Expose mdRipple instance in MdButton components

[kherock]

Bug, feature request, or proposal:

feature request: Developers should have a way to programmatically trigger md-button ripples.

What is the current behavior?

To the best of my knowledge, Angular doesn't provide any way to access directive instances on on descendants of components declared in a template. The closest I can get to accessing the ripple is by getting a reference to the MdButton instance.

What is the use-case or motivation for changing an existing behavior?

I'm making a carousel-type component that allows using the keyboard arrow keys to navigate. As a nice touch of visual feedback, I'd like to trigger a ripple on the buttons on keydown.

Here I've monkeypatched MdButton to demonstrate what I want:
https://embed.plnkr.co/esgkZ5ZtH9Vl8Wlkqx7t/

It's also useful if I want to implement a basic version of the feature discovery pattern:
https://material.io/guidelines/growth-communications/feature-discovery.html

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Latest

Is there anything else we should know?

It might be useful to expose this on other components that implement ripples. It should be as simple as adding a ViewChild and documenting its existence as a component property.

[devversion]

Thanks for the issue. We already have a similar issue: #4179

It would be great if you add your feedback/problem there as well!

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