_send s end end

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defmodule ChatApp.GUI do # .. . defp process_input(state) do new_active_sends = case state.input |> :wxTextCtrl.getValue() |> to_string() do "/" <> command - > process_command(command, state) state.active_send s message when byte_size(message) > 0 - > case ConnectionManager.send_to_all(message) do {ref, name} - > append_message(name, message, state) now = System.monotonic_time(:second) Map.put(state.active_sends, ref, {message, now}) nil - > state.active_send s end "" - > state.active_send s end :wxTextCtrl.clear(state.input) new_active_send s end end

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defmodule ChatApp.GUI do # .. . defp process_input(state) do new_active_sends = case state.input |> :wxTextCtrl.getValue() |> to_string() do "/" <> command - > process_command(command, state) state.active_send s message when byte_size(message) > 0 - > case ConnectionManager.send_to_all(message) do {ref, name} - > append_message(name, message, state) now = System.monotonic_time(:second) Map.put(state.active_sends, ref, {message, now}) nil - > state.active_send s end "" - > state.active_send s end :wxTextCtrl.clear(state.input) new_active_send s end end

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defmodule ChatApp.GUI do # .. . defp process_input(state) do new_active_sends = case state.input |> :wxTextCtrl.getValue() |> to_string() do "/" <> command - > process_command(command, state) state.active_send s message when byte_size(message) > 0 - > case ConnectionManager.send_to_all(message) do {ref, name} - > append_message(name, message, state) now = System.monotonic_time(:second) Map.put(state.active_sends, ref, {message, now}) nil - > state.active_send s end "" - > state.active_send s end :wxTextCtrl.clear(state.input) new_active_send s end end

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defmodule ChatApp.GUI do # .. . defp append_message(name, message, state) do append_text_with_font(state.chat, name, state.bold) append_text(state.chat, ": #{message}\n") end end

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defmodule ChatApp.GUI do # .. . defp append_message(name, message, state) do append_text_with_font(state.chat, name, state.bold) append_text(state.chat, ": #{message}\n") end end

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defmodule ChatApp.GUI do # .. . defp append_message(name, message, state) do append_text_with_font(state.chat, name, state.bold) append_text(state.chat, ": #{message}\n") end end

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defmodule ChatApp.GUI do # .. . defp process_command("listen" <> args, state) do case Regex.named_captures(~r{\A\s+(?\d+)\s+(?\S.*)\z}, args) do %{"port" => port, "name" => name} - > case ConnectionManager.listen(String.to_integer(port), name) do :ok - > append_text_with_font(state.chat, "Listening\n", state.italic) _error - > append_text_with_font( state.chat , "Error: listening failed\n" , state.itali c ) end nil - > append_text_with_font( state.chat , "Usage: /listen PORT NAME\n" , state.itali c ) end end end

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defmodule ChatApp.GUI do # .. . defp process_command("listen" <> args, state) do case Regex.named_captures(~r{\A\s+(?\d+)\s+(?\S.*)\z}, args) do %{"port" => port, "name" => name} - > case ConnectionManager.listen(String.to_integer(port), name) do :ok - > append_text_with_font(state.chat, "Listening\n", state.italic) _error - > append_text_with_font( state.chat , "Error: listening failed\n" , state.itali c ) end nil - > append_text_with_font( state.chat , "Usage: /listen PORT NAME\n" , state.itali c ) end end end

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defmodule ChatApp.GUI do # .. . defp process_command("listen" <> args, state) do case Regex.named_captures(~r{\A\s+(?\d+)\s+(?\S.*)\z}, args) do %{"port" => port, "name" => name} - > case ConnectionManager.listen(String.to_integer(port), name) do :ok - > append_text_with_font(state.chat, "Listening\n", state.italic) _error - > append_text_with_font( state.chat , "Error: listening failed\n" , state.itali c ) end nil - > append_text_with_font( state.chat , "Usage: /listen PORT NAME\n" , state.itali c ) end end end

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defmodule ChatApp.GUI do # .. . defp process_command("listen" <> args, state) do case Regex.named_captures(~r{\A\s+(?\d+)\s+(?\S.*)\z}, args) do %{"port" => port, "name" => name} - > case ConnectionManager.listen(String.to_integer(port), name) do :ok - > append_text_with_font(state.chat, "Listening\n", state.italic) _error - > append_text_with_font( state.chat , "Error: listening failed\n" , state.itali c ) end nil - > append_text_with_font( state.chat , "Usage: /listen PORT NAME\n" , state.itali c ) end end end

