docs: clarify form control usage in form-field w/ chip-list and chip-input

[jonaaix]

Reproduction

Use StackBlitz to reproduce your issue:
https://stackblitz.com/edit/angular-b8qqaf

Steps to reproduce:

1. Create chip list input
   https://material.angular.io/components/chips/overview
2. Disable form field

Expected Behavior

What behavior were you expecting to see?
Should be disabled, light gray color

Actual Behavior

What behavior did you actually see?
Is functionally disabled, but not visibly

Environment

• Angular: 9
• CDK/Material: 9
• Browser(s): Edge chromium
• Operating System (e.g. Windows, macOS, Ubuntu): macOS

[jonaaix]

It seems mat-form-field-disabled ist not added for chip-list form-fields, and form-field ist still focusable

[devversion]

The problem here is that the MatChipList is the actual form-field control. The form control state of the input is not propagated to the chip list, nor to the parent form-field.

A fix would be to capture the disabled state of the input in the chip list, but conceptually it should be the other way around. i.e. the chip-list is the primary source of data, and notifies the chip input if it should be disabled. It's possible that the input is disabled while the chip list is not, right?

In your concrete StackBlitz, a fix would be toggle the disabled state on the chip-list. Either by using the disabled input or by using a form control on the chip list.

We should probably clarify this in the documentation. Happy to chat more about it though.

[jonaaix]

@devversion
Indeed, i just moved the [formControl]="fruitControl" from input to mat-chip-list and disabling works properly. On the other hand adding [disabled]="fruitCtrl.disabled" to mat-chip-list also works. So this is not a bug.

Maybe the example in the docs could have a correctly placed formControl directive or [(ngModel)], to make it obvious. I did not realize that mat-chip-list could be a form field, so I implemented a class member array to manage the chip-list data.

[devversion]

@btxtiger Yeah, I agree. We should make this more obvious in the docs. Feel free to send a PR if you want. Otherwise I can have a look when I get a chance.