Add support for hiding a dialog without closing it

[superMDguy]

Bug, feature request, or proposal:

Only show one dialog at a time.

What is the expected behavior?

When a dialog is opened from within another dialog, all other open dialogs should be hidden.

What is the current behavior?

The new dialog gets layered on top of the old dialog(s)

What are the steps to reproduce?

1. Open a dialog
2. Open another dialog from within that dialog

Plunker: https://plnkr.co/edit/GglOLDnx1uJlZRgD4y1W?p=preview

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

In angularjs material, multiple dialogs weren't allowed by default. See, for example http://plnkr.co/edit/qNu3q5XsIie1fYjKeJRc?p=preview. Ideally, though, there'd be a virtual stack of dialogs maintained, where only the topmost item on the stack is visible.

Is there anything else we should know?

Currently, I can work around the issue by running

Array.from(document.querySelectorAll(".cdk-overlay-container .cdk-global-overlay-wrapper")) .forEach((node, index, array) => (index !== array.length - 1) ? (<HTMLElement> node).style.display = "none" : true);

in ngOnInit, and running

Array.from(document.querySelectorAll(".cdk-overlay-container .cdk-global-overlay-wrapper")) .forEach((node) =>(<HTMLElement> node).style.display = "" );

in ngOnDestroy.

This works, and effectively creates a stack of hidden dialogs. But I don't want to add that snippet in every dialog that could possibly be opened from another dialog.

[jelbourn]

We intentionally support opening multiple dialogs at the same time.

What is missing, however, is a way to hide a dialog without closing it when you only have a DialogRef. Going to re-purpose this issue for that feature.

[jelbourn]

We could potentially do this by adding a hide API to MdDialogRef, but I think that this introduces too much confusion between close and hide. Instead, I think it would make sense to add the ability to add/remove css classes to the dialog via MdDialogRef.

[superMDguy]

How about a way to set an "inactive" class on the overlay pane that's automatically applied to all but the topmost dialog?

[julianobrasil]

If the default behavior is to hide the dialog, maybe the hide API isn't so confusing. An attribute in the parameter object passed to the open function would be simple to use. It would be something like

dialog.open({
  hideOnCascade: false 
});

The default behavior would be true. So the user could specify different when necessary.

[xban1x]

I would be interested to help out with this small feature. But hideOnCascade seems wrong to me...

[willshowell]

#6289 will probably help. It makes openDialogs public so you can access all open DialogRefs and assign unique ids to them for tracking.

Also agree with the dynamic add/remove classes approach since it gives you more flexibility of "inactive" dialogs that just hiding (e.g. lower opacity, transformations).

[superMDguy]

The PR @willshowell's mentioned is a step in the right direction. Additionally, it'd be good to either have a way to add a class to a MdDialogRef, like @jelbourn suggested, or to have an automatically applied "inactive" class, like I suggested earlier. I like the "inactive" class better, because it makes things cleaner for me, but if it's too much of a niche use case it might be better to just use @jelbourn's suggestion.