Best practice for Android MVVM startActivity

I am building an Android App using MVVM and DataBinding. And I have a function inside my ViewModel that starts an Activity. Is it okay to have an onClick call inside a ViewModel?

Like this.

public class MyViewModel {
    public void onClick(View view, long productId) {
        Context context = view.getContext();
        Intent intent = new Intent(context, ProductDetailActivity.class);
        intent.putExtra("productId", productId);
        context.startActivity(intent);
    }
}

And in my XML:

...
android:onClick="@{(v) -> viewModel.onClick(v, viewModel.product.id)}">

Or would it be a best practice to move it to the View and call it from EventBus or Rx and have only POJO in my ViewModel?

The answer to your question is what is your goal?

If you want to use MVVM for separation of concerns so that you can unit test your Viewmodel then you should try to keep everything that requires a Context separate from your Viewmodel. The Viewmodel contains the core business logic of your app and should have no external dependencies.

However I like where you are going :) If the decision which Activity is opened lies in the View, then it is very very hard to write a JUnit test for it. However you can pass an object into the Viewmodel which performs the startActivity() call. Now in your Unit test you can simply mock this object and verify that the correct Activity is opened

The way I do it is, in your ViewModel:

val activityToStart = MutableLiveData<Pair<KClass<*>, Bundle?>>()

This allows you to check the class of Activity started, and the data passed in the Bundle. Then, in your Activity, you can add this code:

viewModel.activityToStart.observe(this, Observer { value ->
    val intent = Intent(this, value.first.java)
    if(value.second != null)
        intent.putExtras(value.second)
    startActivity(intent)
})

That's absolutely perfect to put it inside ViewModel, however you need to set your ViewModel from Activity/Fragment.

Here are some links you can follow to learn MVVM architecture.

Approaching Android with MVVM
Android MVVM
https://github.com/ivacf/archi
People-MVVM
MVVM on Android: What You Need to Know

As the principle of MVVM points out that only View (activity/fragment) holds reference to the ViewModel and the ViewModel shouldn't hold reference to any View.

In your case, to start an activity, I will do like this:

MyViewModel.class

public class MyViewModel {
public static final int START_SOME_ACTIVITY = 123;

 @Bindable
 private int messageId;

 public void onClick() {
  messageId = START_SOME_ACTIVITY;
  notifyPropertyChanged(BR.messageId); //BR class is automatically generated when you rebuild the project
 }

 public int getMessageId() {
        return messageId;
 }

 public void setMessageId(int message) {
        this.messageId = messageId;
 }

}

And in your MainActivity.class

@BindingAdapter({"showMessage"})
public static void runMe(View view, int messageId) {
    if (messageId == Consts.START_SOME_ACTIVITY) {      
        view.getContext().startActivity(new Intent(view.getContext(), SomeActivity.class));      
    }
}

@Override
protected void onPause() {
    super.onPause();
    finish(); //only call if you want to clear this activity after go to other activity
}

finally, in your activity_main.xml

<Button    
  android:onClick="@{()-> myViewModel.onClick()}"    
  bind:showMessage="@{myViewModel.messageId}" />

As per the data binding documentation. There are 2 ways to do that:

1- MethodReferences : You have to pass the view as a parameter to the function, or you will get a compile time error.
If you will use this way do a separate class as example here that handle such events.

MyHandler

public class MyHandler {
   public void onClick(View view, long productId) {
        Context context = view.getContext();
        Intent intent = new Intent(context, ProductDetailActivity.class);
        intent.putExtra("productId", productId);
        context.startActivity(intent);
    }
}

XML

<data>
        <variable
            name="viewModel"
            type="com.example.ViewModel"

        <variable
            name="myHandler"
            type="com.example.MyHandler" />

    </data>android:onClick="@{myHandler.onClick(viewModel.product.id)}">

2- Listener bindings : You don't need to pass the view as the example here.
But if you want to startActivity make your viewModel extends AndroidViewModel and you will use the applicaion object.

ViewModel

public class MyViewModel extends AndroidViewModel {
    public void onClick(long productId) {
        Intent intent = new Intent(getApplication(), ProductDetailActivity.class);
        intent.putExtra("productId", productId);
        context.startActivity(intent);
    }
}

XML

android:onClick="@{() -> viewModel.onClick(viewModel.product.id)}">

Best practice for Android MVVM startActivity

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