feat(select): add mat-select-header component

Adds a `mat-select-header` component, which is a fixed header above the select's options. It allows for the user to project an input to be used for filtering long lists of options.

**Note:** This component only handles the positioning, styling, some basic focus management and exposes the panel id for a11y. The functionality is up to the consumer to handle.

Fixes #2812.

Author: crisbeto

src/demo-app/select/select-demo.html

@@ -140,6 +140,31 @@
     </mat-card-content>
   </mat-card>
 
+  <mat-card>
+    <mat-card-subtitle>Select header</mat-card-subtitle>
+
+    <mat-card-content>
+      <mat-form-field>
+        <mat-select placeholder="Drink" [(ngModel)]="currentDrink" #selectWitHeader="matSelect">
+          <mat-select-header>
+            <input
+              type="search"
+              role="combobox"
+              class="mat-select-header-input"
+              [(ngModel)]="searchTerm"
+              [attr.aria-owns]="selectWitHeader.panelId"
+              (ngModelChange)="filterDrinks()"
+              placeholder="Search for a drink"/>
+          </mat-select-header>
+
+          <mat-option *ngFor="let drink of filteredDrinks" [value]="drink.value" [disabled]="drink.disabled">
+            {{ drink.viewValue }}
+          </mat-option>
+        </mat-select>
+      </mat-form-field>
+    </mat-card-content>
+  </mat-card>
+
   <div *ngIf="showSelect">
     <mat-card>
       <mat-card-subtitle>formControl</mat-card-subtitle>

src/demo-app/select/select-demo.ts

@@ -20,6 +20,7 @@ export class SelectDemo {
   currentPokemon: string[];
   currentPokemonFromGroup: string;
   currentDigimon: string;
+  searchTerm: string;
   latestChangeEvent: MatSelectChange;
   floatPlaceholder: string = 'auto';
   foodControl = new FormControl('pizza-1');
@@ -47,6 +48,8 @@ export class SelectDemo {
     {value: 'milk-8', viewValue: 'Milk'},
   ];
 
+  filteredDrinks = this.drinks.slice();
+
   pokemon = [
     {value: 'bulbasaur-0', viewValue: 'Bulbasaur'},
     {value: 'charizard-1', viewValue: 'Charizard'},
@@ -126,4 +129,10 @@ export class SelectDemo {
   compareByReference(o1: any, o2: any) {
     return o1 === o2;
   }
+
+  filterDrinks() {
+    this.filteredDrinks = this.searchTerm ? this.drinks.filter(item => {
+      return item.viewValue.toLowerCase().indexOf(this.searchTerm.toLowerCase()) > -1;
+    }) : this.drinks.slice();
+  }
 }

src/lib/core/style/_menu-common.scss

@@ -15,8 +15,12 @@ $mat-menu-icon-margin: 16px !default;
 
 @mixin mat-menu-base($default-elevation) {
   @include mat-overridable-elevation($default-elevation);
+  @include mat-menu-scrollable();
   min-width: $mat-menu-overlay-min-width;
   max-width: $mat-menu-overlay-max-width;
+}
+
+@mixin mat-menu-scrollable() {
   overflow: auto;
   -webkit-overflow-scrolling: touch;   // for momentum scroll on mobile
 }

src/lib/select/_select-theme.scss

@@ -31,6 +31,10 @@
     }
   }
 
+  .mat-select-header {
+    color: mat-color($foreground, divider);
+  }
+
   .mat-form-field {
     &.mat-focused {
       &.mat-primary .mat-select-arrow {

src/lib/select/public-api.ts

@@ -9,3 +9,4 @@
 export * from './select-module';
 export * from './select';
 export * from './select-animations';
+export * from './select-header';

src/lib/select/select-animations.ts

@@ -61,9 +61,8 @@ export const transformPanel: AnimationTriggerMetadata = trigger('transformPanel'
  * panel has transformed in.
  */
 export const fadeInContent: AnimationTriggerMetadata = trigger('fadeInContent', [
+  state('void', style({opacity: 0})),
   state('showing', style({opacity: 1})),
-  transition('void => showing', [
-    style({opacity: 0}),
-    animate('150ms 100ms cubic-bezier(0.55, 0, 0.55, 0.2)')
-  ])
+  transition('void => showing', animate('150ms 100ms cubic-bezier(0.55, 0, 0.55, 0.2)')),
+  transition('showing => void', animate('150ms cubic-bezier(0.55, 0, 0.55, 0.2)'))
 ]);

