feat(select): allow for option sorting logic to be customized

Adds an input for a `sortPredicate` to `mat-select`. The new input allows consumers to override the logic that `mat-select` uses to sort its values.

Fixes #11871.

Author: crisbeto

src/lib/select/select.spec.ts

@@ -3802,6 +3802,29 @@ describe('MatSelect', () => {
       expect(fixture.componentInstance.control.value).toEqual(['steak-0', 'pizza-1', 'tacos-2']);
     }));
 
+    it('should be able to customize the value sorting logic', fakeAsync(() => {
+      fixture.componentInstance.sortPredicate = (a, b, optionsArray) => {
+        return optionsArray.indexOf(b) - optionsArray.indexOf(a);
+      };
+      fixture.detectChanges();
+
+      trigger.click();
+      fixture.detectChanges();
+      flush();
+
+      const options = overlayContainerElement.querySelectorAll('mat-option') as
+          NodeListOf<HTMLElement>;
+
+      for (let i = 0; i < 3; i++) {
+        options[i].click();
+      }
+      fixture.detectChanges();
+
+      // Expect the items to be in reverse order.
+      expect(trigger.textContent).toContain('Tacos, Pizza, Steak');
+      expect(fixture.componentInstance.control.value).toEqual(['tacos-2', 'pizza-1', 'steak-0']);
+    }));
+
     it('should sort the values that get set via the model based on the panel order',
         fakeAsync(() => {
           trigger.click();
@@ -4182,7 +4205,8 @@ class FloatLabelSelect {
   selector: 'multi-select',
   template: `
     <mat-form-field>
-      <mat-select multiple placeholder="Food" [formControl]="control">
+      <mat-select multiple placeholder="Food" [formControl]="control"
+        [sortPredicate]="sortPredicate">
         <mat-option *ngFor="let food of foods"
                     [value]="food.value">{{ food.viewValue }}
         </mat-option>
@@ -4205,6 +4229,7 @@ class MultiSelect {
 
   @ViewChild(MatSelect) select: MatSelect;
   @ViewChildren(MatOption) options: QueryList<MatOption>;
+  sortPredicate: (a: MatOption, b: MatOption, options: MatOption[]) => number;
 }
 
 @Component({

src/lib/select/select.ts

@@ -381,7 +381,7 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
   }
 
   /**
-   * A function to compare the option values with the selected values. The first argument
+   * Function to compare the option values with the selected values. The first argument
    * is a value from an option. The second is a value from the selection. A boolean
    * should be returned.
    */
@@ -415,9 +415,16 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
   /** Input that can be used to specify the `aria-labelledby` attribute. */
   @Input('aria-labelledby') ariaLabelledby: string;
 
-  /** An object used to control when error messages are shown. */
+  /** Object used to control when error messages are shown. */
   @Input() errorStateMatcher: ErrorStateMatcher;
 
+  /**
+   * Function used to sort the values inside a select in multiple mode.
+   * Follows the same logic as `Array.prototype.sort`.
+   */
+  @Input() sortPredicate: (a: MatOption, b: MatOption, options: MatOption[]) => number =
+      (a, b, options) => options.indexOf(a) - options.indexOf(b)
+
   /** Unique id of the element. */
   @Input()
   get id(): string { return this._id; }
@@ -912,7 +919,7 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
   private _sortValues() {
     if (this.multiple) {
       const options = this.options.toArray();
-      this._selectionModel.sort((a, b) => options.indexOf(a) - options.indexOf(b));
+      this._selectionModel.sort((a, b) => this.sortPredicate(a, b, options));
       this.stateChanges.next();
     }
   }