Linking ipython widget button and slider values

Question

I'm trying to figure out how to link the value of a counter controlled by button widgets to the value of a slider widget.

The goal here is use ipython widgets to create a simple "vcr-like" interface with three widgets: an IntSlider and two Buttons that increment a counter and decrement a counter. This is what I've got:

import ipywidgets as widgets
from functools import partial
from IPython.display import display
import traitlets

class Counter:
   def __init__(self, initial=0):
      self.value = initial

   def increment(self, amount=1):
      self.value += amount
      return self.value

def button_plus(counter, w):
    counter.increment(+1)  

def button_minus(counter, w):
    counter.increment(-1) 

counter = Counter()
# 1 step forward button
wplus = widgets.Button(description='>')
wplus.on_click(partial(button_plus, counter))
# 1 step backward button
wminus = widgets.Button(description='<')
wminus.on_click(partial(button_minus, counter))
# integer slider
wpick = widgets.IntSlider(value=0,min=0,max=10,step=1,description="time step")

display(wminus, wpick, wplus)

print(counter.value)
print(wpick.value)

and here's a screen grab where I've moved the IntSlider to 1 and clicked twice on the increment button:

I'd obviously like there to be a single integer value being controlled by and be in sync with all 3 widgets.

I read about widget linking but I don't see how to do this since my button widgets don't have a value -- the counter object has the value I want to link.

This doesn't work:

l = traitlets.link((counter, 'value'), (wpick, 'value'))

because counter is not HasTraits.

How can I get counter.value to be linked to wpick.value so that clicking on one of the buttons will adjust the int on the slider?

Answer 1

Following this guide, you need the Counter class to inheret from the DOMWidget class like this:

from traitlets import CInt, link
class Counter(widgets.DOMWidget):
    value = CInt(0, sync=True)

You can then define your counter widget and button callback methods:

counter = Counter()
def button_plus(name):
    counter.value += 1 if counter.value < 10 else 0
def button_minus(name):
    counter.value -= 1 if counter.value > 0 else 0

link the slider and counter widgets:

link((wpick, 'value'), (counter, 'value'))

and register the events on the buttons:

wplus.on_click(button_plus)
wminus.on_click(button_minus)

Clicking the buttons will now in/decrease the value of the counter.