在 window-ref.ts 的实现里，定义了一个每隔 300 毫秒，通过 fromEvent 发射一个 resize event 的Observable：

/**
   * Returns an observable for the window resize event and emits an event
   * every 300ms in case of resizing. An event is simulated initially.
   *
   * If there's no window object available (i.e. in SSR), a null value is emitted.
   */
  get resize$(): Observable<any> {
    if (!this.nativeWindow) {
      return of(null);
    } else {
      return fromEvent(this.nativeWindow, 'resize').pipe(
        debounceTime(300),
        startWith({ target: this.nativeWindow }),
        distinctUntilChanged()
      );
    }
  }

加上 distingctUntilChanged 操作符后，能过滤掉完全一致的 resize event. 下面的例子，展示了如何使用 distingctUntilChanged，将数字序列里重复的数字过滤掉。distingctUntilChanged 默认会将当前元素和前一元素做比较。

import { of } from 'rxjs';
import { distinctUntilChanged } from 'rxjs/operators';

of(1, 1, 2, 2, 2, 1, 1, 2, 3, 3, 4).pipe(
    distinctUntilChanged(),
  )
  .subscribe(x => console.log(x)); // 1, 2, 1, 2, 3, 4

下列例子展示了如何将自定义的比较逻辑，通过箭头函数作为参数，传入 distinctUntilChanged 里。

import { of } from 'rxjs';
import { distinctUntilChanged } from 'rxjs/operators';

interface Person {
   age: number,
   name: string
}

of<Person>(
    { age: 4, name: 'Foo'},
    { age: 7, name: 'Bar'},
    { age: 5, name: 'Foo'},
    { age: 6, name: 'Foo'},
  ).pipe(
    distinctUntilChanged((p: Person, q: Person) => p.name === q.name),
  )
  .subscribe(x => console.log(x));

// displays:
// { age: 4, name: 'Foo' }
// { age: 7, name: 'Bar' }
// { age: 5, name: 'Foo' }