RxJava network requests and caching

Question

I am seeking an example of a flow I'm trying to implement with help of RxJava.

Suppose I want to show a list of data. The flow should look something like this:

1. Read cache. If it contains the data, show it;

2. Send an API request to the server:

   If it returned the data, then cache it and show it.

   If it returned and error and there was no cached data, then show an error.

   If it returned and error and there was something cached, then do nothing.

Right now I have a method that does something similar (with lots of inspiration from Jake's u2020). The main difference is that it uses in-memory caching, which means there's no need for a separate Observable for reading from cache and it can be done synchronously.

I don't know how to combine two observables (one for reading from cache and the other for API call) and obtain the flow described above.

Any suggestions?

Answer 1

I think I solved my problem. The observable chain looks like so:

apiCall()
        .map(data -> dataInMemory = data)
        .onErrorResumeNext(t -> data == null ?
                Observable.just(Data.empty()) : Observable.empty())
        .startWith(readDataFromCache().map(data -> dataInMemory = data))
        .subscribeOn(ioScheduler)
        .observeOn(uiScheduler)
        .subscribe(dataRequest);

The main point is, that if readDataFromCache() throws an error, it will call onCompleted() without calling onError(). So it should be a custom Observable which you can control.

Data.empty() is a stub for my data - the Subscriber should treat it as an error.

dataInMemory is a member in my controller which acts as in-memory cache.

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EDIT: the solution doesn't work properly. The completion of one use case (see comment) is not achieved.

EDIT 2: well, the solution does work properly after some tweaking. The fix was returning different types of observables depending on the state of in-memory cache. Kind of dirty.