Structured concurrency_ Manuel Vicente Vivo_Codemotion Madrid 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.

About: Manuel Vicente Vivo, Developer Relations, Google

Manuel is an Android Developer in the Developer Relations team at Google where he works to help other developers succeed.

Transcript

1. 24-25 September, 2019
2. Structured Concurrency Asynchronous shift in Kotlin 24-25 September, 2019 Manuel Vicente Vivo - @manuelvicnt
3. Structured Concurrency
4. What problems are coroutines trying to solve?
5. Coroutines simplify async programming
6. Sync blocking fun loadData() { val data = networkRequest() show(data) } networkRequest show onDraw onDraw onDraw
7. Sync blocking fun networkRequest(): Data { // Blocking network request code }
8. fun loadData() { networkRequest { data -> show(data) } } Async with callbacks networkRequest show onDraw onDraw onDraw Network thread
9. fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks
10. fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks
11. fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks
12. fun loadData() { networkRequest { data -> show(data) } } Async with callbacks
13. fun loadData() { networkRequest { data -> anotherRequest(data) { otherData -> // ... } } } Async with callbacks
14. 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 ->
15. Best of both worlds?
16. suspend fun loadData() { val data = networkRequest() show(data) } Async with coroutines onDraw onDraw onDraw Network thread networkRequest show
17. suspend fun loadData() { val data = networkRequest() show(data) }
18. suspend fun loadData() { val data = networkRequest() show(data) } suspend
19. suspend fun loadData() { val data = networkRequest() show(data) } suspend resume
20. suspend fun loadData() { val data = networkRequest() show(data) } suspend Callback resume
21. Callbacks? The Kotlin compiler writes them under the hood when the computation can suspend
22. Continuation Coroutines call those “callbacks” Continuation
23. Continuation suspend fun loadData() { val data = networkRequest() show(data) }
24. Continuation fun loadData(continuation: Continuation) { val data = networkRequest(continuation) show(data) }
25. State Machine Continuations form a state machine
26. State Machine LoadData Network Request State 0 - init
27. State Machine LoadData Network Request State 1 suspend
28. State Machine LoadData Network Request State 2 resume
29. State Machine LoadData Network Request State 3 - exit
30. Continuation-Passing Style (CPS) Disclaimer Not Covered
31. With Coroutines… Computation gets suspended without blocking the thread
32. suspend fun loadData() { val data = networkRequest() show(data) }
33. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data
34. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { }
35. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code } Dispatchers.IO
36. .Default .Main Dispatchers .IO
37. Dispatchers .IO .Main .Default
38. .IO .Main .Default Network & Disk
39. .IO .Main .Default Network & Disk CPU
40. .IO .Main .Default Network & Disk UI/Non-blocking CPU
41. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code }
42. suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code } Main Safe
43. But… what is a coroutine?
44. Coroutine Runnable with super powers Takes a block of code to run in a thread
45. Asynchronicity expressed as sequential code that is easy to read and reason about Other perks: exception handling and cancellation ❤ Coroutine
46. suspend fun loadData() { val data = networkRequest() show(data) }
47. suspend fun loadData() { val data = networkRequest() show(data) } fun onButtonClicked() { loadData() } Suspend fun ‘loadData’ must be called from a coroutine
48. suspend fun loadData() { val data = networkRequest() show(data) } fun onButtonClicked() { launch { loadData() } }
49. 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?
50. Structured Concurrency
51. Scopes Keep track of coroutines Ability to cancel them Gets exceptions
52. fun onButtonClicked() { launch { loadData() } } Launch must be called in a scope // MyViewModel.kt
53. fun onButtonClicked() { launch { loadData() } } // MyViewModel.kt
54. val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { launch { loadData() } } // MyViewModel.kt
55. val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { scope.launch { loadData() } } // MyViewModel.kt
56. val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { scope.launch { loadData() } } When loadData throws, the scope gets the exception
57. val scope = CoroutineScope(Dispatchers.Main) Parent fun onButtonClicked() { scope.launch { loadData() } }
58. val scope = CoroutineScope(Dispatchers.Main) Parent Child fun onButtonClicked() { scope.launch { loadData() } }
59. fun onButtonClicked() { scope.launch { loadData() } } Child fun onCleared() { scope.cancel() }
60. Cancelling a Scope Cancels all children coroutines Useless, cannot start more coroutines
61. suspend fun loadData() { val data = networkRequest() show(data) }
