Intro to Reactive Programming and RxJava: How to React 😮😱🤪

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Intro to Reactive Programming and RxJava: How to React Jordan Jozwiak

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Reactive Programming Allows us to manipulate a stream of data with functions. 

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Reactive Programming Allows us to manipulate a stream of data with functions.  (asynchronously)

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Reactive Programming

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Reactive Programming 1. Data streams 

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Reactive Programming 1. Data streams  2. Functional Programming 

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Reactive Programming 1. Data streams  2. Functional Programming  3. Asynchronous observers

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applications ! Retrieving data from a database and then filtering out some results based on user settings.

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applications ! Retrieving data from a database and then filtering out some results based on user settings. 2 3 7

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applications ! Retrieving data from a database and then filtering out some results based on user settings. ⏳

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applications ! Retrieving data from a database and then filtering out some results based on user settings. a, b, c -> a, c

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applications ! Retrieve data from a database and then filtering out some results based on user settings. ! Render a UI that combines data from multiple data sources ! Create a real-time model for stock prices ! Show auto-complete search results to a user ! Log data from a temperature, wind, or pressure sensor

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Who actually uses this?

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Who actually uses this? ! Very popular in the Android community

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Who actually uses this? ! Very popular in the Android community ! Some usage at Google for database requests

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Who actually uses this? ! Very popular in the Android community ! Some usage at Google for database requests ! Microsoft, Netflix, GitHub, SoundCloud, and more (see reactivex.io)

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Reactive Programming Theory

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Reactive Programming Theory Implementation Reactive Extensions (reactivex.io) (libraries to make normal languages reactive)

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Reactive Programming Theory Implementation Reactive Extensions (reactivex.io) (libraries to make normal languages reactive) RxJava RxSwift RxJS …

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4 2 1 7 6 filter(x -> (x % 2) == 0)

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4 2 1 7 6 filter(x -> (x % 2) == 0)

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4 2 1 7 6 filter(x -> (x % 2) == 0) 4

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4 2 1 7 6 filter(x -> (x % 2) == 0) 4 2

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4 2 1 7 6 filter(x -> (x % 2) == 0) 4 2 6

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4 2 1 7 6 filter(x -> (x % 2) == 0) 4 2 6

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4 2 1 7 6 SUBJECT subject - the stream that emits data 4 2 6

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4 2 6 OBSERVER observer - “subscribes to” or “consumes” data. There can be multiple observers for a single subject. 4 2 1 7 6

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4 2 1 7 6 filter(x -> (x % 2) == 0) 4 2 6 SUBJECT OBSERVER

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4 2 1 7 6 filter(x -> (x % 2) == 0) 4 2 6 SUBJECT OBSERVER 4 2 6 SERVER 2

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4 2 1 7 6 filter / map / count /   max / min / delay / ... SUBJECT OBSERVER

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4 2 1 7 6 some function that takes ⏳ SUBJECT OBSERVER

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4 2 1 7 6 some function that takes ⏳ SUBJECT OBSERVER

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a d z a h some function that takes ⏳ SUBJECT OBSERVER

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some function that takes ⏳ SUBJECT OBSERVER

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? ? ? ? ? some function that takes ⏳ ? ? ? ? SUBJECT OBSERVER

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some function that takes ⏳ SUBJECT OBSERVER another function

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? ? ? ? ? some function that takes ⏳ ? ? ? ? SUBJECT OBSERVER another function ? ? ? ? ?

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What creates the subject data stream?

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What creates the subject data stream? ! These streams are called Observables in RxJava

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What creates the subject data stream? ! These streams are called Observables in RxJava ! Observable.create(), fromIterable(), etc.

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What creates the subject data stream? ! These streams are called Observables in RxJava ! Observable.create(), fromIterable(), etc. ! Retrofitting existing object-oriented design into Observable data streams can be messy ! Many, many open-source projects that wrap other implementations into an Rx package (an easy way to get ⭐ ⭐ ⭐ on GitHub)

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introduction background examples live demo wrap-up

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What languages are you familiar with?

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~80 % Percent of the most popular languages are imperative   or multi-paradigmatic languages Source: StackOverflow Developer Survey 2018

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What's a language paradigm?

