重複運算子

repeat 運算子允許從源 Observable 重複整個序列。

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

以上示例的輸出

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

這個序列重複無數次，永遠不會完成。

要重複序列有限次數，只需將整數作為引數傳遞給 repeat 運算子。

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")
    );

這個例子列印

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

瞭解 repeat 運算子在源 Observable 序列完成時重新訂閱源 Observable 非常重要。讓我們用 Observable.create 重寫上面的例子。

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")
        );

這個例子列印

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

使用運算子連結時，重要的是要知道 repeat 運算子重複整個序列而不是前面的運算子。

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")
        );

這個例子列印

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

這個例子表明 repeat 運算子重複整個序列重新訂閱 Observable 而不是重複最後的 map 運算子，並且 repeat 運算子序列中的哪個位置無關緊要。

這個序列

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

等於這個序列

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