src/lib/select/select-header.html

@@ -0,0 +1,3 @@
+<span cdkTrapFocus>
+  <ng-content></ng-content>
+</span>

src/lib/select/select-header.ts

@@ -0,0 +1,33 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Component, ViewEncapsulation, ChangeDetectionStrategy, ViewChild} from '@angular/core';
+import {FocusTrapDirective} from '@angular/cdk/a11y';
+
+/**
+ * Fixed header that will be rendered above a select's options.
+ * Can be used as a bar for filtering out options.
+ */
+@Component({
+  moduleId: module.id,
+  selector: 'mat-select-header',
+  changeDetection: ChangeDetectionStrategy.OnPush,
+  encapsulation: ViewEncapsulation.None,
+  preserveWhitespaces: false,
+  templateUrl: 'select-header.html',
+  host: {
+    'class': 'mat-select-header',
+  }
+})
+export class MatSelectHeader {
+  @ViewChild(FocusTrapDirective) _focusTrap: FocusTrapDirective;
+
+  _trapFocus() {
+    this._focusTrap.focusTrap.focusFirstTabbableElementWhenReady();
+  }
+}

src/lib/select/select-module.ts

@@ -8,10 +8,12 @@
 import {NgModule} from '@angular/core';
 import {CommonModule} from '@angular/common';
 import {MatSelect, MatSelectTrigger, MAT_SELECT_SCROLL_STRATEGY_PROVIDER} from './select';
+import {MatSelectHeader} from './select-header';
 import {MatCommonModule, MatOptionModule} from '@angular/material/core';
 import {OverlayModule} from '@angular/cdk/overlay';
 import {MatFormFieldModule} from '@angular/material/form-field';
 import {ErrorStateMatcher} from '@angular/material/core';
+import {A11yModule} from '@angular/cdk/a11y';
 
 
 @NgModule({
@@ -20,9 +22,17 @@ import {ErrorStateMatcher} from '@angular/material/core';
     OverlayModule,
     MatOptionModule,
     MatCommonModule,
+    A11yModule,
   ],
-  exports: [MatFormFieldModule, MatSelect, MatSelectTrigger, MatOptionModule, MatCommonModule],
-  declarations: [MatSelect, MatSelectTrigger],
+  exports: [
+    MatFormFieldModule,
+    MatSelect,
+    MatSelectTrigger,
+    MatSelectHeader,
+    MatOptionModule,
+    MatCommonModule,
+  ],
+  declarations: [MatSelect, MatSelectTrigger, MatSelectHeader],
   providers: [MAT_SELECT_SCROLL_STRATEGY_PROVIDER, ErrorStateMatcher]
 })
 export class MatSelectModule {}

src/lib/select/select.html

@@ -34,20 +34,21 @@
   (detach)="close()">
 
   <div
-    #panel
     class="mat-select-panel {{ _getPanelTheme() }}"
     [ngClass]="panelClass"
     [@transformPanel]="multiple ? 'showing-multiple' : 'showing'"
     (@transformPanel.done)="_onPanelDone()"
     [style.transformOrigin]="_transformOrigin"
     [class.mat-select-panel-done-animating]="_panelDoneAnimating"
-    [style.font-size.px]="_triggerFontSize">
+    [style.font-size.px]="_triggerFontSize"
+    (keydown)="_handleKeydown($event)">
 
-    <div
-      class="mat-select-content"
-      [@fadeInContent]="'showing'"
-      (@fadeInContent.done)="_onFadeInDone()">
-      <ng-content></ng-content>
+    <div [@fadeInContent]="'showing'" (@fadeInContent.done)="_onFadeInDone()">
+      <ng-content select="mat-select-header"></ng-content>
+
+      <div #panel class="mat-select-content" [attr.id]="panelId">
+        <ng-content></ng-content>
+      </div>
     </div>
   </div>
 </ng-template>