Clientside callbacks

[waralex]

ClientSide callbacks.

Allows you to add Client Side callbacks using

callback!(ClientsideFunction("namespace", "func_name"),app, callid"my-id.value => my-div.children")

or

callback!(
        """
        function(input_value) {
            return (
                parseFloat(input_value_1, 10)
            );
        }
        """
        , app, callid"my-id.value => my-div.children"
        )

Based on hot_reload branch and should be reviewed after it

Corresponding integration tests: https://github.com/plotly/Dash.jl/blob/clientside_callbacks/test/integration/clientside/test_clientside.py

[alexcjohnson]

It's not clear to me exactly what's being tested by some of these @test_throws - at a minimum it would be nice to have a comment like "my-div2.children is already an output", but better would be for the code to somehow inspect the error to ensure it's actually the one you intended to be thrown, as opposed to something else like a typo in the callid string causing a regexp failure.

Minor, and not new in this PR, you're just rearranging, but it's the first I'm looking at this code 😉

[alexcjohnson]

This looks great, very clean. No comments on the source code, and aside from my one minor comment on the exception testing, all I noticed is it's perhaps overkill to copy all the Python clientside callback tests here.

I wouldn't bother pruning them, but as a general rule all we need to test here is the behavior of the back end. Once a few clientside callbacks (single/multi output, with or without state...) have made it to the front end, everything else is testing specific behavior of the renderer, and we can assume that's tested in the main dash repo, where the renderer code lives. Aside from the extra time to port the tests to Julia it's both extra test execution time and more maintenance cost should any of those tests need to change in the future.

Anyway neither of these comments is blocking - nice job! 💃

[waralex]

@alexcjohnson About copying Python tests. I thought about this, but decided to continue copying its for the following reason. I have very little experience of using dash on the user's side, so I assume that Python tests handle some of the pitfalls that Python users have already encountered. Plus for me, this is a kind of tutorial on how dash is used.

With exceptions, the problem is more global - I created a separate issue(#37) for it