How to use RxJava / RxAndroid in Android to make slow network call and update the UI? - rx-android

Question

I have started reading about RxJava / RxAndroid, but I can't find simple tutorial that covers typical thing, like getting network data and updating UI with the result.
Many tutorials cover scenario like running one or more background tasks, that take a parameter and return nothing.
Lets say I have a slow function that may return data or throw an Exception, like this:
private String getNetworkData(Integer parameter) throws Exception {
Thread.sleep(1000); // simulated delay
switch (parameter) {
case 0: return "Bill";
case 1: return "Joe";
case 2: return "Bob";
case 3: return "Alex";
case 4: return "Mary";
default: throw new Exception("No such user");
}
}
So far, I have written something like this: in my MainActivity I have a button with onClick set like this:
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
Observable<Integer> observable = Observable
.just(0, 1, 3, 4, 8) // these are call parameters, right?
.subscribeOn(Schedulers.io()) // this is where I do slow work, right?
.observeOn(AndroidSchedulers.mainThread()); // this is where I get results, right?
observable.subscribe(new Observer<Integer>() {
#Override
public void onSubscribe(#NonNull Disposable d) {
Log.d(TAG,"onSubscribe on " + Thread.currentThread().getName());
}
#Override
public void onNext(#NonNull Integer i) {
// what to do with returned?
// how do I catch errors?
String returnedData = getNetworkData(i);
Log.d(TAG,"onNext on " + Thread.currentThread().getName());
}
#Override
public void onError(#NonNull Throwable e) {
// how and where to throw errors that can be processed here?
Log.d(TAG,"onSubscribe on " + Thread.currentThread().getName());
}
#Override
public void onComplete() {
Log.d(TAG,"onComplete on " + Thread.currentThread().getName());
}
});
}
});
The question is: how can I update UI and receive returned data?
I have tried to understand something from this:
How to return value in rxJava
but it does not explain anything to me, I have no idea what type is youtubeApi (is it Observable or what?).
After some discussion in comments under another question I changed my button handler to this:
Callable callable = new Callable<String>() {
#Override
public String call() throws Exception {
Log.d(TAG, "callable called on " + Thread.currentThread().getName());
Thread.sleep(1000); // simulated delay
throw new Exception("Exception!");
// return "Bill";
// this is not what I want, because I can't get any parameter from here
}
};
SingleObserver<String> observer = new SingleObserver<String>() {
#Override
public void onSubscribe(#NonNull Disposable d) {
Log.d(TAG, "onSubscribe called on " + Thread.currentThread().getName());
}
#Override
public void onSuccess(#NonNull String s) {
Log.d(TAG, "onSuccess called on " + Thread.currentThread().getName());
}
#Override
public void onError(#NonNull Throwable e) {
Log.d(TAG, "onError called on " + Thread.currentThread().getName());
}
};
Single.fromCallable(callable)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(observer);
But now I can't pass a parameter to my slow function.

Answer 1

OK. I got this.
So we need an observer, which will update our view.
Observer<String> observer = new Observer<String>() {
#Override
public void onSubscribe(#NonNull Disposable d) {
Log.d(TAG, "onSubscribe called on thread " + Thread.currentThread().getName());
}
#Override
public void onNext(#NonNull String s) {
Log.d(TAG, "onNext called on thread " + Thread.currentThread().getName() + " returned data " + s);
}
#Override
public void onError(#NonNull Throwable e) {
Log.d(TAG, "onError called on thread " + Thread.currentThread().getName() + " with message " + e.getMessage());
}
#Override
public void onComplete() {
Log.d(TAG, "onComplete called on thread " + Thread.currentThread().getName());
}
};
I need a function, that takes Integer as parameter (this will be my database Id) and returns String (this will be data returned from database).
Function<Integer, String> getNameByIdFunction = new Function<Integer, String>() {
#Override
public String apply(Integer integer) throws Throwable {
Thread.sleep(1000); // simulated delay
switch (integer) {
case 0:
return "Bill";
case 1:
return "Joe";
case 2:
return "Bob";
case 3:
return "Alex";
case 4:
return "Mary";
default:
throw new Exception("No such user");
}
}
};
And finally we need to connect everything together with Observable.
Observable
.just(1,3,5,0,4) // I want 5 function calls with diffrent parameters
.map(getNameByIdFunction) // this is my function
.subscribeOn(Schedulers.io()) // this is thread for a function
.observeOn(AndroidSchedulers.mainThread()) // this is thread that will update my UI
.subscribe(observer); // everything will start after we subscribe