fix(breakpoints): emit only one event for adjacent breakpoint changes.

src/cdk/layout/breakpoints-observer.spec.ts

@@ -8,14 +8,14 @@
 import {LayoutModule} from './layout-module';
 import {BreakpointObserver, BreakpointState} from './breakpoints-observer';
 import {MediaMatcher} from './media-matcher';
-import {async, TestBed, inject} from '@angular/core/testing';
+import {fakeAsync, TestBed, inject, flush} from '@angular/core/testing';
 import {Injectable} from '@angular/core';
 
 describe('BreakpointObserver', () => {
   let breakpointManager: BreakpointObserver;
   let mediaMatcher: FakeMediaMatcher;
 
-  beforeEach(async(() => {
+  beforeEach(fakeAsync(() => {
     TestBed.configureTestingModule({
       imports: [LayoutModule],
       providers: [{provide: MediaMatcher, useClass: FakeMediaMatcher}]
@@ -33,12 +33,12 @@ describe('BreakpointObserver', () => {
     mediaMatcher.clear();
   });
 
-  it('retrieves the whether a query is currently matched', () => {
+  it('retrieves the whether a query is currently matched', fakeAsync(() => {
     let query = 'everything starts as true in the FakeMediaMatcher';
     expect(breakpointManager.isMatched(query)).toBeTruthy();
-  });
+  }));
 
-  it('reuses the same MediaQueryList for matching queries', () => {
+  it('reuses the same MediaQueryList for matching queries', fakeAsync(() => {
     expect(mediaMatcher.queryCount).toBe(0);
     breakpointManager.observe('query1');
     expect(mediaMatcher.queryCount).toBe(1);
@@ -48,62 +48,68 @@ describe('BreakpointObserver', () => {
     expect(mediaMatcher.queryCount).toBe(2);
     breakpointManager.observe('query1');
     expect(mediaMatcher.queryCount).toBe(2);
-  });
+  }));
 
-  it('splits combined query strings into individual matchMedia listeners', () => {
+  it('splits combined query strings into individual matchMedia listeners', fakeAsync(() => {
     expect(mediaMatcher.queryCount).toBe(0);
     breakpointManager.observe('query1, query2');
     expect(mediaMatcher.queryCount).toBe(2);
     breakpointManager.observe('query1');
     expect(mediaMatcher.queryCount).toBe(2);
     breakpointManager.observe('query2, query3');
     expect(mediaMatcher.queryCount).toBe(3);
-  });
+  }));
 
-  it('accepts an array of queries', () => {
+  it('accepts an array of queries', fakeAsync(() => {
     let queries = ['1 query', '2 query', 'red query', 'blue query'];
     breakpointManager.observe(queries);
     expect(mediaMatcher.queryCount).toBe(queries.length);
-  });
+  }));
 
-  it('completes all events when the breakpoint manager is destroyed', () => {
+  it('completes all events when the breakpoint manager is destroyed', fakeAsync(() => {
     let firstTest = jasmine.createSpy('test1');
     breakpointManager.observe('test1').subscribe(undefined, undefined, firstTest);
     let secondTest = jasmine.createSpy('test2');
     breakpointManager.observe('test2').subscribe(undefined, undefined, secondTest);
 
+    flush();
     expect(firstTest).not.toHaveBeenCalled();
     expect(secondTest).not.toHaveBeenCalled();
 
     breakpointManager.ngOnDestroy();
+    flush();
 
     expect(firstTest).toHaveBeenCalled();
     expect(secondTest).toHaveBeenCalled();
-  });
+  }));
 
-  it('emits an event on the observable when values change', () => {
+  it('emits an event on the observable when values change', fakeAsync(() => {
     let query = '(width: 999px)';
     let queryMatchState: boolean = false;
     breakpointManager.observe(query).subscribe((state: BreakpointState) => {
       queryMatchState = state.matches;
     });
 
+    flush();
     expect(queryMatchState).toBeTruthy();
     mediaMatcher.setMatchesQuery(query, false);
+    flush();
     expect(queryMatchState).toBeFalsy();
-  });
+  }));
 
-  it('emits a true matches state when the query is matched', () => {
+  it('emits a true matches state when the query is matched', fakeAsync(() => {
     let query = '(width: 999px)';
+    breakpointManager.observe(query).subscribe();
     mediaMatcher.setMatchesQuery(query, true);
     expect(breakpointManager.isMatched(query)).toBeTruthy();
-  });
+  }));
 
-  it('emits a false matches state when the query is not matched', () => {
+  it('emits a false matches state when the query is not matched', fakeAsync(() => {
     let query = '(width: 999px)';
+    breakpointManager.observe(query).subscribe();
     mediaMatcher.setMatchesQuery(query, false);
-    expect(breakpointManager.isMatched(query)).toBeTruthy();
-  });
+    expect(breakpointManager.isMatched(query)).toBeFalsy();
+  }));
 });
 
 export class FakeMediaQueryList implements MediaQueryList {
@@ -153,6 +159,8 @@ export class FakeMediaMatcher {
   setMatchesQuery(query: string, matches: boolean) {
     if (this.queries.has(query)) {
       this.queries.get(query)!.setMatches(matches);
+    } else {
+      throw Error('This query is not being observed.');
     }
   }
 }

src/cdk/layout/breakpoints-observer.ts

@@ -7,8 +7,8 @@
  */
 import {Injectable, NgZone, OnDestroy} from '@angular/core';
 import {MediaMatcher} from './media-matcher';
-import {combineLatest, fromEventPattern, Observable, Subject} from 'rxjs';
-import {map, startWith, takeUntil} from 'rxjs/operators';
+import {asapScheduler, combineLatest, fromEventPattern, Observable, Subject} from 'rxjs';
+import {debounceTime, map, startWith, takeUntil} from 'rxjs/operators';
 import {coerceArray} from '@angular/cdk/coercion';
 
 
@@ -59,11 +59,13 @@ export class BreakpointObserver implements OnDestroy {
     const queries = splitQueries(coerceArray(value));
     const observables = queries.map(query => this._registerQuery(query).observable);
 
-    return combineLatest(observables).pipe(map((breakpointStates: BreakpointState[]) => {
-      return {
-        matches: breakpointStates.some(state => state && state.matches)
-      };
-    }));
+    return combineLatest(observables).pipe(
+      debounceTime(0, asapScheduler),
+      map((breakpointStates: BreakpointState[]) => {
+        return {
+          matches: breakpointStates.some(state => state && state.matches)
+        };
+      }));
   }
 
   /** Registers a specific query to be listened for. */

src/lib/tooltip/tooltip.html

@@ -1,6 +1,6 @@
 <div class="mat-tooltip"
      [ngClass]="tooltipClass"
-     [class.mat-tooltip-handset]="(_isHandset | async)!.matches"
+     [class.mat-tooltip-handset]="(_isHandset | async)?.matches"
      [@state]="_visibility"
      (@state.start)="_animationStart()"
      (@state.done)="_animationDone($event)">{{message}}</div>