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What's a language paradigm? ! Computer languages resemble linguistic language

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What's a language paradigm? ! Computer languages resemble linguistic language ○ Grammatical structures

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What's a language paradigm? ! Computer languages resemble linguistic language ○ Grammatical structures ○ Tenses: past, present, future

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What's a language paradigm? ! Computer languages resemble linguistic language ○ Grammatical structures ○ Tenses: past, present, future ○ Vocabulary

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What's a language paradigm? ! Computer languages resemble linguistic language ○ Grammatical structures ○ Tenses: past, present, future ○ Vocabulary ○ Dialects

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What's a language paradigm? ! Computer languages resemble linguistic language ○ Grammatical structures ○ Tenses: past, present, future ○ Vocabulary ○ Dialects ! Languages are multi-faceted and evolve over time. They are often multi- paradigmatic languages. ! Examples ○ Imperative (what you’re probably used to)

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Imperative vs. Functional

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Imperative vs. Functional Imperative Programming

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Imperative vs. Functional Imperative Programming ! Describes how the program operates

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Imperative vs. Functional Imperative Programming ! Describes how the program operates ! Uses statements to change state of the program

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Imperative vs. Functional Imperative Programming ! Describes how the program operates ! Uses statements to change state of the program ! Conditional statements, loops, inheritance

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Imperative vs. Functional Imperative Programming ! Describes how the program operates ! Uses statements to change state of the program ! Conditional statements, loops, inheritance ! Easier initial learning curve ! Examples: C, PHP, Java Functional Programming ! Describes what the program should accomplish ! Ideally stateless ! Evaluate functions and create expressions instead of statements if (value % 2 == 0) { // do something }

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History of Imperative Programming ! Old computers utilized vacuum tubes and very limited memory ! Programs written as step-by-step instructions ! FORTRAN introduced in 1954. Compiled, variables, complex expressions. ! 60s and 70s: COBOL, BASIC, C ! Object-oriented code grew in popularity in the 1980s. Smalltalk. C++ ! 90s: Perl, Python, Javascript, Ruby, Java

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History of Functional Programming ! Based on lambda calculus (Church 1936) ○ Study of computation with functions ! Lisp was developed by IBM in the 1950s ○ LISP = LISt Processor ○ Still in use today ○ Only a year younger than FORTRAN ! John Backus gave a 1978 Turing Award lecture on why imperative programming is “bad” and functional programming is “good” ! 80s-00s - Object-Oriented programming dominated based on principles like encapsulation. Academics and theoreticians care about Functional Programming. ! 2010s - JavaScript made passing around functions cool again

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Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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A flow of data objects from one function to the next Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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! Functions are meant to be pure Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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! Functions are meant to be pure ! All functions have an input and output data types Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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! Functions are meant to be pure ! All functions have an input and output data types ! No external state is modified (no side-effects) Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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! Functions are meant to be pure ! All functions have an input and output data types ! No external state is modified (no side-effects) Example:  add(x, y) -> return x + y Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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Uses the Observer pattern with Subject data sources that notify Observers Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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Uses the Observer pattern with Subject data sources that notify Observers Errors propagated through the flow of data Reactive Programming  1. Data streams    2. Functional programming    3. Asynchronous observers 

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Reactive Programming is NOT

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Reactive Programming is NOT ! It's own language ! React, ReactJs, ReactNative

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RxJava

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RxJava ! The Java library for the Reactive Extensions specification ○ This specification is applied to other languages: RxJs, RxSwift, etc. - reactivex.io/languages

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RxJava ! The Java library for the Reactive Extensions specification ○ This specification is applied to other languages: RxJs, RxSwift, etc. - reactivex.io/languages ! Helpful due to complicated stream and asynchronous apis ! Brings functional design to an object-oriented language ! Very helpful for mobile development, which often involves asynchronously requesting data from a server, transforming it, and displaying it in the UI

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introduction background examples live demo wrap-up

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... to subscribe to data Imperative List data =   new DataSource().getData(); for (Integer value : data) { System.out.println(value); } Output: 4, 2, 1, 7, 6, ... Data: 4, 2, 1, 7, 6, ...

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... to subscribe to data Imperative List data =   new DataSource().getData(); for (Integer value : data) { System.out.println(value); } Output: 4, 2, 1, 7, 6, ... Data: 4, 2, 1, 7, 6, ...

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... to subscribe to data Imperative List data =   new DataSource().getData(); for (Integer value : data) { System.out.println(value); } Output: 4, 2, 1, 7, 6, ... Data: 4, 2, 1, 7, 6, ...

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... to subscribe to data Reactive new DataSource().getDataStream() .subscribe(new Consumer() { @Override public void accept(Integer value) { System.out.println(value); } }); Imperative List data =   new DataSource().getData(); for (Integer value : data) { System.out.println(value); } Output: 4, 2, 1, 7, 6, ... Data: 4, 2, 1, 7, 6, ...

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... to subscribe to data Imperative List data =   new DataSource().getData(); for (Integer value : data) { System.out.println(value); } Reactive new DataSource().getDataStream() .subscribe(new Consumer() { @Override public void accept(Integer value) { System.out.println(value); } }); Output: 4, 2, 1, 7, 6, ... Data: 4, 2, 1, 7, 6, ...

