How to use mvvm pattern using databinding

Thumb Rule:

No package from android.* should lie in ViewModel. You can ignore package import for ViewModel

Also, you can do it with normal ViewModel as well.

How to proceed?

Lets make it simple. Suppose, you are making Login page.

Lets have below things in ViewModel itself:

• Fields of email & password with two-way binding
• Form Validation logic
• Button Click Event
• Api Call

All these things lie in your ViewModel.

Now, your Activity needs to react to the result of your Api Call. So, let your ViewModel have a separate LiveData where T is the type of Response from your Api Call.

For example:

val loginApiStatus = MutableLiveData<LoginResponse>()

And then, let your Activity observe this LiveData. It should be observed in onStart() method of Activity. I will tell you the reason why to observe in onStart().

viewModel.loginApiStatus.observe(this, Observer{ loginResponse->
    // respond accordingly
})

Now, once you receive response from Api, simply update the LiveData in your ViewModel as:

loginApiStatus.value = loginResponse // Login Api Response

With this structure, you have total control over handling the Api Response. Even if, your activity goes into background, after launching Api Call, you will still have the state of what happened to that Api call. Now, when you return to Login screen again from background, you start observing the LiveData again (because we are observing the state LiveData in onStart() as I said earlier), and you will get the state to react upon.

Life becomes a lot easier when you start storing states of your View / Fragment / Activity, in your ViewModel itself.