I want to to be notified if the logged in user in MSAL changes. I therefore used msalClient.getCurrentAccountAsync(callback) but the callback is only called once initially, not after changes like logging in. Is that the intended behaviour? How can I observe the logged in user? Do I have to build my own wrapper around MSAL so I know a user logged in/logged out etc?
val msalClient = PublicClientApplication.createSingleAccountPublicClientApplication(
applicationContext,
R.raw.msal_config
)
val callback = object : CurrentAccountCallback {
override fun onAccountLoaded(activeAccount: IAccount?) {
Timber.i("MSAL monitor login: getCurrentAccountAsync callback onAccountLoaded")
}
override fun onAccountChanged(
priorAccount: IAccount?,
currentAccount: IAccount?
) {
Timber.i("MSAL monitor login: getCurrentAccountAsync callback onAccountChanged")
}
override fun onError(exception: MsalException) {
Timber.e("MSAL monitor login: getCurrentAccountAsync callback onError, exception follows:")
}
}
msalClient.getCurrentAccountAsync(callback)
// callback is fired once directly after registering it
// call msalClient.signIn(...) and sign in
// I would expect the callback to be fired again after the login but it is not
Context
I'm building a Flutter Plugin above the DJK SDK. For that, I have to implement the communication with the aircraft on the native side, and I'm doing it with Java. I'm also doing it only for Android.
One of the methods of the API is boolean connectToAircraft(), which must return if the connection with the aircraft succeeded.
Expected/current behavior
After I call connectToAircraft() - which invokes the DJISDKManager.getInstance().startConnectionToProduct() method, I expected to be able to use anything related to aircraft immediately, but this doesn't happen. I have to wait a few seconds before I can retrieve data from the aircraft.
Some code
public class UavApi implements IUavApi, DJISDKManager.SDKManagerCallback {
...
private final CountDownLatch onConnectToUavFinishedSignal = new CountDownLatch(1);
...
public boolean connectToUav() throws InterruptedException {
Logger.v("connectToUav()");
DJISDKManager.getInstance().startConnectionToProduct();
synchronized (onConnectToUavFinishedSignal) {
onConnectToUavFinishedSignal.await();
}
return DJISDKManager.getInstance().getProduct() instanceof Aircraft;
}
...
#Override
public void onProductConnect(#Nullable final BaseProduct baseProduct) {
Logger.v(MessageFormat.format("onProductConnect(product: {0})", baseProduct));
if (baseProduct != null) {
handleProductConnected(baseProduct);
}
}
#Override
public void onProductChanged(#Nullable final BaseProduct baseProduct) {
Logger.v(MessageFormat.format("onProductChanged(product: {0})", baseProduct));
if (baseProduct != null) {
handleProductConnected(baseProduct);
}
}
...
private void handleProductConnected(#NonNull final BaseProduct baseProduct) {
Logger.d(MessageFormat.format("Is null? {0}", baseProduct == null ? "Yes" : "No"));
Logger.d(MessageFormat.format("Type: {0}", baseProduct.getClass().getSimpleName()));
onConnectToUavFinishedSignal.countDown();
}
...
}
Problem
The code above is what I tried to do, but it's not working and guess it's because I'm misunderstanding the use of the onProductChange() and onProductConnect() methods.
The DJISDKManager.getInstance().getProduct() is always returning null.
OBS: It's always returning null immediately after the onConnectToUavFinishedSignal.await() call finishes. After a few seconds, I get a valid instance of the aircraft.
Something I've also noticed is that sometimes the onProductChange() is called with some value that the log outputs as Unknwoun and None. What are those and how can I test for them? Like if (baseProduct == ???) doSomething()
Environment
Android 9
MSDK 4.13.1
Phantom 4 Pro
Difference
According to the SDK Docs onProductChanged is primarily used to detect when the connection status changes from only remote controller connected to a full connection between the aircraft and the SDK running on your device.
