pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/generators.dart

// 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/dev_compiler/issues/245 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;

final _jsIterator = JS('', 'Symbol("_jsIterator")');
final _current = JS('', 'Symbol("_current")');

syncStar(gen, E, @rest args) => JS('', '''(() => {
  const SyncIterable_E = ${getGenericClass(SyncIterable)}($E);
  return new SyncIterable_E($gen, $args);
})()''');

@JSExportName('async')
async_(gen, T, @rest args) => JS('', '''(() => {
  let iter;
  function onValue(res) {
    if (res === void 0) res = null;
    return next(iter.next(res));
  }
  function onError(err) {
    // If the awaited Future throws, we want to convert this to an exception
    // thrown from the `yield` point, as if it was thrown there.
    //
    // If the exception is not caught inside `gen`, it will emerge here, which
    // will send it to anyone listening on this async function's Future<T>.
    //
    // In essence, we are giving the code inside the generator a chance to
    // use try-catch-finally.
    return next(iter.throw(err));
  }
  function next(ret) {
    if (ret.done) return ret.value;
    // Checks if the awaited value is a Future.
    let future = ret.value;
    if (!$instanceOf(future, ${getGenericClass(Future)})) {
      future = $Future.value(future);
    }
    // Chain the Future so `await` receives the Future's value.
    return future.then($dynamic)(onValue, {onError: onError});
  }
  return ${getGenericClass(Future)}($T).new(function() {
    iter = $gen.apply(null, $args)[Symbol.iterator]();
    return onValue();
  });
})()''');

// 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(ochafik): Port back to Dart (which it used to be in the past).
final _AsyncStarStreamController = JS('', '''
  class _AsyncStarStreamController {
    constructor(generator, T, args) {
      this.isAdding = false;
      this.isWaiting = false;
      this.isScheduled = false;
      this.isSuspendedAtYield = false;
      this.canceler = null;
      this.iterator = generator(this, ...args)[Symbol.iterator]();
      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;
      if (!$instanceOf(future, ${getGenericClass(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();
      }, { 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(gen, T, @rest args) => JS('', '''(() => {
  return new $_AsyncStarStreamController($gen, $T, $args).controller.stream;
})()''');