Android MVVM - 如何在 ViewModel 中引用 Activity

Question

MVVM architecture,

this is my View (Activity):

private MyApp app;
private MainActivityVM viewModel;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
app = (MyApp) this.getApplication();
ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);

MainActivityVM.Factory factory = new MainActivityVM.Factory(app);

final MainActivityVM model = ViewModelProviders.of(this, factory)
.get(MainActivityVM.class);

viewModel = model;
binding.setVm(viewModel);
viewModel.onCreate();

and View Model:

public class MainActivityVM extends AndroidViewModel implements ViewModel {

    public MainActivityVM(@NonNull Application application) {
        super(application);
    }

    @Override public void onCreate() {
         model = new MyService();
         model.getData(); /* <-- how do i pass the activity here? */
    }

    @Override public void onPause() { }

    @Override public void onResume() { }

    @Override public void onDestroy() { }

    public static class Factory extends ViewModelProvider.NewInstanceFactory {

        @NonNull
        private final Application mApplication;

        public Factory(@NonNull Application application) {
            mApplication = application;
        }

        @Override
        public <T extends android.arch.lifecycle.ViewModel> T create(Class<T> modelClass) {
            return (T) new MainActivityVM(mApplication);
        }
    }
}

and Model:

public class myService{

    public getData(){
        if(permissionacquired(){
            getdata()
        }else{
            requestPermission();
        }
    }

    private void requestPermission() {
        PermissionKey permKey = new PermissionKey(HealthConstants.StepCount.HEALTH_DATA_TYPE, PermissionType.READ);
        HealthPermissionManager pmsManager = new HealthPermissionManager(mStore);
        try {
            // Show user permission UI for allowing user to change options

            /* BELOW CODE REQUIRE Activity reference to PASS */

            pmsManager.requestPermissions(Collections.singleton(permKey), MainActivity.this).setResultListener(result -> {

            /* ABOVE CODE REQUIRE Activity reference to PASS */

            Log.d(APP_TAG, "Permission callback is received.");
            Map<PermissionKey, Boolean> resultMap = result.getResultMap();

                if (resultMap.containsValue(Boolean.FALSE)) {
                    updateStepCountView("");
                    showPermissionAlarmDialog();
                } else {
                    // Get the current step count and display it
                    mReporter.start(mStepCountObserver);
                }
            });
        } catch (Exception e) { Log.e(APP_TAG, "Permission setting fails.", e); }
    }

}

EDIT: if you see my request permission in my Model, the API require activity to be pass - how can i pass activity reference to the request permission?

I have a get permission method that comes from Model. this get permission method from my service provider require activity eg requestPermission(Activity)

so in my ModelView, i have the model object which is the dataService from another source.

then, how I can reference Activity in my ViewModel so I can call: model.requestPermission(Activity); in my ViewModel?

understanding from here that:

Caution : A ViewModel must never reference a view, Lifecycle, or any class that may hold a reference to the activity context.

Answer 1

只要您在 onCreate() 方法中需要权限，您就可以将带有权限请求的逻辑移动到活动中，并将请求结果传递到视图模型中。

Answer 2

In my case I also added Activity into ViewModel for permissions and strings, but it's not a good idea. When I disabled location permission in one Fragment , an application crashed, because it restarted, then restored FragmentManager with fragment stack and later started MainActivity . So ViewModel got location status too early (in constructor) and threw an exception. But when I moved getting location status to a function, then the application restarted normally.

So, using Dagger, you can write something like:

AppModule:
    @JvmStatic
    @Provides
    fun provideActivity(app: MainApplication): AppCompatActivity = app.mainActivity

In MainApplication hold mainActivity and in MainActivity set in onCreate :

application.mainActivity = this

In onDestroy :

application.mainActivity = null

In any ViewModel add:

class SomeViewModel @Inject constructor(
    private val activity: Provider<AppCompatActivity>
)

Then use it: activity.get().getString(R.string.some_string) .