fix(input): hints not being read out by screen readers

src/lib/input/input-container.html

@@ -30,6 +30,6 @@
           [class.md-warn]="dividerColor == 'warn'"></span>
   </div>
 
-  <div *ngIf="hintLabel != ''" class="md-hint">{{hintLabel}}</div>
+  <div *ngIf="hintLabel != ''" [attr.id]="_hintLabelId" class="md-hint">{{hintLabel}}</div>
   <ng-content select="md-hint"></ng-content>
 </div>

src/lib/input/input-container.spec.ts

@@ -46,7 +46,9 @@ describe('MdInputContainer', function () {
         MdInputContainerWithValueBinding,
         MdInputContainerWithFormControl,
         MdInputContainerWithStaticPlaceholder,
-        MdInputContainerMissingMdInputTestController
+        MdInputContainerMissingMdInputTestController,
+        MdInputContainerMultipleHintTestController,
+        MdInputContainerMultipleHintMixedTestController
       ],
     });
 
@@ -271,6 +273,17 @@ describe('MdInputContainer', function () {
     expect(fixture.debugElement.query(By.css('.md-hint'))).not.toBeNull();
   });
 
+  it('sets an id on hint labels', () => {
+    let fixture = TestBed.createComponent(MdInputContainerHintLabelTestController);
+
+    fixture.componentInstance.label = 'label';
+    fixture.detectChanges();
+
+    let hint = fixture.debugElement.query(By.css('.md-hint')).nativeElement;
+
+    expect(hint.getAttribute('id')).toBeTruthy();
+  });
+
   it('supports hint labels elements', () => {
     let fixture = TestBed.createComponent(MdInputContainerHintLabel2TestController);
     fixture.detectChanges();
@@ -285,6 +298,17 @@ describe('MdInputContainer', function () {
     expect(el.textContent).toBe('label');
   });
 
+  it('sets an id on the hint element', () => {
+    let fixture = TestBed.createComponent(MdInputContainerHintLabel2TestController);
+
+    fixture.componentInstance.label = 'label';
+    fixture.detectChanges();
+
+    let hint = fixture.debugElement.query(By.css('md-hint')).nativeElement;
+
+    expect(hint.getAttribute('id')).toBeTruthy();
+  });
+
   it('supports placeholder attribute', async(() => {
     let fixture = TestBed.createComponent(MdInputContainerPlaceholderAttrTestComponent);
     fixture.detectChanges();
@@ -404,6 +428,55 @@ describe('MdInputContainer', function () {
     const textarea: HTMLTextAreaElement = fixture.nativeElement.querySelector('textarea');
     expect(textarea).not.toBeNull();
   });
+
+  it('sets the aria-describedby when a hintLabel is set', () => {
+    let fixture = TestBed.createComponent(MdInputContainerHintLabelTestController);
+
+    fixture.componentInstance.label = 'label';
+    fixture.detectChanges();
+
+    let hint = fixture.debugElement.query(By.css('.md-hint')).nativeElement;
+    let input = fixture.debugElement.query(By.css('input')).nativeElement;
+
+    expect(input.getAttribute('aria-describedby')).toBe(hint.getAttribute('id'));
+  });
+
+  it('sets the aria-describedby to the id of the md-hint', () => {
+    let fixture = TestBed.createComponent(MdInputContainerHintLabel2TestController);
+
+    fixture.componentInstance.label = 'label';
+    fixture.detectChanges();
+
+    let hint = fixture.debugElement.query(By.css('.md-hint')).nativeElement;
+    let input = fixture.debugElement.query(By.css('input')).nativeElement;
+
+    expect(input.getAttribute('aria-describedby')).toBe(hint.getAttribute('id'));
+  });
+
+  it('sets the aria-describedby with multiple md-hint instances', () => {
+    let fixture = TestBed.createComponent(MdInputContainerMultipleHintTestController);
+
+    fixture.componentInstance.startId = 'start';
+    fixture.componentInstance.endId = 'end';
+    fixture.detectChanges();
+
+    let input = fixture.debugElement.query(By.css('input')).nativeElement;
+
+    expect(input.getAttribute('aria-describedby')).toBe('start end');
+  });
+
+  it('sets the aria-describedby when a hintLabel is set, in addition to a md-hint', () => {
+    let fixture = TestBed.createComponent(MdInputContainerMultipleHintMixedTestController);
+
+    fixture.detectChanges();
+
+    let hintLabel = fixture.debugElement.query(By.css('.md-hint')).nativeElement;
+    let endLabel = fixture.debugElement.query(By.css('.md-hint[align="end"]')).nativeElement;
+    let input = fixture.debugElement.query(By.css('input')).nativeElement;
+    let ariaValue = input.getAttribute('aria-describedby');
+
+    expect(ariaValue).toBe(`${hintLabel.getAttribute('id')} ${endLabel.getAttribute('id')}`);
+  });
 });
 
 @Component({
@@ -512,6 +585,28 @@ class MdInputContainerInvalidHint2TestController {}
 })
 class MdInputContainerInvalidHintTestController {}
 
