LiveData remove Observer after first callback

Question

How do I remove the observer after I receive the first result? Below are two code ways I've tried, but they both keep receiving updates even though I have removed the observer.

Observer observer = new Observer<DownloadItem>() {         @Override         public void onChanged(@Nullable DownloadItem downloadItem) {             if(downloadItem!= null) {                 DownloadManager.this.downloadManagerListener.onDownloadManagerFailed(null, "this item already exists");                 return;             }             startDownload();             model.getDownloadByContentId(contentId).removeObservers((AppCompatActivity)context);         }     };     model.getDownloadByContentId(contentId).observeForever(observer); 

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 model.getDownloadByContentId(contentId).observe((AppCompatActivity)context, downloadItem-> {              if(downloadItem!= null) {                 this.downloadManagerListener.onDownloadManagerFailed(null, "this item already exists");                 return;             }             startDownload();             model.getDownloadByContentId(contentId).removeObserver(downloadItem-> {});         } ); 

Answer 1

There is a more convenient solution for Kotlin with extensions:

fun <T> LiveData<T>.observeOnce(lifecycleOwner: LifecycleOwner, observer: Observer<T>) {     observe(lifecycleOwner, object : Observer<T> {         override fun onChanged(t: T?) {             observer.onChanged(t)             removeObserver(this)         }     }) } 

This extension permit us to do that:

liveData.observeOnce(this, Observer<Password> {     if (it != null) {         // do something     } }) 

So to answer your original question, we can do that:

val livedata = model.getDownloadByContentId(contentId) livedata.observeOnce((AppCompatActivity) context, Observer<T> {     if (it != null) {         DownloadManager.this.downloadManagerListener.onDownloadManagerFailed(null, "this item already exists");     }     startDownload(); }) 

The original source is here: https://code.luasoftware.com/tutorials/android/android-livedata-observe-once-only-kotlin/

Update: @Hakem-Zaied is right, we need to use observe instead of observeForever.