Consider adding a layer to remove coupling with views and BlueskyRun API

[danielballan]

The view code knows about of the concept of a BlueskyRun and its particular interface and signals:

bluesky-widgets/bluesky_widgets/_matplotlib_axes.py

Lines 104 to 105 in 434046b

	run = line_spec.run
	x, y = line_spec.func(run)

bluesky-widgets/bluesky_widgets/_matplotlib_axes.py

Lines 121 to 125 in 434046b

	self.connect(run.events.new_data, update)
	self.connect(
	run.events.completed,
	lambda event: run.events.new_data.disconnect(update),
	)

What if, instead, we separate the bluesky-aware aspect into a separate components, perhaps by subclassing like so:

class LineSpec:
    "Not bluesky-aware, just knows about an abstract `transform` (callable) and `data`"
    def __init__(self, update: callable, label, style=None):
        "Expected signature of update is: update() -> x: Array, y: Array"
        ...
        self.events = EmitterGroup(new_data=Event, completed=Event)

class BlueskyLineSpec(LineSpec):
    "Hooks up LineSpec's signals to BlueskyRun signals."
    def __init__(self, transform: callable, run: BlueskyRun, label, style=None):

        run.events.new_data.connect(self.events.new_data)
        run.events.completed.connect(self.events.completed)

        def update():
            return transform(data)

        super().__init__(update, label, style)

then LineSpec and the views become generic. That would extend the utility of these components at the cost of an additional layer of signaling.

[danielballan]

Another important consequence of this is that the bottom layer LineSpec is not bound to a single BlueskyRun, and you could generate LineSpecs joining data from multiple Runs. And of course from things that have nothing to do with BlueskyRun at all.

[danielballan]

Also, if transform always returns a dict of kwargs rather than an artist-specific construct (like (x, y) for lines) we can simplify the view code and reduce the artist-specific code.

h/t @tacaswell

[danielballan]

#97 does this almost entirely. It moves the 1:1 correspondence to BlueskyRun into a classmethod. That could easily be moved off to a subclass in the future.

[danielballan]

@tacaswell shared this snippet of work related to matplotlib 4.0 https://gist.github.com/tacaswell/95177903175dbc28be5353b4a0e5118f. Keeping this here so I don't lose it.