Handle defered handling of exceptions (where the exception is thrown before the

corelib/src/implementation/future_implementation.dart

 // Copyright 2011 Google Inc. All Rights Reserved.
 // Dart core library.
 
 /**
  * Thrown if client tries to obtain value or exception
  * before a future has completed.
  */
 class FutureNotCompleteException implements Exception {
   FutureNotCompleteException() {}
   String toString() => "Exception: future has not been completed";
@@ skipping 67 matching lines @@
 
   bool get hasValue() {
     return isComplete && _exception === null;
   }
 
   void then(void onComplete(T value)) {
     if (hasValue) {
       onComplete(value);
     } else if (!isComplete) {
       _listeners.add(onComplete);
-    } else if (!_exceptionHandled) {

[Siggi Cherem (dart-lang), 2012/01/10 20:39:27]

I believe we don't want to do this part of the change.

Basically we are worried about users of [Future] that don't register any
exception handlers at first (it wasn't initially part of their design), but
later end up inadvertently ignoring exceptions. 

It's unfortunate that this requires an ordering (add exception handlers before
calling 'then'), but I still believe exposing exceptions out-weights this.

-      throw _exception;
     }
   }
 
   void handleException(void onException(Object exception)) {
-    _exceptionHandlers.add(onException);
+    if (null !== _exception) {

[Siggi Cherem (dart-lang), 2012/01/10 20:39:27]

(nit): keeping the style consistent with the rest of the dart code, swap the 2
expressions (_expression !== null)

+      if (!_exceptionHandled) _exceptionHandled = onException(exception);
+    } else {
+      _exceptionHandlers.add(onException);
+    }
   }
 
   void _complete() {
     _isComplete = true;
     if (_exception !== null) {
       for (Function handler in _exceptionHandlers) {
         if (handler(_exception)) {
           _exceptionHandled = true;
           break;
         }
@@ skipping 42 matching lines @@
   }
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(var exception) {
     _futureImpl._setException(exception);
   }
 }