+@Component({
+  template: `
+    <md-input-container>
+      <input mdInput>
+      <md-hint align="start" [id]="startId">Hello</md-hint>
+      <md-hint align="end" [id]="endId">World</md-hint>
+    </md-input-container>`
+})
+class MdInputContainerMultipleHintTestController {
+  startId: string;
+  endId: string;
+}
+
+@Component({
+  template: `
+    <md-input-container hintLabel="Hello">
+      <input mdInput>
+      <md-hint align="end">World</md-hint>
+    </md-input-container>`
+})
+class MdInputContainerMultipleHintMixedTestController {}
+
 @Component({
   template: `<md-input-container><input mdInput [(ngModel)]="model"></md-input-container>`
 })

src/lib/input/input-container.ts

@@ -58,11 +58,15 @@ export class MdPlaceholder {}
   host: {
     'class': 'md-hint',
     '[class.md-right]': 'align == "end"',
+    '[attr.id]': 'id',
   }
 })
 export class MdHint {
   // Whether to align the hint label at the start or end of the line.
   @Input() align: 'start' | 'end' = 'start';
+
+  // Unique ID for the hint. Used for the aria-describedby on the input.
+  @Input() id: string = `md-input-hint-${nextUniqueId++}`;
 }
 
 
@@ -77,9 +81,10 @@ export class MdHint {
     '[placeholder]': 'placeholder',
     '[disabled]': 'disabled',
     '[required]': 'required',
+    '[attr.aria-describedby]': 'ariaDescribedby',
     '(blur)': '_onBlur()',
     '(focus)': '_onFocus()',
-    '(input)': '_onInput()'
+    '(input)': '_onInput()',
   }
 })
 export class MdInputDirective {
@@ -95,6 +100,9 @@ export class MdInputDirective {
   /** Whether the element is focused or not. */
   focused = false;
 
+  /** Sets the aria-describedby attribute on the input for improved a11y. */
+  ariaDescribedby: string;
+
   /** Whether the element is disabled. */
   @Input()
   get disabled() {
@@ -119,6 +127,7 @@ export class MdInputDirective {
       this._placeholderChange.emit(this._placeholder);
     }
   }
+
   /** Whether the element is required. */
   @Input()
   get required() { return this._required; }
@@ -249,10 +258,13 @@ export class MdInputContainer implements AfterContentInit {
   get hintLabel() { return this._hintLabel; }
   set hintLabel(value: string) {
     this._hintLabel = value;
-    this._validateHints();
+    this._processHints();
   }
   private _hintLabel = '';
 
+  // Unique id for the hint label.
+  _hintLabelId: string = `md-input-hint-${nextUniqueId++}`;
+
   /** Text or the floating placeholder. */
   @Input()
   get floatingPlaceholder(): boolean { return this._floatingPlaceholder; }
@@ -270,11 +282,11 @@ export class MdInputContainer implements AfterContentInit {
       throw new MdInputContainerMissingMdInputError();
     }
 
-    this._validateHints();
+    this._processHints();
     this._validatePlaceholders();
 
     // Re-validate when things change.
-    this._hintChildren.changes.subscribe(() => this._validateHints());
+    this._hintChildren.changes.subscribe(() => this._processHints());
     this._mdInputChild._placeholderChange.subscribe(() => this._validatePlaceholders());
   }
 
@@ -287,6 +299,7 @@ export class MdInputContainer implements AfterContentInit {
   /** Whether the input has a placeholder. */
   _hasPlaceholder() { return !!(this._mdInputChild.placeholder || this._placeholderChild); }
 
+  /** Focuses the underlying input. */
   _focusInput() { this._mdInputChild.focus(); }
 
   /**
@@ -299,6 +312,14 @@ export class MdInputContainer implements AfterContentInit {
     }
   }
 
+  /**
+   * Does any extra processing that is required when handling the hints.
+   */
+  private _processHints() {
+    this._validateHints();
+    this._syncAriaDescribedby();
+  }
+
   /**
    * Ensure that there is a maximum of one of each `<md-hint>` alignment specified, with the
    * attribute being considered as `align="start"`.
@@ -322,4 +343,28 @@ export class MdInputContainer implements AfterContentInit {
       });
     }
   }
+
+  /**
+   * Sets the child input's `aria-describedby` to a space-separated list of the ids
+   * of the currently-specified hints, as well as a generated id for the hint label.
+   */
+  private _syncAriaDescribedby() {
+    let ids: string[] = [];
+    let startHint = this._hintChildren ?
+        this._hintChildren.find(hint => hint.align === 'start') : null;
+    let endHint = this._hintChildren ?
+        this._hintChildren.find(hint => hint.align === 'end') : null;
+
+    if (startHint) {
+      ids.push(startHint.id);
+    } else if (this._hintLabel) {
+      ids.push(this._hintLabelId);
+    }
+
+    if (endHint) {
+      ids.push(endHint.id);
+    }
+
+    this._mdInputChild.ariaDescribedby = ids.join(' ');
+  }
 }