I have some confusion on subscribers and when they react to observers. Lets say i have the following simple observer with a subscriber that does an action:
Observable.just(preferences.getBoolean(C"vibrate", false))
.subscribeOn(Schedulers.io())//observe on new thread
.observeOn(AndroidSchedulers.mainThread()) //subscribe(listen) on main thread
.subscribe(new Action1<Boolean>() {
#Override
public void call(Boolean shouldVibrate) {
if (shouldVibrate)
Toast.makeText(context,"i should vibrate now",Toast.SHORT).show();
}
});
I realize the observer gets called right away when this code is first seen. But what if the shared preference is changed again afterwards, will this code run again automatically or does it only run everytime i call subscribe ? What if i wanted it to run everytime the shared preference was altered (sort of like a watcher).
It really depends on the observable. I would suggest reading "Hot" and "Cold" Observables on the reactive Observable docs.
In your case, this is a Cold observable. It will resubscribe each time it is subscribed to. However, you only subscribe to it once. Your code snippet will actually block on the preferences fetch (probably not a huge problem), but it will only emit one preference.
I would suggest using the ContentObservable class in the RxAndroid extension lib for RxJava, which you are already using (because of AndroidSchedulers).
It would look something like this (This is back-of-napkin code, I have not compiled or ran this):
// Defer the observable so it gets a fresh preference value. Also, we'll
// be using it a few times.
final Observable<Boolean> vibratePreference = Observable.defer(
new Func0<Observable<Boolean>>() {
#Override
public Observable<Boolean> call() {
return Observable.just(preferences.getBoolean("vibrate", false));
}
});
vibratePreference
.concatWith(ContentObservable.fromSharedPreferencesChanges(preferences)
// Only consider changes to the vibrate preference.
.filter(new Func1<String, Boolean>() {
#Override
public Boolean call(final String key) {
return "vibrate".equals(key);
}
})
// Each time the preference changes, get the latest value.
.flatMap(new Func1<String, Observable<Boolean>>() {
#Override
public Observable<Boolean>(final String unusedKey) {
return vibratePreference;
}
}))
.scheduleOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe( /* ...and so on. */ );
Also, if you are doing this on an activity or a fragment, I would strongly suggest looking into bindActivity and bindFragment in AppObservable in RxAndroid to make sure you are binding this observable to the lifecycle. You also may want to store a CompositeSubscription that you can empty in onPause and restore subscriptions in onResume. Those are slightly off-topic but will most likely be useful very soon.
Related
I'm learning RxJava so please be gentle. I've watched the tutorials, done the reading, searched SO, however, I'm still having some problems transforming my AsyncTaskLoader. For some reason, I can't find a pattern of operators to achieve my task (although I think it's a common one). What I'm trying to do is the following: return an Observable my fragment could subscribe to. The observable should do the following on subscribe:
1) Fetch data from the local database by doing 2 queries, running some logic and returning results;
2) Fetching data from API;
3) Synchronising the new API data with the database;
4) Repeating step one and returning results;
So far I've transformed my db calls and my API calls to return observables. I'm trying to understand how I can emit the cold results and continue with the chain. I could probably keep the two operations separately, and use the same subscriber to subscribe to both? But I'm not sure how that would work if my new loader-replacement class returns an observable... Also I don't really need to process the results from the second observable - I just need for the first one to replay when the second one finished.
So far I have the following:
public Observable<StuffFetchResult> getColdStuff() {
return Observable.zip(mDataSource.listStuff(), mDataSource.listOtherStuff(),
(stuff, moreStuff) -> {
List<Stuff> mergedList = new ArrayList<>();
// do some merging stuff
return new StuffFetchResult(mergedList);
}).subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}
Assume I also have getHotStuff() that will do the API call and the synchronisation with the database, if that's the right approach, and return the same Observable. However, I'm stuck on the next step - how can I restart the first observable to replay once hotStuff has completed, without adding another subscriber?
EDIT:
I've made some progress and I think all I need now is to join it all up. I have my two methods:
1) getColdStuff() is pretty much as described above
2) getHotStuff() will do call to the API, synchronise with the database, and return an Observable. The idea was to call getColdStuff() again after getHotStuff() has finished in order to refresh the UI, so actual result returned from getHotStuff() can be ignored. All it needs to do is to trigger getColdStuff() once done.