Keep in mind that when the aircraft is disconnected, this method will be called with an instance of an aircraft, but this instance will come with property isConnected as false. If you print the aircraft object to the console you will notice that if isConnected is true, it will print the aircraft name, otherwise, it will print "None".
As long for the onProductConnect, it will be called always after DJISDKManager.getInstance().registerApp() succeeded or after you manually connect to the aircraft with success using DJISDKManager.getInstance().startConnectionToProduct(). In my tests, even though the app registration succeeds, the method will return false, so you might need to check if the SDKManagerCallback::onRegister results in DJISDKError.REGISTRATION_SUCCESS.
Solution
You need to listen to component change events. Unfortunately just because the product is connected it does not mean that the individual components, such as the flight controller, camera etc are connected. You will need to implement onComponentChange and add a listener to detect when a component is connected. These don't always connect in the same order and may start to connect before or after the product is connected.
#Override
public void onComponentChange(
BaseProduct.ComponentKey componentKey,
BaseComponent oldBaseComponent,
BaseComponent newBaseComponent
) {
newBaseComponent.setComponentListener(isConnected -> {
// check if component connected and access data
if (isConnected) {
if(componentKey == ComponentKey.FLIGHT_CONTROLLER) {
// DJISDKManager.getInstance().getProduct() should no longer be null
DJISDKManager.getInstance().getProduct().getModel();
}
}
})
}
I am using ActivityScenarioRule for Espresso UI Testing and I wanted to get access to the method getStringArray(), calling which requires the Activity . So, is there any way to retrieve the Activity by the ActivityScenarioRule , maybe something similar to getActivity in ActivityTestRule.
#Rule
public ActivityScenarioRule activityScenarioRule = new ActivityScenarioRule<>(MainActivity.class);
I am not using ActivityTestRule, because it is deprecated!
Since it appears you're using Java, here's how you'd do it:
#Rule
ActivityScenarioRule<MainActivity> activityScenarioRule = new ActivityScenarioRule<>(MainActivity.class);
#Test
public void test() {
activityScenarioRule.getScenario().onActivity(activity -> {
// use 'activity'.
});
}
Please read the documentation for more info on these new ways of interacting with the activity under test.
For anyone who wants Activity, but that without need to re-write all tests to run on UI-thread, a fairly straightforward Java way to get it:
Waiting for UI
Assume you want to test if a dialog is shown after some delay, the onActivity(...) hook runs on UI-thread, which means waiting in there would cause the dialog to be nerver shown.
In such cases you need to keep a strong-reference to ActivityScenario (as that prevents Activity close).
Test should wait for onActivity(...) hook to be called, then keep passed Activity's reference.
Finally, move test logic out of onActivity(...) hook.
Example
private ActivityScenario mActivityScenario;
#After
public void tearDown() throws Exception {
if (mActivityScenario != null) {
mActivityScenario.close();
}
mActivityScenario = null;
}
#Override
public Activity getActivity() {
if (mActivityScenario == null) {
mActivityScenario = ActivityScenario.launch(getActivityClassForScenario());
}
return tryAcquireScenarioActivity(mActivityScenario);
}
protected static Activity tryAcquireScenarioActivity(ActivityScenario activityScenario) {
Semaphore activityResource = new Semaphore(0);
Activity[] scenarioActivity = new Activity[1];
activityScenario.onActivity(activity -> {
scenarioActivity[0] = activity;
activityResource.release();
});
try {
activityResource.tryAcquire(15000, TimeUnit.MILLISECONDS);
} catch (InterruptedException e) {
Assert.fail("Failed to acquire activity scenario semaphore");
}
Assert.assertNotNull("Scenario Activity should be non-null", scenarioActivity[0]);
return scenarioActivity[0];
}
Espresso states the following:
At the same time, the framework prevents direct access to activities
and views of the application because holding on to these objects and
operating on them off the UI thread is a major source of test
flakiness.
