feat(a11y): add basic focus-trap directive (#1311)

* feat(a11y): add simple focus-trapping directive

* fix too soon return

* add unit tests

* add todo

src/lib/core/a11y/focus-trap.spec.ts

@@ -0,0 +1,58 @@
+import {inject, ComponentFixture, TestBed} from '@angular/core/testing';
+import {By} from '@angular/platform-browser';
+import {Component} from '@angular/core';
+import {FocusTrap} from './focus-trap';
+import {InteractivityChecker} from './interactivity-checker';
+
+
+describe('FocusTrap', () => {
+  let checker: InteractivityChecker;
+  let fixture: ComponentFixture<FocusTrapTestApp>;
+
+  describe('with default element', () => {
+    beforeEach(() => TestBed.configureTestingModule({
+      declarations: [FocusTrap, FocusTrapTestApp],
+      providers: [InteractivityChecker]
+    }));
+
+    beforeEach(inject([InteractivityChecker], (c: InteractivityChecker) => {
+      checker = c;
+      fixture = TestBed.createComponent(FocusTrapTestApp);
+    }));
+
+    it('wrap focus from end to start', () => {
+      let focusTrap = fixture.debugElement.query(By.directive(FocusTrap));
+      let focusTrapInstance = focusTrap.componentInstance as FocusTrap;
+
+      // Because we can't mimic a real tab press focus change in a unit test, just call the
+      // focus event handler directly.
+      focusTrapInstance.wrapFocus();
+
+      expect(document.activeElement.nodeName.toLowerCase())
+          .toBe('input', 'Expected input element to be focused');
+    });
+
+    it('should wrap focus from start to end', () => {
+      let focusTrap = fixture.debugElement.query(By.directive(FocusTrap));
+      let focusTrapInstance = focusTrap.componentInstance as FocusTrap;
+
+      // Because we can't mimic a real tab press focus change in a unit test, just call the
+      // focus event handler directly.
+      focusTrapInstance.reverseWrapFocus();
+
+      expect(document.activeElement.nodeName.toLowerCase())
+          .toBe('button', 'Expected button element to be focused');
+    });
+  });
+});
+
+
+@Component({
+  template: `
+    <focus-trap>
+      <input>
+      <button>SAVE</button>
+    </focus-trap>
+    `
+})
+class FocusTrapTestApp { }

src/lib/core/a11y/focus-trap.ts

@@ -0,0 +1,78 @@
+import {Component, ViewEncapsulation, ViewChild, ElementRef} from '@angular/core';
+import {InteractivityChecker} from './interactivity-checker';
+
+
+/**
+ * Directive for trapping focus within a region.
+ *
+ * NOTE: This directive currently uses a very simple (naive) approach to focus trapping.
+ * It assumes that the tab order is the same as DOM order, which is not necessarily true.
+ * Things like tabIndex > 0, flex `order`, and shadow roots can cause to two to misalign.
+ * This will be replaced with a more intelligent solution before the library is considered stable.
+ */
+@Component({
+  moduleId: module.id,
+  selector: 'focus-trap',
+  // TODO(jelbourn): move this to a separate file.
+  template: `
+  <div tabindex="0" (focus)="reverseWrapFocus()"></div>
+  <div #trappedContent><ng-content></ng-content></div>
+  <div tabindex="0" (focus)="wrapFocus()"></div>`,
+  encapsulation: ViewEncapsulation.None,
+})
+export class FocusTrap {
+  @ViewChild('trappedContent') trappedContent: ElementRef;
+
+  constructor(private _checker: InteractivityChecker) { }
+
+  /** Wrap focus from the end of the trapped region to the beginning. */
+  wrapFocus() {
+    let redirectToElement = this._getFirstTabbableElement(this.trappedContent.nativeElement);
+    if (redirectToElement) {
+      redirectToElement.focus();
+    }
+  }
+
+  /** Wrap focus from the beginning of the trapped region to the end. */
+  reverseWrapFocus() {
+    let redirectToElement = this._getLastTabbableElement(this.trappedContent.nativeElement);
+    if (redirectToElement) {
+      redirectToElement.focus();
+    }
+  }
+
+  /** Get the first tabbable element from a DOM subtree (inclusive). */
+  private _getFirstTabbableElement(root: HTMLElement): HTMLElement {
+    if (this._checker.isFocusable(root) && this._checker.isTabbable(root)) {
+      return root;
+    }
+
+    // Iterate in DOM order.
+    let childCount = root.children.length;
+    for (let i = 0; i < childCount; i++) {
+      let tabbableChild = this._getFirstTabbableElement(root.children[i] as HTMLElement);
+      if (tabbableChild) {
+        return tabbableChild;
+      }
+    }
+
+    return null;
+  }
+
+  /** Get the last tabbable element from a DOM subtree (inclusive). */
+  private _getLastTabbableElement(root: HTMLElement): HTMLElement {
+    if (this._checker.isFocusable(root) && this._checker.isTabbable(root)) {
+      return root;
+    }
+
+    // Iterate in reverse DOM order.
+    for (let i = root.children.length - 1; i >= 0; i--) {
+      let tabbableChild = this._getLastTabbableElement(root.children[i] as HTMLElement);
+      if (tabbableChild) {
+        return tabbableChild;
+      }
+    }
+
+    return null;
+  }
+}

src/lib/core/core.ts

@@ -50,6 +50,9 @@ export {
   LIVE_ANNOUNCER_ELEMENT_TOKEN,
 } from './a11y/live-announcer';
 
+export {FocusTrap} from './a11y/focus-trap';
+export {InteractivityChecker} from './a11y/interactivity-checker';
+
 export {
   MdUniqueSelectionDispatcher,
   MdUniqueSelectionDispatcherListener