I've tried the suggestion in the answer to and created the following:
BehaviorRelay<Observable<StuffFetchResult>> callSequence = BehaviorRelay.create();
Observable<StuffFetchResult> valueSequence = Observable.switchOnNextDelayError(callSequence.toSerialized());
valueSequence.subscribe(new Subscriber<StuffFetchResult>() {
#Override
public void onCompleted() {}
#Override
public void onError(Throwable e) {}
#Override
public void onNext(StuffFetchResult result) {
// UI stuff
}
});
callSequence.call(loader.getColdStuff());
I can subscribe to valueSequence here and use callSequence.call(loader.getColdStuff());, which will run the first method and produce results in onNext() of my subscription, which I can use for my UI. However, I'm not sure how to run getHotStuff() in parallel and also do a different action on it when it returns. Also getHotStuff() returns a different type of Observable so I can't really use the same callSequence?
EDIT 2
Using two subscribers, I can achieve the required behaviour I think. Not really sure if that's the right way to go about it though.
loader.getHotStuff()
.subscribeOn(Schedulers.io())
.subscribe( new Subscriber<Object>() {
#Override
public void onCompleted() {}
#Override
public void onError(Throwable e) {}
#Override
public void onNext(Object stuffWeDontCareAbout) {
callSequence.call(loader.getColdStuff());
}
});
if i understand your scenario correctly, you may want something like that -
BehaviorSubject<Observable<T> callSequence = BehaviorSubject.create();
Observable<T> valueSequence = Observable.swithOnNextDelayError(callSequence.toSerialized());
your subscriber will be listening to the valueSequence, and whenever you need to "restart", you will call this -
callSequence.onNext(call.cache()); // *call* is Observable<T>
(i leave the .subscribeOn/.observeOn configuration to you)
Keeping it simple and short, how to subscribe to an observable in another subscriber's onNext() method so that we can only make nested subscription when the current observable completes its execution. Both the subscriptions will be made on separate threads and the requirement is that the first thread must complete its execution before the second thread is started.
makeObservable()
.subscribeOn(Schedulers.newThread())
.subscribe(new Subscriber<User> {
#override
void onNext(User user){
//do something
//make another subscription here
});
Don't make a new subscription, return another Observable and subscribe to it.
apiCall()
.subscribeOn(<scheduler>)
.observeOn(<scheduler>)
.flatMap(new Func1<User, Observable<Something>() {
#Override
public Observable<Something> call(User user) {
return Observable.just(<example>);
}
});
edit: when the api call returns, flatMap will intercept the stream, and from there, either return an Observable or call a function that returns an Observable (ie another api call).
If they're both emitting the same items, use Observable.concat(), which subscribes to observable N+1 after observable N completes. But it might be worth it to describe your use case in more detail.
Edit: You should be able to do something like:
userClient
.saveUser(user)
.flatMap(userSaveResult ->
userClient
.saveUserDetails(userSaveResult.id, seuser.getDetails))
.onError(...)
I have an Observable and subscribe to it. I need to not miss any emitted result, so I use onBackpressureBuffer like following:
Observable<Data> observable = observable.onBackpressureBuffer();
if (BuildConfig.DEBUG)
{
observable
.subscribeOn(HandlerScheduler.from(dataManager.getBackgroundHandler()))
.observeOn(HandlerScheduler.from(dataManager.getBackgroundHandler()))
.subscribe(new MeasuringSubscriber(...));
}
// Here is the real observer that I need in my app
observable
.subscribeOn(HandlerScheduler.from(dataManager.getBackgroundHandler()))
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Observer<Data>()
{
#Override
public void onCompleted() {
}
#Override
public void onError(Throwable e) {
}
#Override
public void onNext(Data data) {
}
});
The MeasuringSubscriber is a custom subscriber that just logs how long a task needs, that's all.
Problem
If I add the MeasuringSubscriber, the subscribers do not work anymore and never emit a result. Why? And how can I make that working?
EDIT - NEW PROBLEM
Currently it's working, but the MeasuringSubscriber is somehow blocking, meaning, first all items are emitted one by one to the MeasuringSubscriber and only afterwards all items are emitted one by one to the main subscriber... Any ideas what could cause that?
I have a solution for that - I can extend my main observalbe from the MeasuringObservable - but I rather would like to know why this happens and how to avoid this...
I tried using publish + connect, but still it does emit all items to the first subscriber before emitting them to the second one...
Rx way of doing things can be very complex for none and for many reasons...
but I feel there ARE simple ways to do simple things with RX...
How would I simply perform this statement on a background thread and receive the response on the ui thread?
All functions of this object need to run on a background thread. Get, put, clear, and delete.
String city = Paper.get("city");
The base object in Rx is Observable. That object usually wraps an OnSubscribe object, which is simply an extension of Action1 that takes a Subscriber as a parameter.
