Add package with iterable and stream "zipping" functionality.

pkg/sequence_zip/lib/stream_zip.dart

+// 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.
+
+library stream_zip;
+
+import "dart:async";
+
+/**
+ * A stream that combines the values of other streams.
+ */
+class StreamZip extends Stream<List> {
+  final Iterable<Stream> _streams;
+  StreamZip(Iterable<Stream> streams) : _streams = streams;
+
+  StreamSubscription<List> listen(void onData(List data), {
+                                  void onError(Object error),
+                                  void onDone(),
+                                  bool cancelOnError}) {
+    cancelOnError = identical(true, cancelOnError);
+    List<StreamSubscription> subscriptions = <StreamSubscription>[];
+    StreamController controller;
+    List current;
+    int dataCount = 0;
+
+    /// Called for each data from a subscription in [subscriptions].
+    void handleData(int index, data) {
+      current[index] = data;
+      dataCount++;
+      if (dataCount == subscriptions.length) {
+        List data = current;
+        current = new List(subscriptions.length);
+        dataCount = 0;
+        for (int i = 0; i < subscriptions.length; i++) {
+          if (i != index) subscriptions[i].resume();
+        }
+        controller.add(data);
+      } else {
+        subscriptions[index].pause();
+      }
+    }
+
+    /// Called for each error from a subscription in [subscriptons].
+    /// Except if [cancelOnError] is true, in which case the function below
+    /// is used insted.

[floitsch, 2013/07/04 14:31:22]

instead

[Lasse Reichstein Nielsen, 2013/07/09 06:16:00]

Done.

+    void handleError(Object error) {
+      controller.addError(error);

[floitsch, 2013/07/04 14:31:22]

Should you also increase the dataCount and pause this stream?

[Lasse Reichstein Nielsen, 2013/07/09 06:16:00]

No. I decided that an error is not a replacement for data. We are still waiting
for the data.

I really, really recommend using cancelOnError when using this in any case.

+    }
+
+    /// Called when a subscription has an error and [cancelOnError] is true.
+    ///
+    /// Prematurely cancels all subscriptions since we know that we won't
+    /// be needing any more values.
+    void handleErrorCancel(Object error) {
+      for (int i = 0; i < subscriptions.length; i++) {
+        subscriptions[i].cancel();
+      }
+      controller.addError(error);
+    }
+
+    void handleDone() {
+      for (int i = 0; i < subscriptions.length; i++) {
+        subscriptions[i].cancel();
+      }
+      controller.close();
+    }
+
+    try {
+      for (Stream stream in _streams) {
+        int index = subscriptions.length;
+        subscriptions.add(stream.listen(
+            (data) { handleData(index, data); },
+            onError: cancelOnError ? handleError : handleErrorCancel,
+            onDone: handleDone,
+            cancelOnError: cancelOnError));
+      }
+    } catch (e) {
+      for (int i = subscriptions.length - 1; i >= 0; i--) {
+        subscriptions[i].cancel();
+      }
+      rethrow;
+    }
+
+    current = new List(subscriptions.length);
+
+    controller = new StreamController<List>(
+      onPause: () {
+        for (int i = 0; i < subscriptions.length; i++) {
+          // This may pause some subscriptions more than once.
+          // These will not be resumed by onResume below, but must wait for the

[floitsch, 2013/07/04 14:31:22]

good thing pause/resume is counting ;)

[Lasse Reichstein Nielsen, 2013/07/09 06:16:00]

Yes! I finally found a case where it is exactly what I needed!

+          // next round.
+          subscriptions[i].pause();
+        }
+      },
+      onResume: () {
+        for (int i = 0; i < subscriptions.length; i++) {
+          subscriptions[i].pause();

[floitsch, 2013/07/04 14:31:22]

resume.

[Lasse Reichstein Nielsen, 2013/07/09 06:16:00]

Hmm. That does look like a failure of the testing.

+        }
+      },
+      onCancel: () {
+        for (int i = 0; i < subscriptions.length; i++) {
+          // Canceling more than once is safe.
+          subscriptions[i].cancel();
+        }
+      }
+    );
+
+    if (subscriptions.isEmpty) {
+      controller.close();
+    }
+    return controller.stream.listen(onData,
+                                    onError: onError,
+                                    onDone: onDone,
+                                    cancelOnError: cancelOnError);
+  }
+}