When there is no other way I use the following method to get an arbitrary activity from an ActivityScenarioRule. It uses onActivity mentioned in the accepted answer:
private <T extends Activity> T getActivity(ActivityScenarioRule<T> activityScenarioRule) {
AtomicReference<T> activityRef = new AtomicReference<>();
activityScenarioRule.getScenario().onActivity(activityRef::set);
return activityRef.get();
}
Any onView(...) code inside onActivity led to a timeout in my testcases. So, I extracted the activity and used it with success outside the onActivity. Beware tho! See the statement above.
#Test
fun checkForUpdate() {
val scenario = ActivityScenario.launch(MainActivity::class.java)
scenario.onActivity {
UpdateTool.checkForUpdate(it)
}
}
(Disclaimer: There are a ton of questions which arise from people asking about data being null/incorrect when using asynchronous operations through requests such as facebook,firebase, etc. My intention for this question was to provide a simple answer for that problem to everyone starting out with asynchronous operations in android)
I'm trying to get data from one of my operations, when I debug it using breakpoints or logs, the values are there, but when I run it they are always null, how can I solve this ?
Firebase
firebaseFirestore.collection("some collection").get()
.addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
#Override
public void onSuccess(QuerySnapshot documentSnapshots) {
//I want to return these values I receive here...
});
//...and use the returned value here.
Facebook
GraphRequest request = GraphRequest.newGraphPathRequest(
accessToken,
"some path",
new GraphRequest.Callback() {
#Override
public void onCompleted(GraphResponse response) {
//I want to return these values I receive here...
}
});
request.executeAsync();
//...and use the returned value here.
Kotlin coroutine
var result: SomeResultType? = null
someScope.launch {
result = someSuspendFunctionToRetrieveSomething()
//I want to return the value I received here...
}
Log.d("result", result.toString()) //...but it is still null here.
Etc.
What is a Synchronous/Asynchronous operation ?
Well, Synchronous waits until the task has completed. Your code executes "top-down" in this situation.
Asynchronous completes a task in the background and can notify you when it is complete.
If you want to return the values from an async operation through a method/function, you can define your own callbacks in your method/function to use these values as they are returned from these operations.
Here's how for Java
Start off by defining an interface :
interface Callback {
void myResponseCallback(YourReturnType result);//whatever your return type is: string, integer, etc.
}
next, change your method signature to be like this :
public void foo(final Callback callback) { // make your method, which was previously returning something, return void, and add in the new callback interface.
next up, wherever you previously wanted to use those values, add this line :
callback.myResponseCallback(yourResponseObject);
as an example :
#Override
public void onSuccess(QuerySnapshot documentSnapshots) {
// create your object you want to return here
String bar = document.get("something").toString();
callback.myResponseCallback(bar);
})
now, where you were previously calling your method called foo:
foo(new Callback() {
#Override
public void myResponseCallback(YourReturnType result) {
//here, this result parameter that comes through is your api call result to use, so use this result right here to do any operation you previously wanted to do.
}
});
}
How do you do this for Kotlin ?
(as a basic example where you only care for a single result)
start off by changing your method signature to something like this:
fun foo(callback:(YourReturnType) -> Unit) {
.....
then, inside your asynchronous operation's result :
firestore.collection("something")
.document("document").get()
.addOnSuccessListener {
val bar = it.get("something").toString()
callback(bar)
}
then, where you would have previously called your method called foo, you now do this :
foo() { result->
// here, this result parameter that comes through is
// whatever you passed to the callback in the code aboce,
// so use this result right here to do any operation
// you previously wanted to do.
}
// Be aware that code outside the callback here will run
// BEFORE the code above, and cannot rely on any data that may
// be set inside the callback.
if your foo method previously took in parameters :
fun foo(value:SomeType, callback:(YourType) -> Unit)
you simply change it to :
foo(yourValueHere) { result ->
// here, this result parameter that comes through is
// whatever you passed to the callback in the code aboce,
// so use this result right here to do any operation
// you previously wanted to do.
