Structured Concurrency - Droidcon Lisbon 2019

Before Kotlin Coroutines (async programming in Kotlin) was stable, they were launched in a way that made them difficult to maintain and track down.

Structured Concurrency introduced changes to solve these problems but also involved an ideological shift in order to use them.

In this talk, you'll learn how to do async programming in Kotlin using Structured Concurrency principles covering topics such as launching coroutines, scopes, error handling and testing.

Transcript

1. Structured Concurrency Asynchronous Shift in Kotlin @manuelvicnt Manuel Vicente Vivo
2. What problems are coroutines trying to solve?
3. Coroutines simplify async programming
4. Sync blocking fun loadData() { val data = networkRequest() show(data) } networkRequest show onDraw onDraw onDraw
5. Sync blocking fun networkRequest(): Data { // Blocking network request code }
6. fun loadData() { networkRequest { data -> show(data) } } Async with callbacks networkRequest show onDraw onDraw onDraw Network thread
7. fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks
8. fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks
9. fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks
10. fun loadData() { networkRequest { data -> show(data) } } Async with callbacks
11. fun loadData() { networkRequest { data -> anotherRequest(data) { otherData -> // ... } } } Async with callbacks
12. fun loadData() { networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> // Hey there! You want more? } } } } } Callback Hell fun loadData() { networkRequest { data -> anotherRequest(data) { otherData ->
13. Best of both worlds?
14. suspend fun loadData() { val data = networkRequest() show(data) } Async with coroutines onDraw onDraw onDraw Network thread networkRequest show
15. suspend fun loadData() { val data = networkRequest() show(data) }
16. suspend fun loadData() { val data = networkRequest() show(data) } suspend
17. suspend fun loadData() { val data = networkRequest() show(data) } suspend resume
18. suspend fun loadData() { val data = networkRequest() show(data) } suspend Callback resume
19. Callbacks? The Kotlin compiler writes them under the hood when the computation can suspend
20. Continuation Coroutines call those “callbacks” Continuation
21. Continuation suspend fun loadData() { val data = networkRequest() show(data) }
22. Continuation fun loadData(continuation: Continuation) { val data = networkRequest(continuation) show(data) }
23. State Machine Continuations form a state machine
24. State Machine LoadData Network Request State 0 - init
25. State Machine LoadData Network Request State 1 suspend
26. State Machine LoadData Network Request State 2 resume
27. State Machine LoadData Network Request State 3 - exit
28. Continuation-Passing Style (CPS) Disclaimer Not Covered
29. With Coroutines… Computation gets suspended without blocking the thread
30. suspend fun loadData() { val data = networkRequest() show(data) }
31. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data
32. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { }
33. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code } Dispatchers.IO
34. .Default .Main Dispatchers .IO
35. Dispatchers .IO .Main .Default
36. .IO .Main .Default Network & Disk
37. .IO .Main .Default Network & Disk CPU
38. .IO .Main .Default Network & Disk UI/Non-blocking CPU
39. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code }
40. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code } Main Safe
41. But… what is a coroutine?
42. Coroutine Runnable with super powers Takes a block of code to run in a thread
43. Asynchronicity expressed as sequential code that is easy to read and reason about Other perks: exception handling and cancellation ❤ Coroutine
44. suspend fun loadData() { val data = networkRequest() show(data) }
45. suspend fun loadData() { val data = networkRequest() show(data) } fun onButtonClicked() { loadData() } Suspend fun ‘loadData’ must be called from a coroutine
46. suspend fun loadData() { val data = networkRequest() show(data) } fun onButtonClicked() { launch { loadData() } }
47. suspend fun loadData() { val data = networkRequest() show(data) } fun onButtonClicked() { launch { loadData() } } Who can cancel this execution? Does it follow a particular lifecycle? Who gets exceptions if it fails?
48. Structured Concurrency
49. Scopes Keep track of coroutines Ability to cancel them Gets exceptions
50. fun onButtonClicked() { launch { loadData() } } Launch must be called in a scope // MyViewModel.kt
51. fun onButtonClicked() { launch { loadData() } } // MyViewModel.kt
52. val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { launch { loadData() } } // MyViewModel.kt
53. val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { scope.launch { loadData() } } // MyViewModel.kt
54. val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { scope.launch { loadData() } } When loadData throws, the scope gets the exception
55. val scope = CoroutineScope(Dispatchers.Main) Parent fun onButtonClicked() { scope.launch { loadData() } }
56. val scope = CoroutineScope(Dispatchers.Main) Parent Child fun onButtonClicked() { scope.launch { loadData() } }
57. fun onButtonClicked() { scope.launch { loadData() } } Child fun onCleared() { scope.cancel() }
58. Cancelling a Scope Cancels all children coroutines Useless, cannot start more coroutines
59. suspend fun loadData() { val data = networkRequest() show(data) }
60. suspend fun loadData() { val data = networkRequest() show(data) } Runs in a scope
61. When a function returns, it has completed all work
62. When a function returns, it has completed all work suspend
63. suspend fun loadData() { val data = networkRequest() show(data) }
64. Scopes Exception Handling
65. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope(Dispatchers.Main) // MyViewModel.kt
66. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + Job() ) // MyViewModel.kt
67. Scope with a Job When a child fails, it propagates cancellation to other (scope) children When a failure is notified, the scope propagates the exception up
68. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + Job() ) // MyViewModel.kt
69. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) // MyViewModel.kt
70. Scope with a SupervisorJob The failure of a child doesn’t affect other (scope) children When a failure is notified, the scope doesn’t do anything
71. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) // MyViewModel.kt
72. TL;DR; work is completed suspend fun returns
73. TL;DR; work is completed suspend fun returns scope cancelled children cancel
74. TL;DR; work is completed suspend fun returns scope cancelled children cancel coroutine errors scope notified
75. Create Coroutines
76. Launch Async v s
77. Launch Async
78. Creates a new Coroutine Creates a new Coroutine Launch Async
79. Launch Creates a new Coroutine Fire and Forget scope.launch(Dispatchers.IO) { loggingService.upload(logs) }
80. Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }
81. Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }
82. Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }
83. Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }
84. Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }
85. Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }
86. Creates a new Coroutine Creates a new Coroutine Fire and Forget Returns a value Launch Async
87. Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Fire and Forget Returns a value Launch Async
88. Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Executed in a Scope Executed in a Scope Fire and Forget Returns a value Launch Async
89. Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Executed in a Scope Executed in a Scope Not a suspend function Not a suspend function Fire and Forget Returns a value Launch Async
90. Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Executed in a Scope Executed in a Scope Re-throws exceptions Not a suspend function Not a suspend function Fire and Forget Returns a value Launch Async
91. Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Executed in a Scope Executed in a Scope Not a suspend function Not a suspend function Holds on exceptions until await is called Fire and Forget Returns a value Launch Async Re-throws exceptions
92. Exception handling
93. Wrapping in a try-catch block scope.launch(Dispatchers.Default) { try { loggingService.upload(logs) } catch(e: Exception) { // Handle Exception } }
94. suspend fun getUser(userId: String): User { coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } try { deferred.await() } catch(e: Exception) { // Handle exception } } }
95. suspend fun getUser(userId: String): User { coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } try { deferred.await() } catch(e: Exception) { // Handle exception } } }
96. suspend fun getUser(userId: String): User { coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } try { deferred.await() } catch(e: Exception) { // Handle exception } } }
97. scope.launch(Dispatchers.IO) { for (name in files) { readFile(name) } }
98. Cancellation requires co-operation
99. scope.launch(Dispatchers.IO) { for (name in files) { readFile(name) } }
100. Check if the coroutine is Active scope.launch(Dispatchers.IO) { for (name in files) { if (!isActive) break readFile(name) } }
101. When to mark a function as suspend
102. suspend fun loadData() { val data = networkRequest() show(data) } When it calls other suspend functions
103. suspend fun loadData() { val data = networkRequest() show(data) } When it calls other suspend functions
104. When NOT to mark a function as suspend
105. When it doesn’t call suspend functions fun onButtonClicked() { scope.launch { loadData() } }
106. Tip Don’t mark a function suspend unless you’re forced to
107. Unit Testing Coroutines
108. Use Case 1 The test doesn’t trigger new coroutines
109. suspend fun loadData() { val data = networkRequest() show(data) } // MyViewModel.kt
110. @Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.loadData() // Assert show did something } // MyViewModelTest.kt
111. Use Case 2 The test triggers new coroutines
112. class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }
113. class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }
114. @Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
115. @Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
116. @Test fun `Test loadData happy path`() { val viewModel = MyViewModel() viewModel.onButtonClicked() // Wait for the result -> using a CountDownLatch // LiveDataTestUtil, Mockito await, etc. } // MyViewModelTest.kt
117. @Test fun `Test loadData happy path`() { val viewModel = MyViewModel() viewModel.onButtonClicked() // Wait for the result -> using a CountDownLatch // LiveDataTestUtil, Mockito await, etc. } // MyViewModelTest.kt Code Smell!! Bad practice
118. class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }
119. class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch(dispatcher) { loadData() } } }
120. class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch(dispatcher) { loadData() } } }
121. val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
122. val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
123. val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
124. class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { // Do something else scope.launch(dispatcher) { loadData() } } }
125. val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) testDispatcher.pauseDispatcher() viewModel.onButtonClicked() // Assert onButtonClicked did something else } // MyViewModelTest.kt
126. val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) testDispatcher.pauseDispatcher() viewModel.onButtonClicked() // Assert onButtonClicked did something else testDispatcher.resumeDispatcher() // Assert show did something } // MyViewModelTest.kt
127. Testing Use runBlocking when the test doesn’t create new coroutines
128. Testing As a good practice, inject Dispatchers and use TestCoroutineDispatcher in tests
129. What we covered What problems Coroutines solve Dispatchers & withContext What a Coroutine is How coroutines work under the hood Principles of Structured Concurrency How to create Coroutines Exception Handling When to mark a function as suspend Testing Coroutines & TestCoroutineDispatcher
130. Thank You Questions? @manuelvicnt Manuel Vicente Vivo