62. suspend fun loadData() { val data = networkRequest() show(data) } Runs in a scope
63. When a function returns, it has completed all work
64. When a function returns, it has completed all work suspend
65. suspend fun loadData() { val data = networkRequest() show(data) }
66. Scopes Exception Handling
67. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope(Dispatchers.Main) // MyViewModel.kt
68. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + Job() ) // MyViewModel.kt
69. 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
70. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + Job() ) // MyViewModel.kt
71. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) // MyViewModel.kt
72. 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
73. fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) // MyViewModel.kt
74. TL;DR; work is completed suspend fun returns
75. TL;DR; work is completed suspend fun returns scope cancelled children cancel
76. TL;DR; work is completed suspend fun returns scope cancelled children cancel coroutine errors scope notified
77. Create Coroutines
78. Launch Async v s
79. Launch Async
80. Creates a new Coroutine Creates a new Coroutine Launch Async
81. Launch Creates a new Coroutine Fire and Forget scope.launch(Dispatchers.IO) { loggingService.upload(logs) }
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. 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() }
87. 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() }
88. Creates a new Coroutine Creates a new Coroutine Fire and Forget Returns a value Launch Async
89. Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher 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 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 Fire and Forget Returns a value Launch Async
92. 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
93. 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
94. Exception handling
95. Wrapping in a try-catch block scope.launch(Dispatchers.Default) { try { loggingService.upload(logs) } 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. suspend fun getUser(userId: String): User { coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } try { deferred.await() } catch(e: Exception) { // Handle exception } } }
98. suspend fun getUser(userId: String): User { coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } try { deferred.await() } catch(e: Exception) { // Handle exception } } }
99. scope.launch(Dispatchers.IO) { for (name in files) { readFile(name) } }
100. Cancellation requires co-operation
101. scope.launch(Dispatchers.IO) { for (name in files) { readFile(name) } }
102. Check if the coroutine is Active scope.launch(Dispatchers.IO) { for (name in files) { if (!isActive) break readFile(name) } }
103. When to mark a function as suspend
104. suspend fun loadData() { val data = networkRequest() show(data) } When it calls other suspend functions
105. suspend fun loadData() { val data = networkRequest() show(data) } When it calls other suspend functions
106. When NOT to mark a function as suspend
107. When it doesn’t call suspend functions fun onButtonClicked() { scope.launch { loadData() } }
108. Tip Don’t mark a function suspend unless you’re forced to
109. Unit Testing Coroutines
110. Use Case 1 The test doesn’t trigger new coroutines
111. suspend fun loadData() { val data = networkRequest() show(data) } // MyViewModel.kt
112. @Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.loadData() // Assert show did something } // MyViewModelTest.kt
113. Use Case 2 The test triggers new coroutines
114. class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }
115. class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }
116. @Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
117. @Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
118. @Test fun `Test loadData happy path`() { val viewModel = MyViewModel() viewModel.onButtonClicked() // Wait for the result -> using a CountDownLatch // LiveDataTestUtil, Mockito await, etc. } // MyViewModelTest.kt
119. @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
120. class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }
121. class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch(dispatcher) { loadData() } } }
122. class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch(dispatcher) { loadData() } } }
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. val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
125. val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt
126. class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { // Do something else scope.launch(dispatcher) { loadData() } } }
127. 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
128. 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
129. Testing Use runBlocking when the test doesn’t create new coroutines
130. Testing As a good practice, inject Dispatchers and use TestCoroutineDispatcher in tests
131. 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
132. Thank You Questions? @manuelvicnt Manuel Vicente Vivo