重复运算符

repeat 运算符允许从源 Observable 重复整个序列。

Observable.just(1, 2, 3)
    .repeat()
    .subscribe(
        next -> System.out.println("next: " + next),
        error -> System.out.println("error: " + error),
        () -> System.out.println("complete")
    );

以上示例的输出

next: 1
next: 2
next: 3
next: 1
next: 2
next: 3

这个序列重复无数次，永远不会完成。

要重复序列有限次数，只需将整数作为参数传递给 repeat 运算符。

Observable.just(1, 2, 3)
    // Repeat three times and complete
    .repeat(3)
    .subscribe(
        next -> System.out.println("next: " + next),
        error -> System.out.println("error: " + error),
        () -> System.out.println("complete")
    );

这个例子打印

next: 1
next: 2
next: 3
next: 1
next: 2
next: 3
next: 1
next: 2
next: 3
complete

了解 repeat 运算符在源 Observable 序列完成时重新订阅源 Observable 非常重要。让我们用 Observable.create 重写上面的例子。

Observable.<Integer>create(subscriber -> {

    //Same as Observable.just(1, 2, 3) but with output message
    System.out.println("Subscribed");
    subscriber.onNext(1);
    subscriber.onNext(2);
    subscriber.onNext(3);
    subscriber.onCompleted();
})
        .repeat(3)
        .subscribe(
                next -> System.out.println("next: " + next),
                error -> System.out.println("error: " + error),
                () -> System.out.println("complete")
        );

这个例子打印

Subscribed
next: 1
next: 2
next: 3
Subscribed
next: 1
next: 2
next: 3
Subscribed
next: 1
next: 2
next: 3
complete

使用运算符链接时，重要的是要知道 repeat 运算符重复整个序列而不是前面的运算符。

Observable.<Integer>create(subscriber -> {
    System.out.println("Subscribed");
    subscriber.onNext(1);
    subscriber.onNext(2);
    subscriber.onNext(3);
    subscriber.onCompleted();
})
        .map(value -> value * 2) //First chain operator
        .map(value -> "modified " + value) //Second chain operator
        .repeat(3)
        .subscribe(
                next -> System.out.println("next: " + next),
                error -> System.out.println("error: " + error),
                () -> System.out.println("complete")
        );

这个例子打印

Subscribed
next: modified 2
next: modified 4
next: modified 6
Subscribed
next: modified 2
next: modified 4
next: modified 6
Subscribed
next: modified 2
next: modified 4
next: modified 6
complete

这个例子表明 repeat 运算符重复整个序列重新订阅 Observable 而不是重复最后的 map 运算符，并且 repeat 运算符序列中的哪个位置无关紧要。

这个序列

Observable.<Integer>create(subscriber -> {
        //...
    })
    .map(value -> value * 2) //First chain operator
    .map(value -> "modified " + value) //Second chain operator
    .repeat(3)
    .subscribe(
        /*....*/
    );

等于这个序列

Observable.<Integer>create(subscriber -> {
        //...
    })
    .repeat(3)
    .map(value -> value * 2) //First chain operator
    .map(value -> "modified " + value) //Second chain operator
    .subscribe(
        /*....*/
    );