Android ClassCastException by referencing fragment in viewmodel

I was having some problem when trying to reference the fragment in my viewmodel class. Here is the part where my fragment call the viewmodel:

@Click(R.id.buttonExport)
void buttonExportClicked(View v){
SummaryViewModel summaryViewModel = ViewModelProviders.of(this).get(SummaryViewModel.class);
summaryViewModel.export(webViewResult);
}


In the viewmodel where I tried to access the context as well as Activity:

public void exportSelfTestSummary(WebView webViewResult) {
Activity context = (Activity) getApplication().getApplicationContext();
final ProgressDialog progressDialog=new ProgressDialog(context);
progressDialog.show();
PdfView.createWebPrintJob(context, webViewResult, directory, fileName, new PdfView.Callback() {
....
}


And the error message I am getting:

07-02 02:34:01.922 3563-3563/com.mainapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.mainapp, PID: 3563
java.lang.ClassCastException: com.mainapp.App cannot be cast to android.app.Activity
at com.mainapp.viewmodel.SummaryViewModel.export(SummaryViewModel.java:218)
at android.view.View.performClick(View.java:6294)
at android.view.View$PerformClick.run(View.java:24770)
at android.os.Handler.handleCallback(Handler.java:790)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)


Any ideas? Thanks!

Edit

@Inject
public SummaryViewModel(@NonNull Application application) {
super(application);
}


4 Answers
4

The error says it clear enough, you cannot cast Application to Activity

Application

Activity

What you are doing is get the application context, which will return an Application class:

Application

Activity context = (Activity) getApplication().getApplicationContext();


If you are inside a fragment, just simply use fragment.getActivity()

fragment.getActivity()

ViewModel

getApplication

getApplication

Activity

You are getting a ClassCastException because you are trying to cast the application into an Activity.

Activity context = (Activity) getApplication().getApplicationContext();


getApplication() method returns the App class and so does the getApplicationContext() method, which cannot be used to get the Activity.

You should never use a reference to an Activity or Fragment in your ViewModel. Your UI should never be modified from your ViewModel. ViewModel should only contain data used to modify your UI.

As an example:

Fragment/Activity:

SummaryViewModel summaryViewModel = ViewModelProviders.of(this).get(SummaryViewModel.class);
LiveData<WebResult> data = summaryViewModel.getWebViewDataInBackgroundThread();

//observe on this data for changes in your activity/fragment
data.observe(this, Observer {
@Override
public void onChanged(@Nullable final WebResult result) {
// Update the UI,
Activity context = (Activity) getApplication().getApplicationContext();
final ProgressDialog progressDialog=new ProgressDialog(context);
progressDialog.show();
PdfView.createWebPrintJob(context, webViewResult, directory, fileName, new PdfView.Callback() {
....
}
})


In your ViewModel:

public LiveData<WebResult> getWebViewDataInBackgroundThread() {
// getdata and return as LiveData
}


Read more on LiveData and ViewModel

Alternatively you can reach the Activity digging through parent views:

Activity act = (Activity) webViewResult.getParent().getParent().....so on


It not a good practice to use Context and Activity Lifecycle related staff specially view inside ViewModel. ViewModel will only hold data and make a connection between data and view. Create ProgressDialog inside Activity or Fragment. Use ViewModel for your business logic. Create a LiveData for ProgressDialog state in ViewModle. Change LiveData value and observe it in Activity or Fragment. So you can decide when to show ProgressDialog or when to hide it. Your view is inside Activity or Fragment and there is no possibility of MemoryLeak.

Context

ViewModel

ViewModel

ProgressDialog

Activity

Fragment

ViewModel

LiveData

ProgressDialog

ViewModle

LiveData

Activity

Fragment

ProgressDialog

Activity

Fragment

As @tam-huynh mentioned your problem is you trying to cast Application Context to Activity that is not valid.

Activity

Create LiveData in ViewModel

LiveData

ViewModel

private MutableLiveData<Integer> progressDialogState;

public LiveData<Integer> getProgressDialogStateAsObservable(){
return progressDialogState;
}

public void setProgressDialogState(int value){
progressDialogState.setValue(value);
}


public void exportSelfTestSummary(WebView webViewResult) {
//Activity context = (Activity) getApplication().getApplicationContext();
//final ProgressDialog progressDialog=new ProgressDialog(context);
//progressDialog.show();
setProgressDialogState(1)
PdfView.createWebPrintJob(context, webViewResult, directory, fileName, new PdfView.Callback() {
....

// after work done set value to 0
//setProgressDialogState(0)
}


Observe it in Activity or Fragment

Activity

Fragment

viewModel.getProgressDialogStateAsObservable().observe(getActivity(), state -> {
if (state == 1) {
// .....
// show
}else if(state == 0){
// hide
}
});


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