Switching between images to annotate

[emmanuelle]

From @mprostock:

My main question, which I can't seem to figure out: How do you envision
switching between images to annotate, what is the correct process for
that? In all use cases I know, this is one of the main processes:
annotate -> next image -> annotate ->next image etc. All of the portal
use-cases only demonstrate usage with a single image though.

By design, dash-canvas ALWAYS triggers the canvas.json-data callback if
anything is changed. So using the "go_button" (renamed to "next")
within the component practically makes it impossible to trigger image
changes. The following things I have tried:

• change the "image_content" property after using the callback: Doesn't
  work, because it triggers a circular callback (changing image_content
  retriggers the callback...)

• reinstantiate the entire component (putting the entire dash_canvas
  component into an output of type Output('canvas-div', 'children'). This
  should work from my perspective (though it is very inefficient), but it
  always only works for the first few images and then starts the above
  cycle of circular callbacks somehow on its own (so something again
  triggers the json callback apparently, reproducible but not consistent!)

• try to make use of the new dash.callback_context object, which should
  have the actual cause of the callback within its 'triggered' list, but
  apparently the only property in dash-canvas is the entire json of
  changes, so the context is useless in this case.

What am I missing? Is there a way, short of NOT using the dash_canvas
internal buttons (which seems kind of beside the point and I also can't
find a way to hide the go-button, it seems mandatory?)

One additional observation: The json-data always includes the actual
image itself in base64 encoding. Considering that in all "normal"
usecases, the image will be provided by the backend anyways, this is
really inefficient and wasteful for the amount of data permanently send
between backend and client, it would be better to just include the
annotations. I imagine that the react component, which was designed to
sketch, does that by default, but for the annotation usecase that is
really unneccessary, maybe it could be made optional?