}
these solutions show how you can create a method/function to return values from async operations you've performed through the use of callbacks.
However, it is important to understand that, should you not be interested in creating a method/function for these:
#Override
public void onSuccess(SomeApiObjectType someApiResult) {
// here, this `onSuccess` callback provided by the api
// already has the data you're looking for (in this example,
// that data would be `someApiResult`).
// you can simply add all your relevant code which would
// be using this result inside this block here, this will
// include any manipulation of data, populating adapters, etc.
// this is the only place where you will have access to the
// data returned by the api call, assuming your api follows
// this pattern
})
There's a particular pattern of this nature I've seen repeatedly, and I think an explanation of what's happening would help. The pattern is a function/method that calls an API, assigning the result to a variable in the callback, and returns that variable.
The following function/method always returns null, even if the result from the API is not null.
Kotlin
fun foo(): String? {
var myReturnValue: String? = null
someApi.addOnSuccessListener { result ->
myReturnValue = result.value
}.execute()
return myReturnValue
}
Kotlin coroutine
fun foo(): String? {
var myReturnValue: String? = null
lifecycleScope.launch {
myReturnValue = someApiSuspendFunction()
}
return myReturnValue
}
Java 8
private String fooValue = null;
private String foo() {
someApi.addOnSuccessListener(result -> fooValue = result.getValue())
.execute();
return fooValue;
}
Java 7
private String fooValue = null;
private String foo() {
someApi.addOnSuccessListener(new OnSuccessListener<String>() {
public void onSuccess(Result<String> result) {
fooValue = result.getValue();
}
}).execute();
return fooValue;
}
The reason is that when you pass a callback or listener to an API function, that callback code will only be run some time in the future, when the API is done with its work. By passing the callback to the API function, you are queuing up work, but the current function (foo() in this case) returns immediately before that work begins and before that callback code is run.
Or in the case of the coroutine example above, the launched coroutine is very unlikely to complete before the function that started it.
Your function that calls the API cannot return the result that is returned in the callback (unless it's a Kotlin coroutine suspend function). The solution, explained in the other answer, is to make your own function take a callback parameter and not return anything.
Alternatively, if you're working with coroutines, you can make your function suspend instead of launching a separate coroutine. When you have suspend functions, somewhere in your code you must launch a coroutine and handle the results within the coroutine. Typically, you would launch a coroutine in a lifecycle function like onCreate(), or in a UI callback like in an OnClickListener.
Other answer explains how to consume APIs based on callbacks by exposing a similar callbacks-based API in the outer function. However, recently Kotlin coroutines become more and more popular, especially on Android and while using them, callbacks are generally discouraged for such purposes. Kotlin approach is to use suspend functions instead. Therefore, if our application uses coroutines already, I suggest not propagating callbacks APIs from 3rd party libraries to the rest of our code, but converting them to suspend functions.
Converting callbacks to suspend
Let's assume we have this callback API:
interface Service {
fun getData(callback: Callback<String>)
}
interface Callback<in T> {
fun onSuccess(value: T)
fun onFailure(throwable: Throwable)
}
We can convert it to suspend function using suspendCoroutine():
private val service: Service
suspend fun getData(): String {
return suspendCoroutine { cont ->
service.getData(object : Callback<String> {
override fun onSuccess(value: String) {
cont.resume(value)
}
override fun onFailure(throwable: Throwable) {
cont.resumeWithException(throwable)
}
})
}
}
This way getData() can return the data directly and synchronously, so other suspend functions can use it very easily:
suspend fun otherFunction() {
val data = getData()
println(data)
}
Note that we don't have to use withContext(Dispatchers.IO) { ... } here. We can even invoke getData() from the main thread as long as we are inside the coroutine context (e.g. inside Dispatchers.Main) - main thread won't be blocked.
