sdk/pkg/dev_compiler/lib/runtime/_generators.js

// 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.
 */
dart_library.library('dart_runtime/_generators', null, /* Imports */[
], /* Lazy Imports */[
  'dart_runtime/_operations',
  'dart/_js_helper',
  'dart/core',
  'dart/collection',
  'dart/async'
], function(exports, _operations, _js_helper, core, collection, async) {
  'use strict';

  const _jsIterator = Symbol('_jsIterator');
  const _current = Symbol('_current');

  function syncStar(gen, E, ...args) {
    const SyncIterable_E = _js_helper.SyncIterable$(E);
    return new SyncIterable_E(gen, args);
  }
  exports.syncStar = syncStar;

  function async_(gen, T, ...args) {
    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 (!_operations.instanceOf(future, async.Future$)) {
        future = async.Future.value(future);
      }
      // Chain the Future so `await` receives the Future's value.
      return future.then(onValue, {onError: onError});
    }
    return async.Future$(T).new(function() {
      iter = gen(...args)[Symbol.iterator]();
      return onValue();
    });
  }
  exports.async = async_;

  // 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());
  //      });
  //    }
  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 = async.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 = async.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;
      async.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, _operations.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 (!_operations.instanceOf(future, async.Future$)) {
        future = async.Future.value(future);
      }
      return future.then((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(() => {
        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. */
  function asyncStar(gen, T, ...args) {
    return new _AsyncStarStreamController(gen, T, args).controller.stream;
  }
  exports.asyncStar = asyncStar;
});