Introduce MainScope factory and CoroutineScope.cancel extension

Fixes #829

Author: qwwdfsad

README.md

@@ -19,9 +19,10 @@ GlobalScope.launch {
 
 ## Modules
 
-* [common](common/README.md) — common coroutines across all backends:
+* [common](common/README.md) — common coroutines across all platforms:
   * `launch` and `async` coroutine builders;
   * `Job` and `Deferred` light-weight future with cancellation support;
+  * `MainScope` for Android and UI applications.
   * `Dispatchers` object with `Main` dispatcher for Android/Swing/JavaFx, and `Default` dispatcher for background coroutines;
   * `delay` and `yield` top-level suspending functions;
   * `Channel` and `Mutex` communication and synchronization primitives;

binary-compatibility-validator/reference-public-api/kotlinx-coroutines-core.txt

@@ -168,6 +168,8 @@ public abstract interface class kotlinx/coroutines/CoroutineScope {
 
 public final class kotlinx/coroutines/CoroutineScopeKt {
 	public static final fun CoroutineScope (Lkotlin/coroutines/CoroutineContext;)Lkotlinx/coroutines/CoroutineScope;
+	public static final fun MainScope ()Lkotlinx/coroutines/CoroutineScope;
+	public static final fun cancel (Lkotlinx/coroutines/CoroutineScope;)V
 	public static final fun coroutineScope (Lkotlin/jvm/functions/Function2;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
 	public static final fun isActive (Lkotlinx/coroutines/CoroutineScope;)Z
 	public static final fun plus (Lkotlinx/coroutines/CoroutineScope;Lkotlin/coroutines/CoroutineContext;)Lkotlinx/coroutines/CoroutineScope;

common/kotlinx-coroutines-core-common/src/CoroutineScope.kt

@@ -73,6 +73,29 @@ public interface CoroutineScope {
 public operator fun CoroutineScope.plus(context: CoroutineContext): CoroutineScope =
     ContextScope(coroutineContext + context)
 
+/**
+ * Creates [CoroutineScope] for a UI components.
+ *
+ * Example of use:
+ * ```
+ * class MyAndroidActivity {
+ *   private val scope = MainScope()
+ *
+ *   override fun onDestroy() {
+ *     super.onDestroy()
+ *     scope.cancel()
+ *   }
+ * }
+ *
+ * ```
+ *
+ * Resulting scope has [SupervisorJob] and [Dispatchers.Main].
+ * If you want to append additional elements to main scope, use [CoroutineScope.plus] operator:
+ * `val scope = MainScope() + CoroutineName("MyActivity") `.
+ */
+@Suppress("FunctionName")
+public fun MainScope(): CoroutineScope = ContextScope(SupervisorJob() + Dispatchers.Main)
+
 /**
  * Returns `true` when current [Job] is still active (has not completed and was not cancelled yet).
  *
@@ -172,3 +195,16 @@ public suspend fun <R> coroutineScope(block: suspend CoroutineScope.() -> R): R
 @Suppress("FunctionName")
 public fun CoroutineScope(context: CoroutineContext): CoroutineScope =
     ContextScope(if (context[Job] != null) context else context + Job())
+
+/**
+ * Cancels this scope, including its job and all its children.
+ * Throws [IllegalStateException] if scope does not have a job in it.
+ *
+ * This API is experimental in order to investigate possible clashes with other cancellation mechanism.
+ */
+@Suppress("NOTHING_TO_INLINE")
+@ExperimentalCoroutinesApi // Experimental and inline until 1.2
+public inline fun CoroutineScope.cancel() {
+    val job = coroutineContext[Job] ?: error("Current scope cannot be cancelled because it does not have a job: $this")
+    job.cancel()
+}

core/kotlinx-coroutines-core/src/Dispatchers.kt

@@ -33,6 +33,7 @@ public actual object Dispatchers {
 
     /**
      * A coroutine dispatcher that is confined to the Main thread operating with UI objects.
+     * This dispatcher can be used either directly or via [MainScope] factory.
      * Usually such dispatcher is single-threaded.
      *
      * Access to this property may throw [IllegalStateException] if no main thread dispatchers are present in the classpath.