Cancellations
If the callback service supports cancelling of background tasks then it is best to cancel when the calling coroutine is itself cancelled. Let's add a cancelling feature to our callback API:
interface Service {
fun getData(callback: Callback<String>): Task
}
interface Task {
fun cancel();
}
Now, Service.getData() returns Task that we can use to cancel the operation. We can consume it almost the same as previously, but with small changes:
suspend fun getData(): String {
return suspendCancellableCoroutine { cont ->
val task = service.getData(object : Callback<String> {
...
})
cont.invokeOnCancellation {
task.cancel()
}
}
}
We only need to switch from suspendCoroutine() to suspendCancellableCoroutine() and add invokeOnCancellation() block.
Example using Retrofit
interface GitHubService {
#GET("users/{user}/repos")
fun listRepos(#Path("user") user: String): Call<List<Repo>>
}
suspend fun listRepos(user: String): List<Repo> {
val retrofit = Retrofit.Builder()
.baseUrl("https://api.github.com/")
.build()
val service = retrofit.create<GitHubService>()
return suspendCancellableCoroutine { cont ->
val call = service.listRepos(user)
call.enqueue(object : Callback<List<Repo>> {
override fun onResponse(call: Call<List<Repo>>, response: Response<List<Repo>>) {
if (response.isSuccessful) {
cont.resume(response.body()!!)
} else {
// just an example
cont.resumeWithException(Exception("Received error response: ${response.message()}"))
}
}
override fun onFailure(call: Call<List<Repo>>, t: Throwable) {
cont.resumeWithException(t)
}
})
cont.invokeOnCancellation {
call.cancel()
}
}
}
Native support
Before we start converting callbacks to suspend functions, it is worth checking whether the library that we use does support suspend functions already: natively or with some extension. Many popular libraries like Retrofit or Firebase support coroutines and suspend functions. Usually, they either provide/handle suspend functions directly or they provide suspendable waiting on top of their asynchronous task/call/etc. object. Such waiting is very often named await().
For example, Retrofit supports suspend functions directly since 2.6.0:
interface GitHubService {
#GET("users/{user}/repos")
suspend fun listRepos(#Path("user") user: String): List<Repo>
}
Note that we not only added suspend, but also we no longer return Call, but the result directly. Now, we can use it without all this enqueue() boilerplate:
val repos = service.listRepos(user)
TL;DR The code you pass to these APIs (e.g. in the onSuccessListener) is a callback, and it runs asynchronously (not in the order it is written in your file). It runs at some point later in the future to "call back" into your code. Without using a coroutine to suspend the program, you cannot "return" data retrieved in a callback from a function.
What is a callback?
A callback is a piece of code you pass to some third party library that it will run later when some event happens (e.g. when it gets data from a server). It is important to remember that the callback is not run in the order you wrote it - it may be run much later in the future, could run multiple times, or may never run at all. The example callback below will run Point A, start the server fetching process, run Point C, exit the function, then some time in the distant future may run Point B when the data is retrieved. The printout at Point C will always be empty.
fun getResult() {
// Point A
var r = ""
doc.get().addOnSuccessListener { result ->
// The code inside the {} here is the "callback"
// Point B - handle result
r = result // don't do this!
}
// Point C - r="" still here, point B hasn't run yet
println(r)
}
How do I get the data from the callback then?
Make your own interface/callback
Making your own custom interface/callback can sometimes make things cleaner looking but it doesn't really help with the core question of how to use the data outside the callback - it just moves the aysnc call to another location. It can help if the primary API call is somewhere else (e.g. in another class).
