Adding support to cancel a headless task

On UWP, when the app is executing a Background task, the OS can request it to cancel the task (for various reasons). If the app does not cancel its background task quickly, the OS might terminate the app.
This change adds support to cancel the headless task which can be used by native code to forward the cancellation request from the OS to the JS code.

Libraries/ReactNative/AppRegistry.js

@@ -21,6 +21,9 @@ const renderApplication = require('renderApplication');
 
 type Task = (taskData: any) => Promise<void>;
 type TaskProvider = () => Task;
+type TaskCanceller = () => void;
+type TaskCancelProvider = () => TaskCanceller;
+
 export type ComponentProvider = () => React$ComponentType<any>;
 export type ComponentProviderInstrumentationHook = (
   component: ComponentProvider,
@@ -47,7 +50,8 @@ export type WrapperComponentProvider = any => React$ComponentType<*>;
 const runnables: Runnables = {};
 let runCount = 1;
 const sections: Runnables = {};
-const tasks: Map<string, TaskProvider> = new Map();
+const taskProviders: Map<string, TaskProvider> = new Map();
+const taskCancelProviders: Map<string, TaskCancelProvider> = new Map();
 let componentProviderInstrumentationHook: ComponentProviderInstrumentationHook = (
   component: ComponentProvider,
 ) => component();
@@ -209,13 +213,23 @@ const AppRegistry = {
    *
    * See http://facebook.github.io/react-native/docs/appregistry.html#registerheadlesstask
    */
-  registerHeadlessTask(taskKey: string, task: TaskProvider): void {
-    if (tasks.has(taskKey)) {
+  registerHeadlessTask(taskKey: string, taskProvider: TaskProvider): void {
+    this.registerCancellableHeadlessTask(taskKey, taskProvider, () => () => { /* Cancel is no-op */ });
+  },
+
+  /**
+   * Register a cancellable headless task. A headless task is a bit of code that runs without a UI.
+   *
+   * See http://facebook.github.io/react-native/docs/appregistry.html#registercancellableheadlesstask
+   */
+  registerCancellableHeadlessTask(taskKey: string, taskProvider: TaskProvider, taskCancelProvider: TaskCancelProvider): void {
+    if (taskProviders.has(taskKey)) {
       console.warn(
-        `registerHeadlessTask called multiple times for same key '${taskKey}'`,
+        `registerHeadlessTask or registerCancellableHeadlessTask called multiple times for same key '${taskKey}'`,
       );
     }
-    tasks.set(taskKey, task);
+    taskProviders.set(taskKey, taskProvider);
+    taskCancelProviders.set(taskKey, taskCancelProvider);
   },
 
   /**
@@ -224,7 +238,7 @@ const AppRegistry = {
    * See http://facebook.github.io/react-native/docs/appregistry.html#startheadlesstask
    */
   startHeadlessTask(taskId: number, taskKey: string, data: any): void {
-    const taskProvider = tasks.get(taskKey);
+    const taskProvider = taskProviders.get(taskKey);
     if (!taskProvider) {
       throw new Error(`No task registered for key ${taskKey}`);
     }
@@ -237,6 +251,19 @@ const AppRegistry = {
         NativeModules.HeadlessJsTaskSupport.notifyTaskFinished(taskId);
       });
   },
+
+  /**
+   * Only called from native code. Cancels a headless task.
+   *
+   * See http://facebook.github.io/react-native/docs/appregistry.html#cancelheadlesstask
+   */
+  cancelHeadlessTask(taskId: number, taskKey: string): void {
+    const taskCancelProvider = taskCancelProviders.get(taskKey);
+    if (!taskCancelProvider) {
+      throw new Error(`No task canceller registered for key '${taskKey}'`);
+    }
+    taskCancelProvider()();
+  },
 };
 
 BatchedBridge.registerCallableModule('AppRegistry', AppRegistry);