| // Copyright (c) 2015, the Dart project authors. Please see the AUTHORS file |
| // for details. All rights reserved. Use of this source code is governed by a |
| // BSD-style license that can be found in the LICENSE file. |
| |
| /// This library adapts ES6 generators to implement Dart's async/await. |
| /// It's designed to interact with Dart's Future/Stream and follow Dart |
| /// async/await semantics. |
| /// See https://github.com/dart-lang/sdk/issues/27315 for ideas on |
| /// reconciling Dart's Future and ES6 Promise. |
| /// Inspired by `co`: https://github.com/tj/co/blob/master/index.js, which is a |
| /// stepping stone for proposed ES7 async/await, and uses ES6 Promises. |
| part of dart._runtime; |
| |
| // TODO(vsm): Remove once this flag we've removed the ability to |
| // whitelist / fallback on the old behavior. |
| bool startAsyncSynchronously = true; |
| void setStartAsyncSynchronously([bool value = true]) { |
| startAsyncSynchronously = value; |
| } |
| |
| final _jsIterator = JS('', 'Symbol("_jsIterator")'); |
| final _current = JS('', 'Symbol("_current")'); |
| |
| /// Implementation inspired by _AsyncStarStreamController in |
| /// dart-lang/sdk's runtime/lib/core_patch.dart |
| /// |
| /// Given input like: |
| /// |
| /// foo() async* { |
| /// yield 1; |
| /// yield* bar(); |
| /// print(await baz()); |
| /// } |
| /// |
| /// This generates as: |
| /// |
| /// function foo() { |
| /// return dart.asyncStar(function*(stream) { |
| /// if (stream.add(1)) return; |
| /// yield; |
| /// if (stream.addStream(bar()) return; |
| /// yield; |
| /// print(yield baz()); |
| /// }); |
| /// } |
| /// |
| // TODO(jmesserly): port back to Dart, based on VM's equivalent class, and move |
| // to dart:async async_patch.dart |
| final _AsyncStarStreamController = JS('', ''' |
| class _AsyncStarStreamController { |
| constructor(T) { |
| this.isAdding = false; |
| this.isWaiting = false; |
| this.isScheduled = false; |
| this.isSuspendedAtYield = false; |
| this.canceler = null; |
| this.controller = ${getGenericClass(StreamController)}(T).new({ |
| onListen: () => this.scheduleGenerator(), |
| onResume: () => this.onResume(), |
| onCancel: () => this.onCancel() |
| }); |
| } |
| |
| onResume() { |
| if (this.isSuspendedAtYield) { |
| this.scheduleGenerator(); |
| } |
| } |
| |
| onCancel() { |
| if (this.controller.isClosed) { |
| return null; |
| } |
| if (this.canceler == null) { |
| this.canceler = $Completer.new(); |
| this.scheduleGenerator(); |
| } |
| return this.canceler.future; |
| } |
| |
| close() { |
| if (this.canceler != null && !this.canceler.isCompleted) { |
| // If the stream has been cancelled, complete the cancellation future |
| // with the error. |
| this.canceler.complete(); |
| } |
| this.controller.close(); |
| } |
| |
| scheduleGenerator() { |
| // TODO(jmesserly): is this paused check in the right place? Assuming the |
| // async* Stream yields, then is paused (by other code), the body will |
| // already be scheduled. This will cause at least one more iteration to |
| // run (adding another data item to the Stream) before actually pausing. |
| // It could be fixed by moving the `isPaused` check inside `runBody`. |
| if (this.isScheduled || this.controller.isPaused || |
| this.isAdding || this.isWaiting) { |
| return; |
| } |
| this.isScheduled = true; |
| $scheduleMicrotask(() => this.runBody()); |
| } |
| |
| runBody(opt_awaitValue) { |
| this.isScheduled = false; |
| this.isSuspendedAtYield = false; |
| this.isWaiting = false; |
| let iter; |
| try { |
| iter = this.iterator.next(opt_awaitValue); |
| } catch (e) { |
| this.addError(e, $stackTrace(e)); |
| this.close(); |
| return; |
| } |
| if (iter.done) { |
| this.close(); |
| return; |
| } |
| |
| // If we're suspended at a yield/yield*, we're done for now. |
| if (this.isSuspendedAtYield || this.isAdding) return; |
| |
| // Handle `await`: if we get a value passed to `yield` it means we are |
| // waiting on this Future. Make sure to prevent scheduling, and pass the |
| // value back as the result of the `yield`. |
| // |
| // TODO(jmesserly): is the timing here correct? The assumption here is |
| // that we should schedule `await` in `async*` the same as in `async`. |
| this.isWaiting = true; |
| let future = iter.value; |
| // TODO(jmesserly): `async` uses a different check that looks for the |
| // (private) implementation type of `Future`, rather than the public type. |
| if (!$Future.is(future)) { |
| future = $Future.value(future); |
| } |
| return future.then($dynamic, (x) => this.runBody(x), |
| { onError: (e, s) => this.throwError(e, s) }); |
| } |
| |
| // Adds element to stream, returns true if the caller should terminate |
| // execution of the generator. |
| add(event) { |
| // If stream is cancelled, tell caller to exit the async generator. |
| if (!this.controller.hasListener) return true; |
| this.controller.add(event); |
| this.scheduleGenerator(); |
| this.isSuspendedAtYield = true; |
| return false; |
| } |
| |
| // Adds the elements of stream into this controller's stream. |
| // The generator will be scheduled again when all of the |
| // elements of the added stream have been consumed. |
| // Returns true if the caller should terminate |
| // execution of the generator. |
| addStream(stream) { |
| // If stream is cancelled, tell caller to exit the async generator. |
| if (!this.controller.hasListener) return true; |
| |
| this.isAdding = true; |
| this.controller.addStream(stream, {cancelOnError: false}).then($dynamic, |
| () => { |
| this.isAdding = false; |
| this.scheduleGenerator(); |
| if (!this.isScheduled) this.isSuspendedAtYield = true; |
| }, { onError: (e, s) => this.throwError(e, s) }); |
| } |
| |
| throwError(error, stackTrace) { |
| try { |
| this.iterator.throw(error); |
| } catch (e) { |
| this.addError(e, stackTrace); |
| } |
| } |
| |
| addError(error, stackTrace) { |
| if ((this.canceler != null) && !this.canceler.isCompleted) { |
| // If the stream has been cancelled, complete the cancellation future |
| // with the error. |
| this.canceler.completeError(error, stackTrace); |
| return; |
| } |
| if (!this.controller.hasListener) return; |
| this.controller.addError(error, stackTrace); |
| } |
| } |
| '''); |
| |
| /// Returns a Stream of T implemented by an async* function. |
| asyncStar<T>(Function(Object) initGenerator) { |
| var stream = JS('', 'new #(#)', _AsyncStarStreamController, T); |
| JS('', '#.iterator = #[Symbol.iterator]()', stream, initGenerator(stream)); |
| return JS('', '#.controller.stream', stream); |
| } |