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源代码：
import { Observable, of, OperatorFunction } from "rxjs";
import { map } from "rxjs/operators";
import { interval } from "rxjs";
import { switchMap } from "rxjs/operators";
/* Projects each source value to an Observable which is merged in the output Observable, emitting values only from the most recently projected Observable.
switchMap将source value映射成一个新的Observable，这个Observable被output Observable自动merge.
只会从最新的被projected过后的Observable里emit数据
给consumer
*/
const add100 = (counter: number) => {
return of(counter + 100);
};
const addFunctionOperator: OperatorFunction<number, number> = switchMap(add100);
const pollTasks = () => {
return interval(1000).pipe(addFunctionOperator);
};
// caller can do subscription and store it as a handle:
let tasksSubscription = pollTasks().subscribe(data =>
console.log("timestamp: " + new Date() + ": " + data)
);
// turn it off at a later time
setTimeout(() => tasksSubscription.unsubscribe(), 3000);
由此可见，switchMap操作符接收一个映射函数，该函数的输入同map操作符一样，都是number，而map的输出类型也是number，而switchMap的输出，是Observable.
switchMap传入一个函数，输出一个OperatorFunction, 这称为函数的operator化。
OperatorFunction才是最后传入Observable.pipe调用里的参数。