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// 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 && !isCanceled) {
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 _inner = Completer<T>();
/// 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;
/// The operation controlled by this completer.
late final operation = CancelableOperation<T>._(this);
/// 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();
});
}
}