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Clean-up of Future documentation and small fix-ups.

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R=floitsch@google.com

Review-Url: https://codereview.chromium.org/2917683002 .
This commit is contained in:
Lasse Reichstein Holst Nielsen 2017-06-02 07:53:42 +02:00
parent eefdf15aa1
commit 2a14e68d78
3 changed files with 165 additions and 143 deletions
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298
sdk/lib/async/future.dart
View file

@ -57,28 +57,38 @@ abstract class FutureOr<T> {
* future.then((value) => handleValue(value))
* .catchError((error) => handleError(error));
*
* A [Future] can complete in two ways:
* A [Future] can be completed in two ways:
* with a value ("the future succeeds")
* or with an error ("the future fails").
* Users can install callbacks for each case.
*
* In some cases we say that a future is completed with another future.
* This is a short way of stating that the future is completed in the same way,
* with the same value or error,
* as the other future once that completes.
* Whenever a function in the core library may complete a future
* (for example [Completer.complete] or [new Future.value]),
* then it also accepts another future and does this work for the developer.
*
* The result of registering a pair of callbacks is a new Future (the
* "successor") which in turn is completed with the result of invoking the
* corresponding callback.
* The successor is completed with an error if the invoked callback throws.
* For example:
*
* Future<int> successor = future.then((int value) {
* // Invoked when the future is completed with a value.
* return 42; // The successor is completed with the value 42.
* },
* onError: (e) {
* // Invoked when the future is completed with an error.
* if (canHandle(e)) {
* return 499; // The successor is completed with the value 499.
* } else {
* throw e; // The successor is completed with the error e.
* }
* });
* ```
* Future<int> successor = future.then((int value) {
* // Invoked when the future is completed with a value.
* return 42; // The successor is completed with the value 42.
* },
* onError: (e) {
* // Invoked when the future is completed with an error.
* if (canHandle(e)) {
* return 499; // The successor is completed with the value 499.
* } else {
* throw e; // The successor is completed with the error e.
* }
* });
* ```
*
* If a future does not have a successor when it completes with an error,
* it forwards the error message to the global error-handler.
@ -86,16 +96,17 @@ abstract class FutureOr<T> {
* However, it also means that error handlers should be installed early,
* so that they are present as soon as a future is completed with an error.
* The following example demonstrates this potential bug:
*
* var future = getFuture();
* new Timer(new Duration(milliseconds: 5), () {
* // The error-handler is not attached until 5 ms after the future has
* // been received. If the future fails before that, the error is
* // forwarded to the global error-handler, even though there is code
* // (just below) to eventually handle the error.
* future.then((value) { useValue(value); },
* onError: (e) { handleError(e); });
* });
* ```
* var future = getFuture();
* new Timer(new Duration(milliseconds: 5), () {
* // The error-handler is not attached until 5 ms after the future has
* // been received. If the future fails before that, the error is
* // forwarded to the global error-handler, even though there is code
* // (just below) to eventually handle the error.
* future.then((value) { useValue(value); },
* onError: (e) { handleError(e); });
* });
* ```
*
* When registering callbacks, it's often more readable to register the two
* callbacks separately, by first using [then] with one argument
@ -107,20 +118,22 @@ abstract class FutureOr<T> {
* Using sequential handlers instead of parallel ones often leads to code that
* is easier to reason about.
* It also makes asynchronous code very similar to synchronous code:
*
* // Synchronous code.
* try {
* int value = foo();
* return bar(value);
* } catch (e) {
* return 499;
* }
* ```
* // Synchronous code.
* try {
* int value = foo();
* return bar(value);
* } catch (e) {
* return 499;
* }
* ```
*
* Equivalent asynchronous code, based on futures:
*
* Future<int> future = new Future(foo); // Result of foo() as a future.
* future.then((int value) => bar(value))
* .catchError((e) => 499);
* ```
* Future<int> future = new Future(foo); // Result of foo() as a future.
* future.then((int value) => bar(value))
* .catchError((e) => 499);
* ```
*
* Similar to the synchronous code, the error handler (registered with
* [catchError]) is handling any errors thrown by either `foo` or `bar`.
@ -134,9 +147,12 @@ abstract class FutureOr<T> {
* called.
*/
abstract class Future<T> {
// The `_nullFuture` is a completed Future with the value `null`.
