feat(MatDatepicker): Add pass-through change and click events to MatDatepickerToggle

[mcalmus]

Feature Description

MatDatepickerToggle is a button, but it doesn't have a mechanism for apps that include it to know when it has been changed to clicked. This would be a useful feature.

Use Case

We do click metrics on the site, but I could see other cases where a developer might want to know when the date picker was opened.

[crisbeto]

You should be able to bind a (click) event to the mat-datepicker-toggle and I'm not sure whether a change event makes sense on a button. There already is a dateChange on the datepicker input. Would that work for you?

[mcalmus]

(click) is not firing for me. Nor is there a click event firing with my @HostListener (not shown).

https://stackblitz.com/edit/components-issue-uosggb?file=src/app/example-component.ts

[Totati]

Hi, the event propegetion is stopped, because it would focus the mat-form-field. https://github.com/angular/components/blob/master/src/material/datepicker/datepicker-toggle.ts#L116

[crisbeto]

I think that we should remove the stopPropagation and find a different way of preventing focus from reaching the form field.