make matTextareaAutosize bindable

[shlomiassaf]

Bug, feature request, or proposal: proposal

What is the expected behavior?

When using matTextareaAutosize, the auto size feature can be enabled/disabled based on bindings.

What is the current behavior?

matTextareaAutosize is not bindable.

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

Controlling the auto-size feature can allow smart layouts based on media queries, or any other logic the user implements. It also allows dynamic forms to set it on/off based on schema selection.

Another thing is that the matTextareaAutosize directive is a bit misleading, one might easily think it is a property of matInput.

This should be fairly easy to implement, I think just adding an @Input() matTextareaAutosize: boolean will do the trick, using true as the initial value and coercion when set.

There is one issue with the duplicate selectors (textarea[mat-autosize], textarea[matTextareaAutosize]) which I have no clue how the end result is when someone will do [mat-autosize]="autoSize

Also related #9831

cc @jelbourn @mmalerba

[mmalerba]

That sounds reasonable to me, want to send a PR?

[pfeigl]

Just started working on this. What is the expected result when autosizing is disabled but the height already changed through the autosizing. I would assume we restore the original height of the element by removing the style.height property again.
For this to work we might have to save the initial value of the height property if it was set.

Also on contrary to the initial description the changes have to be applied in CdkTextareaAutosize, but we should probably extend the functionality into MatTextareaAutosize.

@shlomiassaf Regarding your question with multiple selectors: We can just have two property setter (one for each selector) which internally map to same code (which enables / disables the autosizing)

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