/// A `Future<Null>` completed with `null`.
static final _Future<Null> _nullFuture = new _Future<Null>.value(null);
/// A `Future<bool>` completed with `false`.
static final _Future<bool> _falseFuture = new _Future<bool>.value(false);
/**
* Creates a future containing the result of calling [computation]
* asynchronously with [Timer.run].
@ -226,19 +242,30 @@ abstract class Future<T> {
}
/**
* A future whose value is available in the next event-loop iteration.
* Creates a future completed with [value].
*
* If [result] is not a [Future], using this constructor is equivalent
* to `new Future<T>.sync(() => result)`.
* If [value] is a future, the created future waits for the
* [value] future to complete, and then completes with the same result.
* Since a [value] future can complete with an error, so can the future
* created by [Future.value], even if the name suggests otherwise.
*
* If [value] is not a [Future], the created future is completed
* with the [value] value,
* equivalently to `new Future<T>.sync(() => value)`.
*
* Use [Completer] to create a future and complete it later.
*/
factory Future.value([FutureOr<T> result]) {
return new _Future<T>.immediate(result);
factory Future.value([FutureOr<T> value]) {
return new _Future<T>.immediate(value);
}
/**
* A future that completes with an error in the next event-loop iteration.
* Creates a future that completes with an error.
*
* The created future will be completed with an error in a future microtask.
* This allows enough time for someone to add an error handler on the future.
* If an error handler isn't added before the future completes, the error
* will be considered unhandled.
*
* If [error] is `null`, it is replaced by a [NullThrownError].
*
@ -260,12 +287,14 @@ abstract class Future<T> {
* Creates a future that runs its computation after a delay.
*
* The [computation] will be executed after the given [duration] has passed,
* and the future is completed with the result.
* and the future is completed with the result of the computation,
*
* If the duration is 0 or less,
* it completes no sooner than in the next event-loop iteration.
* it completes no sooner than in the next event-loop iteration,
* after all microtasks have run.
*
* If [computation] is omitted,
* it will be treated as if [computation] was set to `() => null`,
* it will be treated as if [computation] was `() => null`,
* and the future will eventually complete with the `null` value.
*
* If calling [computation] throws, the created future will complete with the
@ -287,17 +316,18 @@ abstract class Future<T> {
}
/**
* Wait for all the given futures to complete and collect their values.
* Waits for multiple futures to complete and collects their results.
*
* Returns a future which will complete once all the futures in a list
* have completed.
* Returns a future which will complete once all the provided futures
* have completed, either with their results, or with an error if either
* of the provided futures fail.
*
* The value of the returned future will be a list of all the values that
* were produced.
*
* If any of the given futures completes with an error, then the returned
* future completes with that error. If other futures complete with errors,
* those errors are discarded.
* If any future completes with an error,
* then the returned future completes with that error.
* If further futures also complete with errors, those errors are discarded.
*
* If `eagerError` is true, the returned future completes with an error
* immediately on the first error from one of the futures. Otherwise all
@ -310,7 +340,7 @@ abstract class Future<T> {
* lost (since the returned future does not provide access to these values).
* The [cleanUp] function is unused if there is no error.
*
* The call to `cleanUp` should not throw. If it does, the error will be an
* The call to [cleanUp] should not throw. If it does, the error will be an
* uncaught asynchronous error.
*/
static Future<List<T>> wait<T>(Iterable<Future<T>> futures,
@ -408,7 +438,8 @@ abstract class Future<T> {
* Returns the result of the first future in [futures] to complete.
*
* The returned future is completed with the result of the first
* future in [futures] to report that it is complete.
* future in [futures] to report that it is complete,
* whether it's with a value or an error.
* The results of all the other futures are discarded.
*
* If [futures] is empty, or if none of its futures complete,
@ -429,13 +460,13 @@ abstract class Future<T> {
}
/**
* Perform an operation for each element of the iterable, in turn.
* Performs an action for each element of the iterable, in turn.
*
* The operation, [f], may be either synchronous or asynchronous.
* The [action] may be either synchronous or asynchronous.
*
* Calls [f] with each element in [input] in order.
