Add Future.of that calls a function and captures the result

sdk/lib/async/future.dart

 // Copyright (c) 2012, 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.
 
 part of dart.async;
 
 /**
  * A [Future] represents a delayed computation. It is used to obtain a not-yet
  * available value, or error, sometime in the future.  Receivers of a
  * [Future] can register callbacks that handle the value or error once it is
@@ skipping 67 matching lines @@
  * [catchError]) is handling the errors for exceptions coming from calls to
  * 'foo', as well as 'bar'. This would not be the case if the error-handler was
  * registered at the same time as the value-handler.
  *
  * Futures can have more than one callback-pairs registered. Each successor is
  * treated independently and is handled as if it was the only successor.
  */
 // TODO(floitsch): document chaining.
 abstract class Future<T> {
   /**
+   * Creates a future containing the result of calling [function].
+   *
+   * The result of computing [:function():] is either a returned value or
+   * a throw.
+   *
+   * If a value is returned, it becomes the result of the created future.
+   *
+   * If calling [function] throws, the created [Future] will be completed
+   * with an async error containing the thrown value and a captured
+   * stacktrace.
+   *
+   * However, if the result of calling [function] is already an asynchronous
+   * result, we treat it specially.
+   *
+   * If the returned value is itself a [Future], completion of
+   * the created future will wait until the returned future completes,
+   * and will then complete with the same result.
+   *
+   * If a thrown value is an [AsyncError], it is used directly as the result
+   * of the created future.
+   */
+  factory Future.of(function()) {
+    try {
+      var result = function();
+      return new _FutureImpl<T>().._setOrChainValue(result);
+    } catch (error, stackTrace) {
+      return new _FutureImpl<T>.immediateError(error, stackTrace);
+    }
+  }
+
+  /**
    * A future whose value is available in the next event-loop iteration.
    *
-   * See [Completer]s, for futures with values that are computed asynchronously.
+   * If [value] is not a [Future], using this constructor is equivalent
+   * to [:new Future.of(() => value):].
+   *
+   * See [Completer] to create a Future and complete it later.
    */
   factory Future.immediate(T value) => new _FutureImpl<T>.immediate(value);
 
   /**
    * A future that completes with an error in the next event-loop iteration.
    *
-   * See [Completer]s, for futures with values that are computed asynchronously.
+   * See [Completer] to create a Future and complete it later.
    */
   factory Future.immediateError(var error, [Object stackTrace]) {
     return new _FutureImpl<T>.immediateError(error, stackTrace);
   }
 
   /**
    * Creates a future that completes after a delay.
    *
    * The future will be completed after [milliseconds] have passed with
    * the result of calling [value]. If [milliseconds] is 0, it completes at the
    * earliest in the next event-loop iteration.
    *
    * If calling [value] throws, the created future will complete with the
    * error.
    *
    * See [Completer]s, for futures with values that are computed asynchronously.
    */
   factory Future.delayed(int milliseconds, T value()) {
-    _ThenFuture<dynamic, T> future = new _ThenFuture<dynamic, T>((_) => value());
+    _ThenFuture<dynamic, T> future =
+        new _ThenFuture<dynamic, T>((_) => value());
     new Timer(milliseconds, (_) => future._sendValue(null));
     return future;
   }
 
   /**
    * Wait for all the given futures to complete and collect their values.
    *
    * Returns a future which will complete once all the futures in a list are
    * complete. If any of the futures in the list completes with an error,
    * the resulting future also completes with an error. Otherwise the value
@@ skipping 181 matching lines @@
    * The argument [exception] should not be `null`.
    *
    * If [exception] is an [AsyncError], it is used directly as the error
    * message sent to the future's listeners, and [stackTrace] is ignored.
    *
    * Otherwise the [exception] and an optional [stackTrace] is combined into an
    * [AsyncError] and sent to this future's listeners.
    */
   void completeError(Object exception, [Object stackTrace]);
 }