Data send multiple times to backend

[maartenbreddels]

While working on #1194, I noticed what I think is a bug. Take the following code:

import ipywidgets
s = ipywidgets.IntSlider(description="hello", min=0, max=2, value=0)
v = ipywidgets.IntText(value=1)
ipywidgets.jslink((v, 'value'), (s, 'value'))
ipywidgets.HBox([s,v])

Now in a next cell, you execute:

s.description = "world"

All is fine, the js logic with state_lock will avoid that this gets send to the python backend again, however, in the js side, s.model.change still recorded that description was changed. If you now enter a digit in the IntText, it will trigger a save_changes in the IntSlider, and it will again send {description: 'world', ..} to the backend. This happens in many other cases. For instance, in my case, this is an array with a ~million entries, not something you'd like to send around multiple times.
I am not sure how to resolve this, although happy to try.

PS: Is there any way to get a reference to a widgets in the js console, in ipyvolume is just set window.last_scatter = this; for debugging, but maybe there is a general way.

[maartenbreddels]

Looking at the source (also of backbone), I suggest changing WidgetModel.set_state, it should remove all key/values from self.changed (after this.set(..)) is called, for which the value didn't change by a change listener that was called by this.set. This follows the same logic as in WidgetModel.sync. For instance state was before {'x': 1, 'y':2}, and say that when x is changed, in the 'change-listener' y is set to 3, 'x' should be removed from this.changes. This may feel uncomfortable poking around in backbonejs' internals, but I don't think backbone can handle this case.

[maartenbreddels]

Actually, changed will be reset on the next .set, it appears that _buffered_state_diff is the guilty one.

[maartenbreddels]

@jasongrout I've demonstated the bug here
For some reason, 'y' enters in last_doubler._buffered_state_diff, since you're working on this now, you may see where it goes wrong. Also included a screenshot.

[jasongrout]

In your doubler example, you'll want to use this.save_changes(), not this.save(). The save_changes function automatically syncs the attributes that have changed since the last time we synced.

[maartenbreddels]

👍 confirmed that works.