Fix binary synchronization and make Image use binary values

[jasongrout]

Fixes #1642 by working around an underscore bug.

This also makes the image use a binary value so that we have at least one core widget using the binary values and messages.

This means that custom deserializers must not return a DataView or ArrayBuffer (other typed arrays like Uint8Array are fine, though).

Test code:

from ipywidgets import Image
from io import BytesIO
import numpy as np
from scipy.misc import imsave
x = np.zeros((255, 255), dtype=np.uint8)
x[:] = np.arange(255)
b = BytesIO()
imsave(b, x, format='png')

im = Image(layout=dict(width='100px', height='100px', border='1px solid red'))
im

Then

im.value=b.getvalue()

and also make sure the image is displayed when the page is refreshed.
CC @maartenbreddels

[maartenbreddels]

I think the createObjectURL is deprecated (firefox gives you a warning on that).
https://developer.mozilla.org/en-US/docs/Web/API/URL/createObjectURL
The proper way would be to use this.el.srcObject = blob I think. We may want to polyfill this with https://github.com/webrtc/adapter , which is what I do in https://github.com/maartenbreddels/ipywebrtc

[maartenbreddels]

Sorry for the noise, that was only for HTMLMediaElement, not HTMLImageElement

[jasongrout]

I took some of this code from https://developer.mozilla.org/en-US/docs/Web/API/Blob

[maartenbreddels]

Yes, it's completely correct.

[jasongrout]

@maartenbreddels - our problem is more serious than my simple workaround. Since underscore is doing the deep compare, any arraybuffer or dataview object anywhere nested in the JSON of a deserialized trait is going to have this issue.

I'm tempted to just override the backbone function and insert a special-case call to our own isequal.

[maartenbreddels]

I guess both Backbone's set and changedAttributes?

[jasongrout]

I think we override set and call our own isEqual.

We're slowly completely reimplementing backbone...

[maartenbreddels]

Or we fork and publish underscore with a fixed _.isEquals, maybe less work.

[jasongrout]

Note that lodash has specific references to typed arrays, indicating they handle that case: https://github.com/lodash/lodash/blob/8e631dfcd496bc355ee7ceeb959421b0788b9bbc/.internal/baseIsEqualDeep.js#L56

[jasongrout]

Or we fork and publish underscore with a fixed _.isEquals, maybe less work.

And then we have to fork backbone and update its dependency. I really don't want to maintain an underscore fork, especially when lodash seems to work in this case.

[jasongrout]

This indicates that we can replace underscore with lodash for backbone, but requires a special webpack config. We can't do that in jlab, though. Too bad.

[jasongrout]

@maartenbreddels - I think this is ready for review and testing. I could also remove underscore from controls, but that's not pressing.