Add ability to use functions with param dependencies in DynamicMap

[philippjfr]

Allows doing things like:

slider = pn.widgets.FloatSlider()

@pn.depends(y=slider.param.value)
def test(y):
    return hv.Curve([0, 1, y])

pn.Column(slider, hv.DynamicMap(test))

• Adds tests
• Added docs

[jlstevens]

Code looks fine but I'm starting to lose track of all the ways you can do things with param watchers, panel etc. I assume the alternative would be a class which is unnecessary for such a simple example?

I guess I am happy to merge but I would also like to see some docs detailing this style among all the other ways you can do things with param nowadays.

[philippjfr]

I guess I am happy to merge but I would also like to see some docs detailing this style among all the other ways you can do things with param nowadays.

That is on my to-do list along with .apply docs, but I don't think I'll get there today and in the meantime I'd like to build examples on top of this.

[jlstevens]

@philippjfr Looks good!

My comments generally relate to expanding the docs in a few places. There is one question about reordering the presentation of the function/class approach but I have no opinion as to whether that should be part of this PR or not.

Another task that shouldn't be part of this PR is to expand the streams user guide to discuss these implicit parameter streams. There are now three types of stream in holoviews imho 1) explicitly defined custom stream objects and classes 2) linked streams 3) implicit parameter driven ones.

Happy to merge once these doc issues are addressed (or postponed for later PRs). And once the tests are passing!

[jbednar]

Looks good now, thanks!

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