Call asyncEnd only on success.

pkg/async_helper/lib/async_helper.dart

Index: pkg/async_helper/lib/async_helper.dart
diff --git a/pkg/async_helper/lib/async_helper.dart b/pkg/async_helper/lib/async_helper.dart
index 84df557a811d6ce40f3643430a3477d7f420260a..1b444815fa10fe3e0ee924e3380cb46b80bcc382 100644
--- a/pkg/async_helper/lib/async_helper.dart
+++ b/pkg/async_helper/lib/async_helper.dart
@@ -32,6 +32,7 @@ Exception _buildException(String msg) {
   return new Exception('Fatal: $msg. This is most likely a bug in your test.');
 }
 
+/// Call this method before an asynchronous test is created.
 void asyncStart() {
   if (_initialized && _asyncLevel == 0) {
     throw _buildException('asyncStart() was called even though we are done '
@@ -45,6 +46,7 @@ void asyncStart() {
   _asyncLevel++;
 }
 
+/// Call this after an asynchronous test has ended successfully.
 void asyncEnd() {
   if (_asyncLevel <= 0) {
     if (!_initialized) {
@@ -62,7 +64,26 @@ void asyncEnd() {
   }
 }

[kustermann, 2013/09/09 12:41:54]

In general I've noticed that people misuse these two methods sometimes. This
test for example is broken
main() {
  asyncStart();
  Timer.run(() { print("A"); asyncEnd(); });
  asyncStart();
  Timer.run(() { print("B"); asyncEnd(); });
}

It could be the case that the first closure was called twice (i.e. the result is
"AA") and the test would pass.

Maybe we should provide a convenience wrapper around asyncStart/asyncEnd, like
this:
main() {
  var asyncEndFn1 = asyncBegin();
  Timer.run(() { print("A"); asyncEndFn1(); });
  var asyncEndFn2 = asyncBegin();
  Timer.run(() { print("B"); asyncEndFn2(); });
}

So returning a closure which makes sure that it's only called once.

 
+/**
+ * Call this after an asynchronous test has ended successfully. This is a helper
+ * for calling [asyncEnd].
+ *
+ * This method intentionally has a signature that matches [:Future.then:] as a
+ * convenience for calling [asyncEnd] when a [:Future:] completes without error,
+ * like this:
+ *
+ *     asyncStart();
+ *     Future result = test();
+ *     result.then(asyncSuccess);
+ */
+void asyncSuccess(_) => asyncEnd();
+
+/**
+ * Helper method for performing asynchronous tests involving [:Future:].
+ *
+ * [f] must return a [:Future:] for the test computation.
+ */
 void asyncTest(f()) {

[kustermann, 2013/09/09 12:41:54]

Either document this on the top of the file or at the method level. be
consistent (i.e. remove the comments on the top or add sections for these
methods as well).

[Johnni Winther, 2013/09/13 07:34:59]

Added a top comment as well.

   asyncStart();
-  f().whenComplete(() => asyncEnd());
+  f().then(asyncSuccess);
 }