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defmodule ChatApp.GUI do # .. . defp process_command("connect" <> args, state) do case Regex.named_captures( ~r{\A\s+(?\S+)\s+(?\d+)\s+(?\S.*)\z} , arg s ) do %{"host" => host, "port" => port, "name" => name} - > case ConnectionManager.connect(host, String.to_integer(port), name) do :ok - > append_text_with_font(state.chat, "Connected\n", state.italic) _error - > append_text_with_font( state.chat , "Error: connecting failed\n" , state.itali c ) end nil - > append_text_with_font( state.chat , "Usage: /connect HOST PORT NAME\n" , state.itali c ) end end end

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defmodule ChatApp.GUI do # .. . defp process_command("connect" <> args, state) do case Regex.named_captures( ~r{\A\s+(?\S+)\s+(?\d+)\s+(?\S.*)\z} , arg s ) do %{"host" => host, "port" => port, "name" => name} - > case ConnectionManager.connect(host, String.to_integer(port), name) do :ok - > append_text_with_font(state.chat, "Connected\n", state.italic) _error - > append_text_with_font( state.chat , "Error: connecting failed\n" , state.itali c ) end nil - > append_text_with_font( state.chat , "Usage: /connect HOST PORT NAME\n" , state.itali c ) end end end

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defmodule ChatApp.GUI do # .. . defp process_command("connect" <> args, state) do case Regex.named_captures( ~r{\A\s+(?\S+)\s+(?\d+)\s+(?\S.*)\z} , arg s ) do %{"host" => host, "port" => port, "name" => name} - > case ConnectionManager.connect(host, String.to_integer(port), name) do :ok - > append_text_with_font(state.chat, "Connected\n", state.italic) _error - > append_text_with_font( state.chat , "Error: connecting failed\n" , state.itali c ) end nil - > append_text_with_font( state.chat , "Usage: /connect HOST PORT NAME\n" , state.itali c ) end end end

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defmodule ChatApp.GUI do # .. . defp process_command("connect" <> args, state) do case Regex.named_captures( ~r{\A\s+(?\S+)\s+(?\d+)\s+(?\S.*)\z} , arg s ) do %{"host" => host, "port" => port, "name" => name} - > case ConnectionManager.connect(host, String.to_integer(port), name) do :ok - > append_text_with_font(state.chat, "Connected\n", state.italic) _error - > append_text_with_font( state.chat , "Error: connecting failed\n" , state.itali c ) end nil - > append_text_with_font( state.chat , "Usage: /connect HOST PORT NAME\n" , state.itali c ) end end end

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Not Very DRY Beware the trap!

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defmodule ChatApp.GUI do # .. . defp process_command("quit", state), do: quit(state) defp process_command(_unknown_command, state) do append_text_with_font( state.chat , "Error: unknown command\n" , state.itali c ) end end

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defmodule ChatApp.GUI do # .. . defp process_command("quit", state), do: quit(state) defp process_command(_unknown_command, state) do append_text_with_font( state.chat , "Error: unknown command\n" , state.itali c ) end end

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defmodule ChatApp.GUI do # .. . defp process_command("quit", state), do: quit(state) defp process_command(_unknown_command, state) do append_text_with_font( state.chat , "Error: unknown command\n" , state.itali c ) end end

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defmodule ChatApp.GUI do # .. . def show_send_failure(ref) do GenServer.cast(__MODULE__, {:show_send_failure, ref}) end def handle_cast({:show_send_failure, ref}, state) do case Map.pop(state.active_sends, ref) do {{message, _timestamp}, new_active_sends} - > append_text_with_font( state.chat , "The following message was not received by all participants: " < > "#{message}\n" , state.itali c ) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} {nil, _active_sends} - > {:noreply, state} end end end

