Document creating a custom sort-header control

[leonardseymore]

Bug, feature request, or proposal:

Proposal

What is the expected behavior?

Be able to add custom controls to headers along with matSort

What is the current behavior?

It is possible to add custom controls e.g.

<ng-container matColumnDef="to">
    <mat-header-cell *matHeaderCellDef>
      <div class="vbox">
        To
        <input [(ngModel)]="filter.to">
      </div>
    </mat-header-cell>
    <mat-cell *matCellDef="let element">{{element.to}}</mat-cell>
  </ng-container>

However this does not work with matSort

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

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

I need individual filter controls for each column in the table, all the examples use a single filter field.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Is there anything else we should know?

[jelbourn]

This is possible today, it's just not documented. You just need to:

1. Have your custom control implement MatSortable
2. Inject MatSort into your custom control and call register
3. Subscribe to the sort state subjects to respond the other control instances changing the sort state