Get the 'undo/redo' functionality to work when using realtime collaboration.

[GoogleCodeExporter]

Note that it seems to work for the remote side. Presumably we just need to 
process the undo/redo events when isLocal is true.

Original issue reported on code.google.com by ma...@google.com on 23 Jan 2014 at 6:39

[GoogleCodeExporter]

It turns out it's not as simple as processing the undo/redo events when isLocal 
is true because we don't know which events are the result of an undo/redo.

Original comment by ma...@google.com on 6 Feb 2014 at 1:15

[GoogleCodeExporter]

I've done it in other Realtime projects by just setting a flag, e.g.

var fromUndo = false;

In the call from the GUI:
fromUndo = true;
model.undo(); (or redo();)

In the event listener:
if(isLocal && fromUndo) {
  (update Blockly)
  fromUndo = false;
}

Not pretty, but it's worked fine so far. Remote events won't remove or be 
affected by the flag, so there shouldn't be a race condition unless the event 
bubbling is so slow that the user has time to trigger another local event. 

Original comment by dinin...@gmail.com on 6 Feb 2014 at 5:43

[GoogleCodeExporter]

Unfortunately, the flag setting technique doesn't work if you use compound 
realtime operations, since the undo (or redo) method may trigger multiple 
events.  In that case, you can't just reset the flag on the first 'fromUndo' 
event you see.

Original comment by ma...@google.com on 6 Feb 2014 at 6:20

[GoogleCodeExporter]

Original comment by ma...@google.com on 14 Feb 2014 at 12:25

• Changed state: Started

[GoogleCodeExporter]

You can surround the undo/redo call with a try-finally block that sets and 
resets the flag.  This works because the realtime API guarantees that all the 
undo/redo generated events will come within the execution of the undo/redo call.

Original comment by ma...@google.com on 14 Feb 2014 at 12:28

[GoogleCodeExporter]

This issue was updated by revision r1637.

We're getting closer to undo/redo support.  There are
still some issues around atomicity of undo which can be fixed with
either a change in how the underlying Google Drive Realtime compound
operations work or by doing some explicit modeling of the undo stack
by our code.

R=neil.fraser@gmail.com

Review URL: https://codereview.appspot.com/66170044

Original comment by ma...@google.com on 21 Feb 2014 at 7:33