// you made your own callback to use in the
// async API
fun getResultImpl(callback: (String)->Unit) {
doc.get().addOnSuccessListener { result ->
callback(result)
}
}
// but if you use it like this, you still have
// the EXACT same problem as before - the printout
// will always be empty
fun getResult() {
var r = ""
getResultImpl { result ->
// this part is STILL an async callback,
// and runs later in the future
r = result
}
println(r) // always empty here
}
// you still have to do things INSIDE the callback,
// you could move getResultImpl to another class now,
// but still have the same potential pitfalls as before
fun getResult() {
getResultImpl { result ->
println(result)
}
}
Some examples of how to properly use a custom callback: example 1, example 2, example 3
Make the callback a suspend function
Another option is to turn the async method into a suspend function using coroutines so it can wait for the callback to complete. This lets you write linear-looking functions again.
suspend fun getResult() {
val result = suspendCoroutine { cont ->
doc.get().addOnSuccessListener { result ->
cont.resume(result)
}
}
// the first line will suspend the coroutine and wait
// until the async method returns a result. If the
// callback could be called multiple times this may not
// be the best pattern to use
println(result)
}
Re-arrange your program into smaller functions
Instead of writing monolithic linear functions, break the work up into several functions and call them from within the callbacks. You should not try to modify local variables within the callback and return or use them after the callback (e.g. Point C). You have to move away from the idea of returning data from a function when it comes from an async API - without a coroutine this generally isn't possible.
For example, you could handle the async data in a separate method (a "processing method") and do as little as possible in the callback itself other than call the processing method with the received result. This helps avoid a lot of the common errors with async APIs where you attempt to modify local variables declared outside the callback scope or try to return things modified from within the callback. When you call getResult it starts the process of getting the data. When that process is complete (some time in the future) the callback calls showResult to show it.
fun getResult() {
doc.get().addOnSuccessListener { result ->
showResult(result)
}
// don't try to show or return the result here!
}
fun showResult(result: String) {
println(result)
}
Example
As a concrete example here is a minimal ViewModel showing how one could include an async API into a program flow to fetch data, process it, and display it in an Activity or Fragment. This is written in Kotlin but is equally applicable to Java.
class MainViewModel : ViewModel() {
private val textLiveData = MutableLiveData<String>()
val text: LiveData<String>
get() = textLiveData
fun fetchData() {
// Use a coroutine here to make a dummy async call,
// this is where you could call Firestore or other API
// Note that this method does not _return_ the requested data!
viewModelScope.launch {
delay(3000)
// pretend this is a slow network call, this part
// won't run until 3000 ms later
val t = Calendar.getInstance().time
processData(t.toString())
}
// anything out here will run immediately, it will not
// wait for the "slow" code above to run first
}
private fun processData(d: String) {
// Once you get the data you may want to modify it before displaying it.
val p = "The time is $d"
textLiveData.postValue(p)
}
}
A real API call in fetchData() might look something more like this
fun fetchData() {
firestoreDB.collection("data")
.document("mydoc")
.get()
.addOnCompleteListener { task ->
if (task.isSuccessful) {
val data = task.result.data
processData(data["time"])
}
else {
textLiveData.postValue("ERROR")
}
}
}
The Activity or Fragment that goes along with this doesn't need to know anything about these calls, it just passes actions in by calling methods on the ViewModel and observes the LiveData to update its views when new data is available. It cannot assume that the data is available immediately after a call to fetchData(), but with this pattern it doesn't need to.
The view layer can also do things like show and hide a progress bar while the data is being loaded so the user knows it's working in the background.