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defmodule ChatApp.GUI do # .. . def show_send_failure(ref) do GenServer.cast(__MODULE__, {:show_send_failure, ref}) end def handle_cast({:show_send_failure, ref}, state) do case Map.pop(state.active_sends, ref) do {{message, _timestamp}, new_active_sends} - > append_text_with_font( state.chat , "The following message was not received by all participants: " < > "#{message}\n" , state.itali c ) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} {nil, _active_sends} - > {:noreply, state} end end end

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defmodule ChatApp.GUI do # .. . def show_send_failure(ref) do GenServer.cast(__MODULE__, {:show_send_failure, ref}) end def handle_cast({:show_send_failure, ref}, state) do case Map.pop(state.active_sends, ref) do {{message, _timestamp}, new_active_sends} - > append_text_with_font( state.chat , "The following message was not received by all participants: " < > "#{message}\n" , state.itali c ) {:noreply, %__MODULE__{state | active_sends: new_active_sends}} {nil, _active_sends} - > {:noreply, state} end end end

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defmodule ChatApp.GUI do # .. . def handle_info(:prune_active_sends, state) do expired = System.monotonic_time(:second) - 5 * 60 new_active_sends = state.active_send s |> Enum.reject(fn {_ref, {_message, timestamp}} - > timestamp < expire d end) |> Map.new() {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end def handle_info(_message, state), do: {:noreply, state} end

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defmodule ChatApp.GUI do # .. . def handle_info(:prune_active_sends, state) do expired = System.monotonic_time(:second) - 5 * 60 new_active_sends = state.active_send s |> Enum.reject(fn {_ref, {_message, timestamp}} - > timestamp < expire d end) |> Map.new() {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end def handle_info(_message, state), do: {:noreply, state} end

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defmodule ChatApp.GUI do # .. . def handle_info(:prune_active_sends, state) do expired = System.monotonic_time(:second) - 5 * 60 new_active_sends = state.active_send s |> Enum.reject(fn {_ref, {_message, timestamp}} - > timestamp < expire d end) |> Map.new() {:noreply, %__MODULE__{state | active_sends: new_active_sends}} end def handle_info(_message, state), do: {:noreply, state} end

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Pardon My Mess I'm a Web Developer, not a GUI expert!

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defmodule ChatApp.GUI do # .. . def show_chat_message(name, content) do GenServer.cast(__MODULE__, {:show_chat_message, name, content}) end def handle_cast({:show_chat_message, name, action}, state) when is_atom(action) do append_text_with_font(state.chat, "#{name} #{action}\n", state.italic) {:noreply, state} end def handle_cast({:show_chat_message, name, content}, state) do append_message(name, content, state) {:noreply, state} end end

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defmodule ChatApp.GUI do # .. . def show_chat_message(name, content) do GenServer.cast(__MODULE__, {:show_chat_message, name, content}) end def handle_cast({:show_chat_message, name, action}, state) when is_atom(action) do append_text_with_font(state.chat, "#{name} #{action}\n", state.italic) {:noreply, state} end def handle_cast({:show_chat_message, name, content}, state) do append_message(name, content, state) {:noreply, state} end end

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defmodule ChatApp.GUI do # .. . def show_chat_message(name, content) do GenServer.cast(__MODULE__, {:show_chat_message, name, content}) end def handle_cast({:show_chat_message, name, action}, state) when is_atom(action) do append_text_with_font(state.chat, "#{name} #{action}\n", state.italic) {:noreply, state} end def handle_cast({:show_chat_message, name, content}, state) do append_message(name, content, state) {:noreply, state} end end

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defmodule ChatApp.GUI do # .. . def show_chat_message(name, content) do GenServer.cast(__MODULE__, {:show_chat_message, name, content}) end def handle_cast({:show_chat_message, name, action}, state) when is_atom(action) do append_text_with_font(state.chat, "#{name} #{action}\n", state.italic) {:noreply, state} end def handle_cast({:show_chat_message, name, content}, state) do append_message(name, content, state) {:noreply, state} end end

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Shall We Try it Out? Let's phone a friend…

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Big Finish Time Let's phone two friends!

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Thank You

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Questions? • These slides • https://speakerdeck.com/jeg2/how-to-level-up-in-elixir • The Code • https://github.com/JEG2/chat_app • Forum Posts • https://elixirforum.com/t/becoming-an-intermediate-elixir-developer/ 37991 • https://elixirforum.com/t/how-to-become-a-senior-in-elixir/40430