I'm trying to apply RX to Android. I want when a button is clicked, to download something from web and display it.
My problem is that HttpClient.connect() executes on the mainThread instead of a background one.
The call to HttpClient.connect() executes as a function passed to Observable.map()
Observable<Integer> dayDeltas = Obs.obsToSequence(Obs.Observable(textView)); //transforms click events to observable
Observable<String> dates = dayDeltas.map(...).map(...)
dates.map(Obs.dateToWebPage()) // calls http.connect()
.map(Obs.parseEvents())
.observeOn(AndroidSchedulers.mainThread())
.subscribeOn(Schedulers.newThread())
.subscribe(updateTextView(textView));
public static Observable<Object> Observable(final TextView text) {
return Observable.create(new Observable.OnSubscribe<Object>() {
#Override
public void call(final Subscriber<? super Object> subscriber) {
final Object event = new Object();
text.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Log.e("click", "click");
subscriber.onNext(event);
}
});
}
});
Now my naive interpretation is that since i have .subscribeOn(Schedulers.newThread()) every function/operator on the observable should execute in a new thread, including .map(f). Clearly this is not what is happening, so what part of this chain does execute on new threads?
subscribeOn is there to trigger subscription side-effects. In your setup, it will register the callback to capture the button press on the new thread but when the press happens, the onNext emission is triggered by the main thread. The chain, including the network connect then executes on the main thread.
You have to put a new observeOn(Schedulers.io()) before the connecting method to make sure the reception of the button press event happens off the main thread.
Edit:
//transforms click events to observable
Observable<Integer> dayDeltas = Obs.obsToSequence(Obs.Observable(textView));
Observable<String> dates = dayDeltas.map(...).map(...)
dates
.observeOn(Schedulers.io()) // <------------------------------------------ add
.map(Obs.dateToWebPage()) // calls http.connect()
.map(Obs.parseEvents())
.observeOn(AndroidSchedulers.mainThread())
//.subscribeOn(Schedulers.newThread()) // <------------------------------- remove
.subscribe(updateTextView(textView));
After carefully reading the Scheduling and Threading RX doc, i have the solution:
dates
.observeOn(Schedulers.io())
.subscribeOn(AndroidSchedulers.mainThread())
.map(Obs.dateToWebPage())
In my original code, the schedulers passed to observeOn/subscribeOn were reversed and in the wrong place.
Related
I have an async method makeRequest() with callback. It called many times from different classes of my application. I need that this calls start one by one and never simultaneously.
I want to implement this using Rx. Like this:
public void execute() { // This method called many times from another classes
Observable.just(true)
// what I need to add here?
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.map(o -> {
internalExecute();
return o;
})
.subscribe();
}
private void internalExecute() { // This method should called only when previous call was finished
makeRequest(this::onRequestFinished);
}
private void onRequestFinished() {
// here is I handle request finish
}
But at now all requests works at parallel. What I need to add here to run requests one by one?
According to comments, you have here separated streams and requests. each client that execute request expect a result from the request. but no requests allowed to run in parallel, in this case I think the easiest way is to limit the Scheduler to an application global background sequential thread Executor, i.e:
Schedulers.from(Executors.newSingleThreadExecutor())
provide somewhere in your app this single thread Executor, in singleton manner of course, it's important that each request stream will use the same object:
private final Scheduler singleThreadScheduler = Schedulers.from(Executors.newSingleThreadExecutor());
public void execute() { // This method called many times from another classes
Observable.just(true)
.map(o -> {
internalExecute();
return o;
})
.subscribeOn(singleThreadScheduler)
.subscribe();
}
private void internalExecute() { // This method should called only when previous call was finished
makeRequest(this::onRequestFinished);
}
private void onRequestFinished() {
//NOTE: you should make sure that the callback execute where you need it (main thread?)
// here is I handle request finish
}
besides that, you're not exposing Observable outside, to the clients, but rather using callback mechanism, you can leverage reactive approach further, by making execute() returning Observable. (and enjoy composition of Obesrvables, operators, proper use of observeOn/subscribeOn, error handling with onError, disposing/unsubscribing etc.), as you're using async api, you can use fromEmitter()/create() (in newer RxJava1 version)), read more here:
private final Scheduler singleThreadScheduler = Schedulers.from(Executors.newSingleThreadExecutor());
public Observable<Result> execute() { // This method called many times from another classes
return Observable.fromEmitter(new Action1<Emitter<? extends Object>>() {
#Override
public void call(Emitter<?> emitter) {
emitter.setCancellation(() -> {
//cancel request on unsubscribing
});
makeRequest(result -> {
emitter.onNext(result);
});
}
})
.subscribeOn(singleThreadScheduler)
}
The Problem
I have an activity which fetches data from an API periodically and displays the data received. The API uses OAuth so I receive a temporary access token which expires after a certain period of time (1 hr). If the app tries to get data with an expired token, obviously the request will fail. In an earlier iteration of my app, I was using AsyncTasks for the network requests and essentially just executed a new AsyncTask that would get a new access token before calling the main AsyncTask that fetches the data from the server. This worked great because the main AsyncTask would wait until the other one was finished before executing.
I recently switched to RxJava and basically just replaced the AsyncTasks with Observables. The problem is that the main Observable that fetches the data doesn't wait for the Observable that refreshes the access token to finish. Here's my code, thanks for your help.
Code
LiveThreadActivity.java
private Subscription subscription;
private Observable<List<CustomComment>> fetchData;
#Override
protected void onResume() {
super.onResume();
if (tokenExpired()) {
auth.refreshToken();
}
subscription = fetchData
.compose(bindToLifecycle())
.retryWhen(new RetryWithDelay(5, 2000))
.subscribe(list -> addNewComments(list), e -> handleFetchDataError(e));
}
// This method gets called in onCreate()
private void dataCollection() {
fetchData = Observable.interval(0, REFRESH_RATE, TimeUnit.MILLISECONDS)
.map(tick -> fetchNewComments()) // Run function every time a tick is emitted
.retryWhen( new RetryWithDelay(2, 2000) ) // Retry twice with 2 second delay
.subscribeOn(Schedulers.io()) // Network stuff in background thread
.observeOn(AndroidSchedulers.mainThread()); // Other stuff on the main thread
}
Auth.java
public class Auth {
...
public void refreshToken() {
Observable.just(1)
.map(y -> refreshAccessToken())
.retryWhen( new RetryWithDelay(3, 2000) )
.subscribeOn(Schedulers.io())
.subscribe();
}
}
Using reactive libraries a new way of thinking is needed. You have to write the code as it is synchronious, but be aware that it evecutes asynchroniously.
Your code just executes synchoniously. It executes two Observable's at the same time.
The function refreshToken() should look like:
public Observable<?> refreshToken() {
return Observable.just(1)
.map(y -> refreshAccessToken())
.retryWhen( new RetryWithDelay(3, 2000) )
.subscribeOn(Schedulers.io());
}
And onResume():
#Override
protected void onResume() {
super.onResume();
Observable obs = fetchData
.compose(bindToLifecycle())
.retryWhen(new RetryWithDelay(5, 2000));
if (tokenExpired()) {
obs = obs.startWith(auth.refreshToken());
}
subscription = obs
.subscribe(list -> addNewComments(list), e -> handleFetchDataError(e));
}
Notice startWith() operator. It allows to executes one Observable (fetching list) after another (refreshing token).
.flatMap() will probably be sufficient, i.e. tokenObservable.flatMap(/* return dataObservable */)
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(...)
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)
}
})