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
val model: MainViewModel by viewModels()
// Observe the LiveData and when it changes, update the
// state of the Views
model.text.observe(this) { processedData ->
binding.text.text = processedData
binding.progress.visibility = View.GONE
}
// When the user clicks the button, pass that action to the
// ViewModel by calling "fetchData()"
binding.getText.setOnClickListener {
binding.progress.visibility = View.VISIBLE
model.fetchData()
}
binding.progress.visibility = View.GONE
}
}
The ViewModel is not strictly necessary for this type of async workflow - here is an example of how to do the same thing in the activity
class MainActivity : AppCompatActivity() {
private lateinit var binding: ActivityMainBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityMainBinding.inflate(layoutInflater)
setContentView(binding.root)
// When the user clicks the button, trigger the async
// data call
binding.getText.setOnClickListener {
binding.progress.visibility = View.VISIBLE
fetchData()
}
binding.progress.visibility = View.GONE
}
private fun fetchData() {
lifecycleScope.launch {
delay(3000)
val t = Calendar.getInstance().time
processData(t.toString())
}
}
private fun processData(d: String) {
binding.progress.visibility = View.GONE
val p = "The time is $d"
binding.text.text = p
}
}
(and, for completeness, the activity XML)
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<TextView
android:id="#+id/text"
android:layout_margin="16dp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"/>
<Button
android:id="#+id/get_text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="16dp"
android:text="Get Text"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="#+id/text"
/>
<ProgressBar
android:id="#+id/progress"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="48dp"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="#+id/get_text"
/>
</androidx.constraintlayout.widget.ConstraintLayout>
When developing Cordova plugins, all of the tutorials I have found go something like this:
File: AwesomePlugin.js
var AwesomePlugin = {
kungfuGripAction = function(target, successCallback, failureCallback) {
return cordova.exec(
successCallback,
failureCallback,
'AwesomePluginClass',
'kungfuGripAction',
[target]
);
}
};
module.exports = AwesomePlugin;
File: AwesomePluginClass.java
#Override
public boolean execute(String action, JSONArray args, CallbackContext callbackContext) throws JSONException {
if (ACTION_KUNGFU_GRIP.equals(action)) {
JSONObject target = args.getJSONObject(0);
if (gripTarget(target)) {
callbackContext.success("Target successfully gripped.");
return true;
} else {
callbackContext.error("Could not grip target.");
return false;
}
}
Log.d(LOG_TAG, "INVALID ACTION! " + action);
callbackContext.error("Invalid action: " + action);
return false;
}
File: clientCode.js
AwesomePlugin.kungfuGripAction(cobraEnemy, function(ok) { }, function(err) { });
In the above code, the callbacks can only be called once and are then disposed. If you attempt to call the .success() or .error() method of the callback context object, it will not work and you will get a log message:
Attempted to send a second callback for ID: AwesomePlugin2982699494<BR>W/CordovaPlugin(976) Result was: "Target successfully gripped."
It seems like it is not possible to write a method with a callback that can be called repeatedly seeing as .success() and .error() are the only documented ways to invoke a callback from within native plugin code. While this is mostly what we want, there are times when we want to have the plugin execute a callback repeatedly. For example:
AwesomePlugin.kungfuGripAction(cobraEnemy, function(ok) {
// After successful grip, punch repeatedly and update life meter.
AwesomePlugin.punchRepeatedly(cobraEnemy, function(hits) {
updateLifeMeter(cobraEnemy, hits);
}, function(err) { });
}, function(err) { });
AwesomePlugin.punchRepeatedly() above will execute repeatedly (maybe in a separate thread) and call function(hits) with each successful execution. If implemented in the de-facto way (using single-use callbacks), you have to either use a loop (which is bad as it is non-async) or tail-call AwesomePlugin.punchRepeatedly() in the callback (error-prone).
What would be the correct way to implement punchRepeatedly() in native code so that it is able register the callback once and then execute it repeatedly?
I think, you can use a pluginResult with the keepCallback property set to true.
PluginResult result = new PluginResult(PluginResult.Status.OK, "YOUR_MESSAGE");
// PluginResult result = new PluginResult(PluginResult.Status.ERROR, "YOUR_ERROR_MESSAGE");
result.setKeepCallback(true);
callbackContext.sendPluginResult(result);
You should be able to invoke the callback several times this way.
In jonas's answer, every time you call sendPluginResult you have to send the same value. So I changed the PluginResult class to add a method like this:
public void setStrMessage(String strMessage){
this.strMessage = strMessage;
}
This way, I can set the message I want to send to the JavaScript side.