How to create an Observable in Android?

Question

What I want to do is to create a simple in-memory cache just to try Observables out. However I got stuck because I don't understand how to create an observable. This is the code I have gotten so far:

public class MovieCache {
    MovieWrapper movieWrapper;

    public Observable<MovieWrapper> getMovies() {
       //How to create and return an Observable<MovieWrapper> here?
    }

    public void setCache(MovieWrapper wrapper) {
        movieWrapper = wrapper;
    }

    public void clearCache() {
        movieWrapper = null;
    }
}

In the getMovies() method I want to create an Observable and return my local field movieWrapper to the subscriber. How can I do this? I tried with using new Observable.just(movieWrapper) but it results in a null exception.

Answer 1

Take a look at this tutorial as it does exactly what you are looking for. Basically you use defer() to make sure you always get the latest version of your cached object:

public class MovieCache {
    MovieWrapper movieWrapper;

    public Observable<MovieWrapper> getMovies() {  
        return Observable.defer(new Func0<Observable<MovieWrapper>>() {
            @Override
            public Observable<MovieWrapper> call() {
                return Observable.just(movieWrapper);
            }
        });
    }

    public void setCache(MovieWrapper wrapper) {
        movieWrapper = wrapper;
    }

    public void clearCache() {
        movieWrapper = null;
    }
}

defer() makes sure that you will get the object upon subscription to the Observable not on creation.

Note however that, according to the author of the post:

The only downside to defer() is that it creates a new Observable each time you get a subscriber. create() can use the same function for each subscriber, so it's more efficient. As always, measure performance and optimize if necessary.