lib/src/cancelable_operation.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.

import 'dart:async';

import 'package:async/async.dart';

import 'utils.dart';

/// An asynchronous operation that can be cancelled.
///
/// The value of this operation is exposed as [value]. When this operation is
/// cancelled, [value] won't complete either successfully or with an error. If
/// [value] has already completed, cancelling the operation does nothing.
class CancelableOperation<T> {
  /// The completer that produced this operation.
  ///
  /// This is canceled when [cancel] is called.
  final CancelableCompleter<T> _completer;

  CancelableOperation._(this._completer);

  /// Creates a [CancelableOperation] wrapping [inner].
  ///
  /// When this operation is canceled, [onCancel] will be called and any value
  /// or error produced by [inner] will be discarded. If [onCancel] returns a
  /// [Future], it will be forwarded to [cancel].
  ///
  /// [onCancel] will be called synchronously when the operation is canceled.
  /// It's guaranteed to only be called once.
  ///
  /// Calling this constructor is equivalent to creating a [CancelableCompleter]
  /// and completing it with [inner]. As such, [isCompleted] is true from the
  /// moment this [CancelableOperation] is created, regardless of whether
  /// [inner] has completed yet or not.
  factory CancelableOperation.fromFuture(Future<T> inner,
      {FutureOr Function()? onCancel}) {
    var completer = CancelableCompleter<T>(onCancel: onCancel);
    completer.complete(inner);
    return completer.operation;
  }

  /// The value returned by the operation.
  Future<T> get value => _completer._inner.future;

  /// Creates a [Stream] containing the result of this operation.
  ///
  /// This is like `value.asStream()`, but if a subscription to the stream is
  /// canceled, this is as well.
  Stream<T> asStream() {
    var controller =
        StreamController<T>(sync: true, onCancel: _completer._cancel);

    value.then((value) {
      controller.add(value);
      controller.close();
    }, onError: (Object error, StackTrace stackTrace) {
      controller.addError(error, stackTrace);
      controller.close();
    });
    return controller.stream;
  }

  /// Creates a [Future] that completes when this operation completes *or* when
  /// it's cancelled.
  ///
  /// If this operation completes, this completes to the same result as [value].
  /// If this operation is cancelled, the returned future waits for the future
  /// returned by [cancel], then completes to [cancellationValue].
  Future<T?> valueOrCancellation([T? cancellationValue]) {
    var completer = Completer<T?>.sync();
    value.then((result) => completer.complete(result),
        onError: completer.completeError);

    _completer._cancelMemo.future.then((_) {
      completer.complete(cancellationValue);
    }, onError: completer.completeError);

    return completer.future;
  }

  /// Registers callbacks to be called when this operation completes.
  ///
  /// [onValue] and [onError] behave in the same way as [Future.then].
  ///
  /// If [onCancel] is provided, and this operation is canceled, the [onCancel]
  /// callback is called and the returned operation completes with the result.
  ///
  /// If [onCancel] is not given, and this operation is canceled, then the
  /// returned operation is canceled.
  ///
  /// If [propagateCancel] is `true` and the returned operation is canceled then
  /// this operation is canceled. The default is `false`.
  CancelableOperation<R> then<R>(FutureOr<R> Function(T) onValue,
      {FutureOr<R> Function(Object, StackTrace)? onError,
      FutureOr<R> Function()? onCancel,
      bool propagateCancel = false}) {
    final completer =
        CancelableCompleter<R>(onCancel: propagateCancel ? cancel : null);

    valueOrCancellation().then((T? result) {
      if (!completer.isCanceled) {
        if (isCompleted) {
          assert(result is T);
          completer.complete(Future.sync(() => onValue(result as T)));
        } else if (onCancel != null) {
          completer.complete(Future.sync(onCancel));
        } else {
          completer._cancel();
        }
      }
    }, onError: (Object error, StackTrace stackTrace) {
      if (!completer.isCanceled) {
        if (onError != null) {
          completer.complete(Future.sync(() => onError(error, stackTrace)));
        } else {
          completer.completeError(error, stackTrace);
        }
      }
    });
    return completer.operation;
  }

  /// Cancels this operation.
  ///
  /// This returns the [Future] returned by the [CancelableCompleter]'s
  /// `onCancel` callback. Unlike [Stream.cancel], it never returns `null`.
  Future cancel() => _completer._cancel();

  /// Whether this operation has been canceled before it completed.
  bool get isCanceled => _completer.isCanceled;

  /// Whether the [CancelableCompleter] backing this operation has been
  /// completed.
  ///
  /// This value being true does not imply that the [value] future has
  /// completed, but merely that it is no longer possible to [cancel] the
  /// operation.
  bool get isCompleted => _completer.isCompleted;
}

/// A completer for a [CancelableOperation].
class CancelableCompleter<T> {
  /// The completer for the wrapped future.
  final Completer<T> _inner;

  /// The callback to call if the future is canceled.
  final FutureOrCallback? _onCancel;

  /// Creates a new completer for a [CancelableOperation].
  ///
  /// When the future operation canceled, as long as the completer hasn't yet
  /// completed, [onCancel] is called. If [onCancel] returns a [Future], it's
  /// forwarded to [CancelableOperation.cancel].
  ///
  /// [onCancel] will be called synchronously when the operation is canceled.
  /// It's guaranteed to only be called once.
  CancelableCompleter({FutureOr Function()? onCancel})
      : _onCancel = onCancel,
        _inner = Completer<T>() {
    operation = CancelableOperation<T>._(this);
  }

  /// The operation controlled by this completer.
  late final CancelableOperation<T> operation;

  /// Whether the completer has completed.
  bool get isCompleted => _isCompleted;
  bool _isCompleted = false;

  /// Whether the completer was canceled before being completed.
  bool get isCanceled => _isCanceled;
  bool _isCanceled = false;

  /// The memoizer for [_cancel].
  final _cancelMemo = AsyncMemoizer();

  /// Completes [operation] to [value].
  ///
  /// If [value] is a [Future], this will complete to the result of that
  /// [Future] once it completes.
  void complete([FutureOr<T>? value]) {
    if (_isCompleted) throw StateError('Operation already completed');
    _isCompleted = true;

    if (value is! Future) {
      if (_isCanceled) return;
      _inner.complete(value);
      return;
    }

    final future = value as Future<T>;
    if (_isCanceled) {
      // Make sure errors from [value] aren't top-leveled.
      future.catchError((_) {});
      return;
    }

    future.then((result) {
      if (_isCanceled) return;
      _inner.complete(result);
    }, onError: (Object error, StackTrace stackTrace) {
      if (_isCanceled) return;
      _inner.completeError(error, stackTrace);
    });
  }

  /// Completes [operation] to [error].
  void completeError(Object error, [StackTrace? stackTrace]) {
    if (_isCompleted) throw StateError('Operation already completed');
    _isCompleted = true;

    if (_isCanceled) return;
    _inner.completeError(error, stackTrace);
  }

  /// Cancel the completer.
  Future _cancel() {
    if (_inner.isCompleted) return Future.value();

    return _cancelMemo.runOnce(() {
      _isCanceled = true;
      var onCancel = _onCancel;
      if (onCancel != null) return onCancel();
    });
  }
}