Widgets: skip intermediate callbacks during throttling

[rossant]

Suppose you have a slider, with a callback function (interact) that takes 2 seconds to display a plot (intensive calculations). If you move the slider for a few seconds, many on_change callbacks will be called successively. The throttle queue becomes longer and longer, the widget goes through many changes, and you have to wait dozens of seconds before the widget stops updating.

In this case, shouldn't the intermediate steps be killed before they are processed, so that only the most recent callbacks in the queue stay alive?

[dawehner]

Maybe it would be possible to use something like an mouse-up event instead.

[jasongrout]

I thought the design was that after 3 queued up messages or so, changes would be merged into a final queued message. It shouldn't be queuing up every change.

[rossant]

I thought the design was that after 3 queued up messages or so, changes would be merged into a final queued message. It shouldn't be queuing up every change.

You're right, it does seem that only three updates occur after the end of the interaction (i.e. when I stop moving the slider). Here is the test script I used:

import time
from IPython.html import widgets
@widgets.interact(x=(0, 255))
def f(x):
    time.sleep(3)
    print(x)

However, when the function takes a few seconds (or more) to complete, it does seem a bit weird to see three updates during the 10 seconds that follow the end of the interaction. Maybe this arbitrary number 3 should depend on the execution duration of the function? Or could there be a single update after the end of the interaction?

[jdfreder]

I thought the design was that after 3 queued up messages or so, changes would be merged into a final queued message. It shouldn't be queuing up every change.

@jasongrout is right, this is how it was designed to work,

There is a property of the JS widget model that allows you to change this, this.msg_throttle = 3;.

[jasongrout]

Perhaps this throttle number should be exposed on the python side.

[minrk]

Perhaps this throttle number should be exposed on the python side.

I was just about to say the same thing. @jdfreder is there any reason not to add msg_throttle = Int(3, sync=True) on the Python side?

[jdfreder]

I was just about to say the same thing. @jdfreder is there any reason not to add msg_throttle = Int(3, sync=True) on the Python side?

Nope, just added it in #5170