Add support for reactive expressions

[anandology]

The current interface to specify a reactive function using decorator taking output and inputs as arguments is too verbose and doesn't feel very elegant.

Wouldn't it be better to say:

app['amount'].children = compute_amount(app['hours'].value, app['rate'].value)

instead of:

@app.callback(Output('amount', 'children'),
              [Input('hours', 'value'), Input('rate', 'value')])
def compute_amount(hours, rate):
    ...

The former one feels more natural way to specify the logic. Also, it makes it easy to use the same function in different scenarios. For example:

app['output1'].children = uppercase(app['input1'].value)
app['output2'].children = uppercase(app['input2'].value)

This would be too cumbersome to do with the current approach.

I've implemented a quick proof-of-concept code to demonstrate that such a thing is possible. The code and notes are available at:

https://gist.github.com/anandology/b93fd555215203977301ecb4994efe66

[chriddyp]

Wow, this is really cool. Thanks for sharing this!

I'm going to keep this issue open to let the idea ferment a little bit longer and to promote discoverability. The verbosity of the current method with decorators isn't enough for me to warrant a new syntax, but I can imagine future versions of this package ending up using something along these lines.

The current method also allows for other types of dependency objects like Event and State, which act in a different way than Input. There is a simple example here: https://gist.github.com/chriddyp/e546a2a2c05df29c868e4cef57fb2105.

[chriddyp]

This type of syntax is similar to what is being proposed for client-side operations in #266

[gvwilson]

Hi - this issue has been sitting for a while, so as part of our effort to tidy up our public repositories I'm going to close it. If it's still a concern, we'd be grateful if you could open a new issue (with a short reproducible example if appropriate) so that we can add it to our stack. Cheers - @gvwilson