* If the call to [f] returns a `Future<T>`, the iteration waits
* until the future is completed before moving to the next element.
* Calls [action] with each element in [elements] in order.
* If the call to [action] returns a `Future<T>`, the iteration waits
* until the future is completed before continuing with the next element.
*
* Returns a [Future] that completes with `null` when all elements have been
* processed.
@ -443,14 +474,14 @@ abstract class Future<T> {
* Non-[Future] return values, and completion-values of returned [Future]s,
* are discarded.
*
* Any error from [f], synchronous or asynchronous, will stop the iteration
* and will be reported in the returned [Future].
* Any error from [action], synchronous or asynchronous,
* will stop the iteration and be reported in the returned [Future].
*/
static Future forEach<T>(Iterable<T> input, FutureOr f(T element)) {
var iterator = input.iterator;
static Future forEach<T>(Iterable<T> elements, FutureOr action(T element)) {
var iterator = elements.iterator;
return doWhile(() {
if (!iterator.moveNext()) return false;
var result = f(iterator.current);
var result = action(iterator.current);
if (result is Future) return result.then(_kTrue);
return true;
});
@ -475,12 +506,13 @@ abstract class Future<T> {
* an error, iteration ends and the future returned by [doWhile]
* completes with the same error.
*
* Calls to [f] may happen at any time, including immediately after calling
* `doWhile`. The only restriction is a new call to [f] won't happen before
* Calls to [action] may happen at any time,
* including immediately after calling `doWhile`.
* The only restriction is a new call to [action] won't happen before
* the previous call has returned, and if it returned a `Future<bool>`, not
* until that future has completed.
*/
static Future doWhile(FutureOr<bool> f()) {
static Future doWhile(FutureOr<bool> action()) {
_Future doneSignal = new _Future();
var nextIteration;
// Bind this callback explicitly so that each iteration isn't bound in the
@ -491,7 +523,7 @@ abstract class Future<T> {
while (keepGoing) {
FutureOr<bool> result;
try {
result = f();
result = action();
} catch (error, stackTrace) {
// Cannot use _completeWithErrorCallback because it completes
// the future synchronously.
@ -587,31 +619,6 @@ abstract class Future<T> {
* added. If the first `catchError` (or `then`) call happens after this future
* has completed with an error then the error is reported as unhandled error.
* See the description on [Future].
*
* Example:
*
* foo
* .catchError(..., test: (e) => e is ArgumentError)
* .catchError(..., test: (e) => e is NoSuchMethodError)
* .then((v) { ... });
*
* This method is equivalent to:
*
* Future catchError(onError(error),
* {bool test(error)}) {
* this.then((v) => v, // Forward the value.
* // But handle errors, if the [test] succeeds.
* onError: (e, stackTrace) {
* if (test == null || test(e)) {
* if (onError is ZoneBinaryCallback) {
* return onError(e, stackTrace);
* }
* return onError(e);
* }
* throw e;
* });
* }
*
*/
// The `Function` below stands for one of two types:
// - (dynamic) -> FutureOr<T>
@ -623,7 +630,7 @@ abstract class Future<T> {
Future<T> catchError(Function onError, {bool test(Object error)});
/**
* Register a function to be called when this future completes.
* Registers a function to be called when this future completes.
*
* The [action] function is called when this future completes, whether it
* does so with a value or with an error.
@ -713,41 +720,42 @@ class TimeoutException implements Exception {
* Most of the time, the simplest way to create a future is to just use
* one of the [Future] constructors to capture the result of a single
* asynchronous computation:
*
* new Future(() { doSomething(); return result; });
*
* ```
* new Future(() { doSomething(); return result; });
* ```
* or, if the future represents the result of a sequence of asynchronous
* computations, they can be chained using [Future.then] or similar functions
* on [Future]:
*
* Future doStuff(){
* return someAsyncOperation().then((result) {
* return someOtherAsyncOperation(result);
* });
* }
*
* ```
* Future doStuff(){
* return someAsyncOperation().then((result) {
* return someOtherAsyncOperation(result);
* });
* }
* ```
* If you do need to create a Future from scratch — for example,
* when you're converting a callback-based API into a Future-based
* one — you can use a Completer as follows:
* ```
* class AsyncOperation {
* Completer _completer = new Completer();
*
* class AsyncOperation {
* Completer _completer = new Completer();
* Future<T> doOperation() {
* _startOperation();
* return _completer.future; // Send future object back to client.
* }
*
* Future<T> doOperation() {
* _startOperation();
* return _completer.future; // Send future object back to client.
* }
* // Something calls this when the value is ready.
