问题
According to LiveData documentation:
The LiveData class provides the following advantages:
...
Always up to date data: If a Lifecycle starts again (like an activity going back to started state from the back stack) it receives the latest location data (if it didn’t already).
But sometimes I don't need this feature.
For example, I have following LiveData in ViewModel and Observer in Activity:
//LiveData
val showDialogLiveData = MutableLiveData<String>()
//Activity
viewModel.showMessageLiveData.observe(this, android.arch.lifecycle.Observer { message ->
AlertDialog.Builder(this)
.setMessage(message)
.setPositiveButton("OK") { _, _ -> }
.show()
})
Now after every rotation old dialog will appear.
Is there a way to clear stored value after it's handled or is it wrong usage of LiveData at all?
回答1:
Update
There are actually a few ways to resolve this issue. They are summarized nicely in the article LiveData with SnackBar, Navigation and other events (the SingleLiveEvent case). This is written by a fellow Googler who works with the Architecture Components team.
TL;DR A more robust approach is to use an Event wrapper class, which you can see an example of at the bottom of the article.
This pattern has made it's way into numerous Android samples, for example:
- Plaid
- Architecture Blueprints
- IOSched
Why is an Event wrapper preferred over SingleLiveEvent?
One issue with SingleLiveEvent is that if there are multiple observers to a SingleLiveEvent, only one of them will be notified when that data has changed - this can introduce subtle bugs and is hard to work around.
Using an Event wrapper class, all of your observers will be notified as normal. You can then choose to either explicitly "handle" the content (content is only "handled" once) or peek at the content, which always returns whatever the latest "content" was. In the dialog example, this means you can always see what the last message was with peek, but ensure that for every new message, the dialog only is triggered once, using getContentIfNotHandled.
Old Response
Alex's response in the comments is I think exactly what you're looking for. There's sample code for a class called SingleLiveEvent. The purpose of this class is described as:
A lifecycle-aware observable that sends only new updates after subscription, used for events like navigation and Snackbar messages.
This avoids a common problem with events: on configuration change (like rotation) an update can be emitted if the observer is active. This LiveData only calls the observable if there's an explicit call to setValue() or call().
回答2:
In my case SingleLiveEvent doesn't help. I use this code:
private MutableLiveData<Boolean> someLiveData;
private final Observer<Boolean> someObserver = new Observer<Boolean>() {
@Override
public void onChanged(@Nullable Boolean aBoolean) {
if (aBoolean != null) {
// doing work
...
// reset LiveData value
someLiveData.postValue(null);
}
}
};
回答3:
You need to use SingleLiveEvent for this case
class SingleLiveEvent<T> : MutableLiveData<T>() {
private val pending = AtomicBoolean(false)
@MainThread
override fun observe(owner: LifecycleOwner, observer: Observer<T>) {
if (hasActiveObservers()) {
Log.w(TAG, "Multiple observers registered but only one will be notified of changes.")
}
// Observe the internal MutableLiveData
super.observe(owner, Observer<T> { t ->
if (pending.compareAndSet(true, false)) {
observer.onChanged(t)
}
})
}
@MainThread
override fun setValue(t: T?) {
pending.set(true)
super.setValue(t)
}
/**
* Used for cases where T is Void, to make calls cleaner.
*/
@MainThread
fun call() {
value = null
}
companion object {
private const val TAG = "SingleLiveEvent"
}
}
And inside you viewmodel class create object like:
val snackbarMessage = SingleLiveEvent<Int>()
回答4:
I`m not sure if it will work in your case, but in my case (increasing/decreasing items amount in Room by click on views) removing Observer and checking if there is active observers let me do the job:
LiveData<MenuItem> menuitem = mViewModel.getMenuItemById(menuid);
menuitem.observe(this, (MenuItem menuItemRoom) ->{
menuitem.removeObservers(this);
if(menuitem.hasObservers())return;
// Do your single job here
});
});
UPDATE 20/03/2019:
Now i prefer this: EventWraper class from Google Samples inside MutableLiveData
/**
* Used as a wrapper for data that is exposed via a LiveData that represents an event.
*/
public class Event<T> {
private T mContent;
private boolean hasBeenHandled = false;
public Event( T content) {
if (content == null) {
throw new IllegalArgumentException("null values in Event are not allowed.");
}
mContent = content;
}
@Nullable
public T getContentIfNotHandled() {
if (hasBeenHandled) {
return null;
} else {
hasBeenHandled = true;
return mContent;
}
}
public boolean hasBeenHandled() {
return hasBeenHandled;
}
}
In ViewModel :
/** expose Save LiveData Event */
public void newSaveEvent() {
saveEvent.setValue(new Event<>(true));
}
private final MutableLiveData<Event<Boolean>> saveEvent = new MutableLiveData<>();
LiveData<Event<Boolean>> onSaveEvent() {
return saveEvent;
}
In Activity/Fragment
mViewModel
.onSaveEvent()
.observe(
getViewLifecycleOwner(),
booleanEvent -> {
if (booleanEvent != null)
final Boolean shouldSave = booleanEvent.getContentIfNotHandled();
if (shouldSave != null && shouldSave) saveData();
}
});
回答5:
If you need simple solution, try this one:
class SingleLiveData<T> : MutableLiveData<T?>() {
override fun observe(owner: LifecycleOwner, observer: Observer<in T?>) {
super.observe(owner, Observer { t ->
if (t != null) {
observer.onChanged(t)
postValue(null)
}
})
}
}
Use it like a regular MutableLiveData
回答6:
The best solution I found is live event library which works perfectly if you have multiple observers:
class LiveEventViewModel : ViewModel() {
private val clickedState = LiveEvent<String>()
val state: LiveData<String> = clickedState
fun clicked() {
clickedState.value = ...
}
}
来源：https://stackoverflow.com/questions/44582019/how-to-clear-livedata-stored-value