jasonaden · May 14, 2019
diff --git a/packages/common/upgrade/src/location_shim.ts b/packages/common/upgrade/src/location_shim.ts
@@ -39,9 +39,16 @@ export class $locationShim {
   private $$search: any = '';
   private $$hash: string = '';
   private $$state: unknown;
+  private $$changeListeners: [
+    ((url: string, state: unknown, oldUrl: string, oldState: unknown, err?: (e: Error) => void) =>
+         void),
+    (e: Error) => void
+  ][] = [];
 
   private cachedState: unknown = null;
 
+
+
   constructor(
       $injector: any, private location: Location, private platformLocation: PlatformLocation,
       private urlCodec: UrlCodec, private locationStrategy: LocationStrategy) {
@@ -313,6 +320,32 @@ export class $locationShim {
     }
   }
 
+  /**
+   * Register URL change listeners. This API can be used to catch updates performed by the
+   * AngularJS framework. These changes are a subset of the `$locationChangeStart/Success` events
+   * as those events fire when AngularJS updates it's internally referenced version of the browser
+   * URL. It's possible for `$locationChange` events to happen, but for the browser URL
+   * (window.location) to remain unchanged. This `onChange` callback will fire only when AngularJS
+   * actually updates the browser URL (window.location).
+   */
+  onChange(
+      fn: (url: string, state: unknown, oldUrl: string, oldState: unknown) => void,
+      err: (e: Error) => void = (e: Error) => {}) {
+    this.$$changeListeners.push([fn, err]);
+  }
+
+  /** @internal */
+  $$notifyChangeListeners(
+      url: string = '', state: unknown, oldUrl: string = '', oldState: unknown) {
+    this.$$changeListeners.forEach(([fn, err]) => {
+      try {
+        fn(url, state, oldUrl, oldState);
+      } catch (e) {
+        err(e);
+      }
+    });
+  }
+
   $$parse(url: string) {
     let pathUrl: string|undefined;
     if (url.startsWith('/')) {
@@ -363,6 +396,7 @@ export class $locationShim {
       // state object; this makes possible quick checking if the state changed in the digest
       // loop. Checking deep equality would be too expensive.
       this.$$state = this.browserState();
+      this.$$notifyChangeListeners(url, state, oldUrl, oldState);
     } catch (e) {
       // Restore old values if pushState fails
       this.url(oldUrl);

diff --git a/packages/common/upgrade/test/upgrade.spec.ts b/packages/common/upgrade/test/upgrade.spec.ts
@@ -624,6 +624,87 @@ describe('New URL Parsing', () => {
   });
 });
 
+describe('$location.onChange()', () => {
+
+  let $location: $locationShim;
+  let upgradeModule: UpgradeModule;
+
+  beforeEach(() => {
+    TestBed.configureTestingModule({
+      imports: [
+        CommonModule,
+        LocationUpgradeTestModule.config({useHash: false, startUrl: 'http://host.com/'}),
+      ],
+      providers: [UpgradeModule],
+    });
+
+    upgradeModule = TestBed.get(UpgradeModule);
+    upgradeModule.$injector = {get: injectorFactory()};
+  });
+
+  beforeEach(inject([$locationShim], (loc: $locationShim) => { $location = loc; }));
+
+  it('should have onChange method', () => { expect(typeof $location.onChange).toBe('function'); });
+
+  it('should add registered functions to changeListeners', () => {
+
+    function changeListener(url: string, state: unknown) { return undefined; }
+    function errorHandler(e: Error) {}
+
+    expect(($location as any).$$changeListeners.length).toBe(0);
+
+    $location.onChange(changeListener, errorHandler);
+
+    expect(($location as any).$$changeListeners.length).toBe(1);
+    expect(($location as any).$$changeListeners[0][0]).toEqual(changeListener);
+    expect(($location as any).$$changeListeners[0][1]).toEqual(errorHandler);
+  });
+
+  it('should call changeListeners when URL is updated', () => {
+
+    const onChangeVals =
+        {url: 'url', state: 'state' as unknown, oldUrl: 'oldUrl', oldState: 'oldState' as unknown};
+
+    function changeListener(url: string, state: unknown, oldUrl: string, oldState: unknown) {
+      onChangeVals.url = url;
+      onChangeVals.state = state;
+      onChangeVals.oldUrl = oldUrl;
+      onChangeVals.oldState = oldState;
+    }
+
+    $location.onChange(changeListener);
+
+    // Mock out setting browserUrl
+    ($location as any).browserUrl = (url: string, replace: boolean, state: unknown) => {};
+
+    const newState = {foo: 'bar'};
+    ($location as any).setBrowserUrlWithFallback('/newUrl', false, newState);
+    expect(onChangeVals.url).toBe('/newUrl');
+    expect(onChangeVals.state).toBe(newState);
+    expect(onChangeVals.oldUrl).toBe('/');
+    expect(onChangeVals.oldState).toBe(null);
+  });
+
+  it('should call forward errors to error handler', () => {
+
+    let error !: Error;
+
+    function changeListener(url: string, state: unknown, oldUrl: string, oldState: unknown) {
+      throw new Error('Handle error');
+    }
+    function errorHandler(e: Error) { error = e; }
+
+    $location.onChange(changeListener, errorHandler);
+
+    // Mock out setting browserUrl
+    ($location as any).browserUrl = (url: string, replace: boolean, state: unknown) => {};
+
+    ($location as any).setBrowserUrlWithFallback('/newUrl');
+    expect(error.message).toBe('Handle error');
+  });
+
+});
+
 function parseLinkAndReturn(location: $locationShim, toUrl: string, relHref?: string) {
   const resetUrl = location.$$parseLinkUrl(toUrl, relHref);
   return resetUrl && location.absUrl() || undefined;

diff --git a/tools/public_api_guard/common/upgrade.d.ts b/tools/public_api_guard/common/upgrade.d.ts
@@ -6,6 +6,7 @@ export declare class $locationShim {
     hash(hash: string | number | null): this;
     hash(): string;
     host(): string;
+    onChange(fn: (url: string, state: unknown, oldUrl: string, oldState: unknown) => void, err?: (e: Error) => void): void;
     path(): string;
     path(path: string | number | null): this;
     port(): number | null;