What all that means is that you just need to define a class that wraps your call and passes the result to the Subscriber:
public class RxPaperGet implements Observable.OnSubscribe<String> {
#Override
public void call(Subscriber<? super String> t1) {
try {
t1.onNext(Paper.get("city"));
} catch (Throwable t) {
t1.onError(t);
return;
}
t1.onCompleted();
}
}
That's a basic example. Now, you would want to wrap that so you can call any function, and not just Paper.get("city"). Something like https://github.com/ReactiveX/RxJavaAsyncUtil/blob/0.x/src/main/java/rx/util/async/operators/OperatorFromFunctionals.java#L44 does that, by allowing you to pass an arbitrary Callable.
Which in your case, would implement as:
Observable<String> res = OperatorFromFunctionals.fromCallable(() -> Paper.get("city"));
(In case you're wondering, this is java8 lambdas brought to android by retrolambda. quite nice to remove the verbosity of Rx)
Once you have your observable, you can subscribe on it, and get results. To execute on the background, and retrieve the results on the ui thread, you would do:
res.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
AndroidSchedulers is provided by rx-android.
Then you can simply be calledback with the result:
.subscribe(city -> Log.d(TAG, city));
That returns a subscription, which is useful if you need to cancel it.
Overall:
OperatorFromFunctionals.fromCallable(() -> Paper.get("city"))
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(city -> Log.d(TAG, city));
EDIT: This is not correct. Will not delete the answer though to preserve the comments.
Very simple example:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getPaper()
.subscribeOn(Schedulers.newThread())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Action1<String>() {
#Override
public void call(String s) {
Log.d("xxx", s);
}
});
}
private Observable<String> getPaper() {
return Observable.just(Paper.get());
}
where Paper.get() is a long running operation that returns a String. Check the docs for Scheduler.
Don't forget to observe on the main thread if you want to change the UI after receiving the result of your operation, else you will get an exception for changing the UI from outside the UI thread.
I am using RxJava to move network access to a separate thread in Android, but my UI still blocks.
I am not using the wrong observable as shown here: Android RxJava, Non Blocking?
The codepoints [A], [B] and [C] in below code are passed in the order [A] -> [C] -> [B] so the current thread is processed fine and RxJava calls [C] once it had a result. This is fine.
Also, blocking is much better compared to doing the network call on the UI thread, but I still have minor blocking. The UI stays fluent after the call is made, but if the server does not respond in a matter of milliseconds, it blocks.
private search; // search is an instance variable in the same class
// [A]
Observable.just(search.find("something")) // search.find calls the REST endpoint
.subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread())
.subscribe(new Action1<Search>() {
#Override public void call(Search search) {
// further processing // [B]
}
}, new Action1<Throwable>() {
#Override public void call(Throwable throwable) {
// error handler
}
});
// [C]
Could it be a problem that search is an instance variable in the same class where the Observable uses it, but the endpoint call is performed from a separate library? It shouldn't matter, right?
Am I doing anything bad that I shouldn't be doing?
--
Find looks like this (removed exception handling for brevity):
public Search find(String searchtext) {
setSearchtext(searchtext);
SearchEndpoint.find(Session.getUser().getId(), searchtext);
return this;
}
SearchEndpoint like this:
public static Search find(final Long userId, final String searchtext) throws IOException {
return ApiService.api().searches().find(userId).setFind(searchtext).execute();
}
and makes a call to the generated Google cloud endpoint library.
Try this:
Observable.create(new Observable.OnSubscribe<Search>() {
#Override
// method signature is from memory - I hope I am correct...
public void call(Subscriber<? super Search> subscriber) {
try {
Search search = search.find("something");
subscriber.onNext(search);
subscriber.onCompleted();
} catch (SomeException e) {
subscriber.onError(e);
}
}
})
// and then continue with your .subscribeOn(...)
To clarify, maybe this makes the problem with your code more obvious:
Observable.just(search.find("something"))
is clearly equivalent to
Search search = search.find("something");
Observable.just(search)
And this makes it obvious that search.find is executed before we ever hand the control over to rxjava and it is executed on whatever thread you are currently on - then the construction of an Observable from the pre-computed value and the delivery of the value happen on another thread but that does not help you much...
I know this is a few months old-- but instead of createing an entirely new Observable (which is relatively error-prone), you can use the map operator to run the search:
String search_input = "something"; // this is where you can specify other search terms
Observable.just(search_input)
.map(s -> search.find(s)) // search.find calls the REST endpoint
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe( // your subscriber goes here
If not using lambdas, that map function should look like:
.map(new Func1<String, Search>() {
#Override
public Search call(String s) {
return search.find(s)
}
})