* void _finishOperation(T result) {
* _completer.complete(result);
* }
*
* // Something calls this when the value is ready.
* void _finishOperation(T result) {
* _completer.complete(result);
* }
*
* // If something goes wrong, call this.
* void _errorHappened(error) {
* _completer.completeError(error);
* }
* }
* // If something goes wrong, call this.
* void _errorHappened(error) {
* _completer.completeError(error);
* }
* }
* ```
*/
abstract class Completer<T> {
/**
@ -758,17 +766,18 @@ abstract class Completer<T> {
* either [complete] or [completeError].
*
* The completer completes the future asynchronously. That means that
* callbacks registered on the future, are not called immediately when
* callbacks registered on the future are not called immediately when
* [complete] or [completeError] is called. Instead the callbacks are
* delayed until a later microtask.
*
* Example:
*
* var completer = new Completer();
* handOut(completer.future);
* later: {
* completer.complete('completion value');
* }
* ```
* var completer = new Completer();
* handOut(completer.future);
* later: {
* completer.complete('completion value');
* }
* ```
*/
factory Completer() => new _AsyncCompleter<T>();
@ -824,7 +833,11 @@ abstract class Completer<T> {
*/
factory Completer.sync() => new _SyncCompleter<T>();
/** The future that will contain the result provided to this completer. */
/**
* The future that is completed by this completer.
*
* The future that is completed when [complete] or [completeError] is called.
*/
Future<T> get future;
/**
@ -837,7 +850,7 @@ abstract class Completer<T> {
* to complete, and complete with the same result, whether it is a success
* or an error.
*
* Calling `complete` or [completeError] must not be done more than once.
* Calling [complete] or [completeError] must be done at most once.
*
* All listeners on the future are informed about the value.
*/
@ -846,7 +859,7 @@ abstract class Completer<T> {
/**
* Complete [future] with an error.
*
* Calling [complete] or `completeError` must not be done more than once.
* Calling [complete] or [completeError] must be done at most once.
*
* Completing a future with an error indicates that an exception was thrown
* while trying to produce a value.
@ -855,18 +868,27 @@ abstract class Completer<T> {
*
* If `error` is a `Future`, the future itself is used as the error value.
* If you want to complete with the result of the future, you can use:
*
* thisCompleter.complete(theFuture)
*
* ```
* thisCompleter.complete(theFuture)
* ```
* or if you only want to handle an error from the future:
*
* theFuture.catchError(thisCompleter.completeError);
*
* ```
* theFuture.catchError(thisCompleter.completeError);
* ```
*/
void completeError(Object error, [StackTrace stackTrace]);
/**
* Whether the future has been completed.
* Whether the [future] has been completed.
*
* Reflects whether [complete] or [completeError] has been called.
* A `true` value doesn't necessarily mean that listeners of this future
* have been invoked yet, either because the completer usually waits until
* a later microtask to propagate the result, or because [complete]
* was called with a future that hasn't completed yet.
*
* When this value is `true`, [complete] and [completeError] must not be
* called again.
*/
bool get isCompleted;
}
@ -882,7 +904,7 @@ void _completeWithErrorCallback(_Future result, error, stackTrace) {
result._completeError(error, stackTrace);
}
// Like [_completeWIthErrorCallback] but completes asynchronously.
// Like [_completeWithErrorCallback] but completes asynchronously.
void _asyncCompleteWithErrorCallback(_Future result, error, stackTrace) {
AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
if (replacement != null) {

8
sdk/lib/async/stream.dart
View file

@ -560,12 +560,12 @@ abstract class Stream<T> {
*
* Returns a new stream where each element of this stream is replaced
* by zero or more data events.
* The event values are proveded as an [Iterable] by a call to [convert]
* The event values are provided as an [Iterable] by a call to [convert]
* with the element as argument, and the elements of that iterable is
* emitted in iteration order.
* If calling [convert] throws, or if iteration of the returned values throws,
* the error is emitted on the returned stream and iteration ends for that
* element of this stream.
* If calling [convert] throws, or if the iteration of the returned values
* throws, the error is emitted on the returned stream and iteration ends
* for that element of this stream.
*
* Error events and the done event of this stream are forwarded directly
* to the returned stream.

2
sdk/lib/async/stream_impl.dart
View file

@ -996,7 +996,7 @@ class _StreamIterator<T> implements StreamIterator<T> {
_stateData = future;
return future;
}
return new _Future<bool>.immediate(false);
return Future._falseFuture;
}
Future cancel() {