ui/coroutines-guide-ui.md

@@ -472,26 +472,19 @@ The natural solution to this problem is to associate a [Job] object with each UI
 all the coroutines in the context of this job. But passing associated job object to every coroutine builder is error-prone, 
 it is easy to forget it. For this purpose, [CoroutineScope] interface should be implemented by UI owner, and then every
 coroutine builder defined as an extension on [CoroutineScope] inherits UI job without explicitly mentioning it.
+For the sake of simplicity, [MainScope()] factory can be used. It automatically provides `Dispatchers.Main` and parent 
+job.
 
 For example, in Android application an `Activity` is initially _created_ and is _destroyed_ when it is no longer 
 needed and when its memory must be released. A natural solution is to attach an 
 instance of a `Job` to an instance of an `Activity`:
 <!--- CLEAR -->
 
 ```kotlin
-abstract class ScopedAppActivity: AppCompatActivity(), CoroutineScope {
-    protected lateinit var job: Job
-    override val coroutineContext: CoroutineContext 
-        get() = job + Dispatchers.Main
-    
-    override fun onCreate(savedInstanceState: Bundle?) {
-        super.onCreate(savedInstanceState)
-        job = Job()
-    }
-        
+abstract class ScopedAppActivity: AppCompatActivity(), CoroutineScope by MainScope() {
     override fun onDestroy() {
         super.onDestroy()
-        job.cancel()
+        cancel() // CoroutineScope.cancel
     } 
 }
 ```
@@ -711,6 +704,7 @@ After delay
 [Job]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html
 [Job.cancel]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/cancel.html
 [CoroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/index.html
+[MainScope()]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-main-scope.html
 [coroutineScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/coroutine-scope.html
 [withContext]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html
 [Dispatchers.Default]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-default.html

ui/kotlinx-coroutines-swing/test/SwingTest.kt

@@ -6,7 +6,10 @@ package kotlinx.coroutines.swing
 
 import kotlinx.coroutines.*
 import org.junit.*
+import org.junit.Test
 import javax.swing.*
+import kotlin.coroutines.*
+import kotlin.test.*
 
 class SwingTest : TestBase() {
     @Before
@@ -29,4 +32,53 @@ class SwingTest : TestBase() {
         job.join()
         finish(6)
     }
+
+    private class SwingComponent(coroutineContext: CoroutineContext = EmptyCoroutineContext) :
+        CoroutineScope by MainScope() + coroutineContext {
+        public var executed = false
+        fun testLaunch(): Job = launch {
+            check(SwingUtilities.isEventDispatchThread())
+            executed = true
+        }
+        fun testFailure(): Job = launch {
+            check(SwingUtilities.isEventDispatchThread())
+            throw TestException()
+        }
+        fun testCancellation() : Job = launch(start = CoroutineStart.ATOMIC) {
+            check(SwingUtilities.isEventDispatchThread())
+            delay(Long.MAX_VALUE)
+        }
+    }
+    @Test
+    fun testLaunchInMainScope() = runTest {
+        val component = SwingComponent()
+        val job = component.testLaunch()
+        job.join()
+        assertTrue(component.executed)
+        component.cancel()
+        component.coroutineContext[Job]!!.join()
+    }
+
+    @Test
+    fun testFailureInMainScope() = runTest {
+        var exception: Throwable? = null
+        val component = SwingComponent(CoroutineExceptionHandler { ctx, e ->  exception = e})
+        val job = component.testFailure()
+        job.join()
+        assertTrue(exception!! is TestException)
+        component.cancel()
+        join(component)
+    }
+
+    @Test
+    fun testCancellationInMainScope() = runTest {
+        val component = SwingComponent()
+        component.cancel()
+        component.testCancellation().join()
+        join(component)
+    }
+
+    private suspend fun join(component: SwingTest.SwingComponent) {
+        component.coroutineContext[Job]!!.join()
+    }
 }