blob: 13e3dd94b288d8e579f3cc8a77eb88430f179356 [file] [log] [blame]
// Copyright (c) 2013, 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.
/// Capture asynchronous results into synchronous values.
/// Capturing a result (either a returned value or a thrown error)
/// means converting it into a [Result] -
/// either a [ValueResult] or an [ErrorResult].
/// This value can release itself by writing itself either to a
/// [EventSink] or a [Completer], or by becoming a [Future].
library dart.pkg.async.results;
import "dart:async";
/// The result of a computation.
abstract class Result<T> {
/// Create a `Result` with the result of calling [computation].
/// This generates either a [ValueResult] with the value returned by
/// calling `computation`, or an [ErrorResult] with an error thrown by
/// the call.
factory Result(T computation()) {
try {
return new ValueResult(computation());
} catch (e, s) {
return new ErrorResult(e, s);
/// Create a `Result` holding a value.
/// Alias for [ValueResult.ValueResult].
factory Result.value(T value) = ValueResult<T>;
/// Create a `Result` holding an error.
/// Alias for [ErrorResult.ErrorResult].
factory Result.error(Object error, [StackTrace stackTrace]) =>
new ErrorResult(error, stackTrace);
// Helper functions.
static _captureValue(value) => new ValueResult(value);
static _captureError(error, stack) => new ErrorResult(error, stack);
static _release(Result v) {
if (v.isValue) return v.asValue.value; // Avoid wrapping in future.
return v.asFuture;
/// Capture the result of a future into a `Result` future.
/// The resulting future will never have an error.
/// Errors have been converted to an [ErrorResult] value.
static Future<Result> capture(Future future) {
return future.then(_captureValue, onError: _captureError);
/// Release the result of a captured future.
/// Converts the [Result] value of the given [future] to a value or error
/// completion of the returned future.
/// If [future] completes with an error, the returned future completes with
/// the same error.
static Future release(Future<Result> future) {
return future.then(_release);
/// Capture the results of a stream into a stream of [Result] values.
/// The returned stream will not have any error events.
/// Errors from the source stream have been converted to [ErrorResult]s.
/// Shorthand for transforming the stream using [CaptureStreamTransformer].
static Stream<Result> captureStream(Stream source) {
return source.transform(const CaptureStreamTransformer());
/// Release a stream of [result] values into a stream of the results.
/// `Result` values of the source stream become value or error events in
/// the returned stream as appropriate.
/// Errors from the source stream become errors in the returned stream.
/// Shorthand for transforming the stream using [ReleaseStreamTransformer].
static Stream releaseStream(Stream<Result> source) {
return source.transform(const ReleaseStreamTransformer());
/// Converts a result of a result to a single result.
/// If the result is an error, or it is a `Result` value
/// which is then an error, then a result with that error is returned.
/// Otherwise both levels of results are value results, and a single
/// result with the value is returned.
static Result flatten(Result<Result> result) {
if (result.isError) return result;
return result.asValue.value;
/// Whether this result is a value result.
/// Always the opposite of [isError].
bool get isValue;
/// Whether this result is an error result.
/// Always the opposite of [isValue].
bool get isError;
/// If this is a value result, return itself.
/// Otherwise return `null`.
ValueResult<T> get asValue;
/// If this is an error result, return itself.
/// Otherwise return `null`.
ErrorResult get asError;
/// Complete a completer with this result.
void complete(Completer<T> completer);
/// Add this result to an [EventSink].
/// Calls the sink's `add` or `addError` method as appropriate.
void addTo(EventSink<T> sink);
/// Creates a future completed with this result as a value or an error.
Future<T> get asFuture;
/// A result representing a returned value.
class ValueResult<T> implements Result<T> {
final T value;
bool get isValue => true;
bool get isError => false;
ValueResult<T> get asValue => this;
ErrorResult get asError => null;
void complete(Completer<T> completer) {
void addTo(EventSink<T> sink) {
Future<T> get asFuture => new Future.value(value);
/// A result representing a thrown error.
class ErrorResult implements Result {
final error;
final StackTrace stackTrace;
ErrorResult(this.error, this.stackTrace);
bool get isValue => false;
bool get isError => true;
ValueResult get asValue => null;
ErrorResult get asError => this;
void complete(Completer completer) {
completer.completeError(error, stackTrace);
void addTo(EventSink sink) {
sink.addError(error, stackTrace);
Future get asFuture => new Future.error(error, stackTrace);
/// Calls an error handler with the error and stacktrace.
/// An async error handler function is either a function expecting two
/// arguments, which will be called with the error and the stack trace,
/// or it has to be a function expecting only one argument,
/// which will be called with only the error.
void handle(Function errorHandler) {
if (errorHandler is ZoneBinaryCallback) {
errorHandler(error, stackTrace);
} else {
/// A stream transformer that captures a stream of events into [Result]s.
/// The result of the transformation is a stream of [Result] values and
/// no error events.
class CaptureStreamTransformer<T> implements StreamTransformer<T, Result<T>> {
const CaptureStreamTransformer();
Stream<Result<T>> bind(Stream<T> source) {
return new Stream<Result<T>>.eventTransformed(source, _createSink);
static EventSink _createSink(EventSink<Result> sink) {
return new CaptureSink(sink);
/// A stream transformer that releases a stream of result events.
/// The result of the transformation is a stream of values and
/// error events.
class ReleaseStreamTransformer<T> implements StreamTransformer<Result<T>, T> {
const ReleaseStreamTransformer();
Stream<T> bind(Stream<Result<T>> source) {
return new Stream<T>.eventTransformed(source, _createSink);
static EventSink<Result> _createSink(EventSink sink) {
return new ReleaseSink(sink);
/// An event sink wrapper that captures the incoming events.
/// Wraps an [EventSink] that expects [Result] values.
/// Accepts any value and error result,
/// and passes them to the wrapped sink as [Result] values.
/// The wrapped sink will never receive an error event.
class CaptureSink<T> implements EventSink<T> {
final EventSink _sink;
CaptureSink(EventSink<Result<T>> sink) : _sink = sink;
void add(T value) { _sink.add(new ValueResult(value)); }
void addError(Object error, [StackTrace stackTrace]) {
_sink.add(new ErrorResult(error, stackTrace));
void close() { _sink.close(); }
/// An event sink wrapper that releases the incoming result events.
/// Wraps an output [EventSink] that expects any result.
/// Accepts [Result] values, and puts the result value or error into the
/// corresponding output sink add method.
class ReleaseSink<T> implements EventSink<Result<T>> {
final EventSink _sink;
ReleaseSink(EventSink<T> sink) : _sink = sink;
void add(Result<T> result) {
if (result.isValue) {
} else {
ErrorResult error = result.asError;
_sink.addError(error.error, error.stackTrace);
void addError(Object error, [StackTrace stackTrace]) {
// Errors may be added by intermediate processing, even if it is never
// added by CaptureSink.
_sink.addError(error, stackTrace);
void close() { _sink.close(); }