等待以前的Rx Observable完成

Question

Hi I'm using RxJava for my disk storage get and set operations. Basically I have a method like this:

public Observable<String> getStorageItem(String id, String type) {
    return Observable.defer(new Func0<Observable<String>>() {
        // Run db operations to get storage item.
    }
}

The problem is that it's possible this method getStorageItem(...) gets subscribed to multiple times in a row. And the DB operations within the observable cannot run concurrently. What's my best option here? Should I manually create some sort've queue? Or does RxJava have some kind of tool that allows me to block the operation until a previous one is complete?

Answer 1

You can use a subscribeOn with a single-threaded scheduler created from an ExecutorService to make sure there's only one DB operation in progress:

ExecutorService exec = Schedulers.newSingleThreadExecutor();
Scheduler s = Schedulers.from(exec);

public Observable<String> getStorageItem(String id, String type) {
    return Observable.fromCallable(() -> {
        // Do DB operations 
    });
}

getStorageItem("1", "2").subscribeOn(s).subscribe(...);
getStorageItem("2", "4").subscribeOn(s).subscribe(...);
getStorageItem("3", "6").subscribeOn(s).subscribe(...);

But note that by moving the computation off the caller's thread, it may execute any time. If you need to wait for it individually (because the getStorageItem is already called on some thread), you can apply toBlocking() after subscribeOn .