feat(mat-select): Call a `beforeOpen` observable that can modify the options before opening

[angelaki]

Feature Description

I'm waiting quite long for this feature and was just about to implement / PR it myself. Haven't found something, but is this already in discussion? And I can't get the npm install to run because of conficts, is this currently broken?

Use Case

Being able to set the items on demand in lazy loading / paged scenarios.

[andrewseguin]

Can you explain more why you would want to change the options just before opening, as opposed to having the options already set that you'd like to see on open?

[angelaki]

I lazy-load my options from the server just before the select opens. Right now I have a set a dummy option and switch them on open, but this causes an unpleasant flickering or much hacky css workaround.

[mmalerba]

I think a good option for now might be to load the options on hover/focus/touchstart of the select trigger element

[statyan]

@andrewseguin Here is one of the possible use cases - let's say we have a form of 10 dropdowns, but user ends up with changing only one of it and saves the form. In this scenario we can see it's really redundant to load/render on the page the options for other 9 dropdowns as we really need options only when a dropdown panel is open. Until then we don't need options as long as we can provide a value and (because of the nature of mat-select/select components) provide a one option the same as walue is so mat-select can render it properly.
This can give a good boost in performance.