rxjs里concatMap operators的用法

Projects each source value to an Observable which is merged in the output Observable, in a serialized fashion waiting for each one to complete before merging the next.

将source Observable里的每个元素施加一个projection函数，这个projection函数返回一个新的Observable，然后将所有这些Observable flatten成另一个Observable.

Returns an Observable that emits items based on applying a function that you supply to each item emitted by the source Observable, where that function returns an (so-called “inner”) Observable. Each new inner Observable is concatenated with the previous inner Observable.

例子：

const clicks = fromEvent(document, 'click');
    const ioe = interval(1000);
    const mapfn = take(4);
    const comp = ioe.pipe(mapfn);
    const fn = ev => comp;
    const result = clicks.pipe( concatMap(fn));
    result.subscribe(x => console.log('diablo: ' +x));

ioe是Observable：

mapfn是OperatorFunction：

comp是一个新的Observable：

fn函数接收任意类型的输入变量，返回一个新的Observable：

测试结果：每点击屏幕一次，顺序打印0～4：

concatMap语义：每点击一次，从interval Observable中拿出4个元素出来，生成4个新的Observable，然后连接在一起。