CODEWITHSUNDEEP
rx-java
aurelius
aurelius

Reputation: 4076

What does .defer() do in RxJava?

I am new to rx java and I cannot understand what Completable.defer brings and why it's a good practice to use it.

so, what's the difference between:

public Completable someMethod1() {
    return Completable.defer(() -> someMethod2());
}

vs

public Completable someMethod1() {
    return someMethod2();
}

I can see that in the method's implementation there is some exception handling but this is surpassing me. Appreciate it.

Upvotes: 10

Views: 11157

Answers (1)

akarnokd
akarnokd

Reputation: 70007

Defer will makes sure each subscriber can get its own source sequence, independent of the other subscribers. Let me illustrate it with two examples:

AtomicInteger index = new AtomicInteger();

Flowable<String> source =
    Flowable.just("a", "b", "c", "d", "e", "f")
    .map(v -> index.incrementAndGet() + "-" + v)
    ;

source.subscribe(System.out:println);

source.subscribe(System.out:println);

prints

1-a
2-b
3-c
4-d
5-e
6-f
7-a
8-b
9-c
10-d
11-e
12-f

versus

Flowable<String> source =
    Flowable.defer(() -> {
        AtomicInteger index = new AtomicInteger();

        return Flowable.just("a", "b", "c", "d", "e", "f")
               .map(v -> index.incrementAndGet() + "-" + v)
               ;
    })
    ;

source.subscribe(System.out:println);

source.subscribe(System.out:println);

prints

1-a
2-b
3-c
4-d
5-e
6-f
1-a
2-b
3-c
4-d
5-e
6-f

In the second example, there is a per subscriber state that would have been otherwise shared across all subscribers. Now since each subscriber gets its own individual sequence created, both index items as one would generally expect.

Upvotes: 34

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