LiveData in MVVM pattern with reteofit kotlin

Question

I am new to MVVVM and retrofit i have successfully implemented MVVM and is able to move data from retrofit to repository then Repository to ViewModel and view.

While doing this i came up to a massive confusion which is mentioned below. In first scenario my code looks like this:

Repository:

 fun iniateOTPprocess() : LiveData<GenericResponse> {


    RetroUtils.getApiManager().listRepos().enqueue(object : RetrofitCallback() {

        override fun onResponse(call: Call<GenericResponse>, response: Response<GenericResponse>) {
            super.onResponse(call, response)

            result.value = response.body()

        }
    }


    )

  return result
}

ViewModel:

 class LoginViewModel2(application: Application) : AndroidViewModel(application) {


lateinit var username: MutableLiveData<String>
lateinit var password: MutableLiveData<String>
var repository: LoginRepository = LoginRepository(application)
var data = MediatorLiveData<GenericResponse>()
var result = MutableLiveData<GenericResponse>()


init {
    data.addSource(result , Observer {
        data.postValue(it)


    })

}

fun onLoginBtnCLicked() {

    initiateOTP()
}

private fun initiateOTP() {
  result =  repository.iniateOTPprocess()
}


  fun getResponse() : MediatorLiveData<GenericResponse>{

   return data

  }



 }

The Mediator live data is never updated this way once the data is updated.

But if i change this code to

Repository

  class LoginRepository(var application: Application) {

var callback: RetrofitCallback = RetrofitCallback()
var result = MutableLiveData<GenericResponse>()


fun iniateOTPprocess() {


    RetroUtils.getApiManager().listRepos().enqueue(object : RetrofitCallback() {

        override fun onResponse(call: Call<GenericResponse>, response: Response<GenericResponse>) {
            super.onResponse(call, response)

            result.value = response.body()

        }
    }


    )


}


fun getData(): MutableLiveData<GenericResponse> {

    return result

}
}

ViewModel

class LoginViewModel2(application: Application) : AndroidViewModel(application) {


lateinit var username: MutableLiveData<String>
lateinit var password: MutableLiveData<String>
var repository: LoginRepository = LoginRepository(application)
var data = MediatorLiveData<GenericResponse>()
var result = MutableLiveData<GenericResponse>()


init {
    data.addSource(repository.getData(), Observer {
        data.postValue(it)


    })

}

fun onLoginBtnCLicked() {

    initiateOTP()
}

private fun initiateOTP() {
    repository.iniateOTPprocess()
}


 fun getResponse() : MediatorLiveData<GenericResponse>{

   return data

 }



}

This code starts working magically. In the view i am observing getResponse() method in both the scenarios. Can anyone here help with the confusion and can explain where the magic is happening. Thanks in advance.

Answer 1

In your first scenario, even though you are updating result through this function:

private fun initiateOTP() {
  result =  repository.iniateOTPprocess()
}

You won't be able to observe the changes in this new result instance since you are already observing the first result instance created when you instantiate your ViewModel :

var result = MutableLiveData<GenericResponse>() // you are observing this instance
init {
    // this result never gets updated
    data.addSource(result , Observer {
        data.postValue(it)


    })

}

...

 fun getResponse() : MediatorLiveData<GenericResponse>{

    return data

 }

Answer 2

your code not following MVVM pattern every single live data must have observer, when you declare at init{} the value its not ready yet because its async. if you change to :

data.addSource(repository.iniateOTPprocess(), Observer {
        data.postValue(it)
})

the code will working,this is the best source to learn MVVM and livedata pattern google codelabs