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... to subscribe to data Reactive new DataSource().getDataStream() .subscribe(  value -> System.out.println(value)  ); Imperative List data =   new DataSource().getData(); for (Integer value : data) { System.out.println(value); } Output: 4, 2, 1, 7, 6, ... Data: 4, 2, 1, 7, 6, ...

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... to subscribe to data Reactive new DataSource().getDataStream() .subscribe( value -> System.out.println(value) ); Imperative List data =   new DataSource().getData(); for (Integer value : data) { System.out.println(value); } Output: 4, 2, 1, 7, 6, ... Data: 4, 2, 1, 7, 6, ...

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... to subscribe to data Reactive new DataSource().getDataStream() .subscribe( value -> updateUI(value) ); Imperative List data =   new DataSource().getData(); for (Integer value : data) { updateUI(value); }

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... to subscribe to data Reactive new DataSource().getDataStream() .subscribe( value -> saveToDatabase(value) ); Imperative List data =   new DataSource().getData(); for (Integer value : data) { saveToDatabase(value); }

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... to subscribe to data changes

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... to subscribe to data changes Imperative ScheduledExecutorService scheduler = // initialize Runnable runner = new Runnable() { public void run() { List data = new DataSource().getData(); for (Integer value : data) { System.out.println(value); } } }; ScheduledFuture future = // initialize scheduler.schedule(new Runnable() { public void run() { future.cancel(/* mayInterruptIfRunning */ true); } }, /* delay */ 0, TimeUnit.SECONDS);

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... to filter our data

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... to filter our data Imperative List data =   new DataSource().getData(); for (Integer value : data) { if (value % 2 == 0) { System.out.println(value); } } Output: 4, 2, 6, ... Data: 4, 2, 1, 7, 6, ...

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... to filter our data http://reactivex.io/documentation/operators/filter

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... to filter our data Reactive new DataSource().getDataStream() .filter(value -> value % 2 == 0) .subscribe(  result -> System.out.println(result)  ); Imperative List data =   new DataSource().getData(); for (Integer value : data) { if (value % 2 == 0) { System.out.println(value); } } Output: 4, 2, 6, ... Data: 4, 2, 1, 7, 6, ...

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… to map our data Output: 8, 4, 2, 14, 12, ... Data: 4, 2, 1, 7, 6, ...

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… to map our data Imperative List data = new DataSource().getData(); for (Integer value : data) { int result = value * 2; System.out.println(result); } Output: 8, 4, 2, 14, 12, ... Data: 4, 2, 1, 7, 6, ...

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... to map our data Imperative List data = new DataSource().getData(); for (Integer value : data) { int result = value * 2; System.out.println(result); } Output: 8, 4, 2, 14, 12, ... Data: 4, 2, 1, 7, 6, ...

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... to map our data http://reactivex.io/documentation/operators/map

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... to map our data Reactive new DataSource().getDataStream() .map(value -> value * 2) .subscribe( result -> System.out.println(result) ); Imperative List data = new DataSource().getData(); for (Integer value : data) { int result = value * 2; System.out.println(result); } Output: 8, 4, 2, 14, 12, ... Data: 4, 2, 1, 7, 6, ...

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... to group our data

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... to group our data List data = new DataSource().getData(); Map> result = new HashMap<>(); String evenKey = "EVEN"; String oddKey = "ODD"; for (Integer value : data) { String key; if (value % 2 == 0) { key = evenKey; } else { key = oddKey; } result.putIfAbsent( key, new ArrayList<>()); result.get(key).add(datum); } System.out.println(result); Output: {EVEN=[4, 2, 6], ODD=[1, 7]} Data: 4, 2, 1, 7, 6

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... to group our data http://reactivex.io/documentation/operators/groupby

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... to group our data new DataSource().getDataStream() .groupBy( value -> (value % 2) == 0 ? "EVEN" : "ODD" ) .collectInto( // put our groups into a hashmap // this is actually still a little messy ) .subscribe( resultMap -> System.out.println(resultMap) ); List data = new DataSource().getData(); Map> result = new HashMap<>(); String evenKey = "EVEN"; String oddKey = "ODD"; for (Integer value : data) { String key; if (value % 2 == 0) { key = evenKey; } else { key = oddKey; } result.putIfAbsent( key, new ArrayList<>()); result.get(key).add(datum); } System.out.println(result); Output: {EVEN=[4, 2, 6], ODD=[1, 7]} Data: 4, 2, 1, 7, 6

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So many more operators ! Transforming ! Buffer ! Scan ! Filtering ! Debounce ! Distinct ! First ! Last ! Take ! Min ! Combining ! Join ! Merge ! Zip ! Math ! Average ! Concat ! Max

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Some operations can get complicated http://reactivex.io/documentation/operators/join

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introduction background examples live demo wrap-up

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introduction background examples live demo wrap-up

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Reactive Programming 1. Data streams  2. Functional Programming  3. Asynchronous observers

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Thanks!