View Model

Model for your views such as Activity or Fragment.

View Models are lifecycle aware.

Data required for your screen is stored at one place i.e. ViewModel.

It may involve formatting that data in a particular format, accumulating data, any logic in displaying this data in your Ul.

Dependency

To Implement View Model We need to use its Dependency


// View Model Dependency 
implementation("androidx.lifecycle:lifecycle-viewmodel-ktx:2.7.0")

Implement View Model in Android Project

To use viewModel we need to create class that inherits ViewModel Class


package com.example.androidarchitecture

import androidx.lifecycle.ViewModel

class MainViewModel : ViewModel() {
  
}

⚠️

Important Note: - model classes are independent from view , There are primarily used to hold data. - They Do not represent View Or How You Use data in View .

So Make Sure That , You Only Hold Data in ViewModel, Do Not Hold Any Reference to any particular View In it ( Because It Will cause Memory leak )

To Use in Our Main Activity We Need to create instance of our ViewModal Class In Our MainActivity


// create instance 
private lateinit var viewModel: MainViewModel

// Intialise inside onCreate 
 viewModel = ViewModelProvider(this)[MainViewModel::class.java]
// or 
 viewModel = ViewModelProvider(this).get(MainViewModel::class.java)
// both initialization method are same

View Model is Life cycle aware so we need to define which activity lifecycle it need to observe

so we have passed this@MainActivity inside ViewModelProvider() constructor to refer to the current instance of activity

Example

Our View Model class will hold data and its current state when changes are made to activity life cycle

then we can access in our application using viewModel Object

MainViewModel.kt


package com.example.androidarchitecture
import androidx.lifecycle.ViewModel
class MainViewModel : ViewModel() {

    var count: Int = 0
    
    fun increment() {
        count++
    }
    
}

MainActivity.kt


package com.example.androidarchitecture
import android.os.Bundle
import android.util.Log
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.ViewModelProvider
import com.example.androidarchitecture.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {
    private val TAG: String = "===="

    private lateinit var viewModel: MainViewModel

    private val binding: ActivityMainBinding by lazy {
        ActivityMainBinding.inflate(layoutInflater)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)

        viewModel = ViewModelProvider(this)[MainViewModel::class.java]

        binding.textView.text = viewModel.count.toString()

        binding.btnIncrement.setOnClickListener {
            viewModel.increment()
            binding.textView.text = viewModel.count.toString()
        }
    }
}

View Model Factory

so we use ViewModelProvider() class to create viewModel object

But problem is that we can not pass argument or value to create viewModel instance with our initial value

Problem : Can not Initialize our viewModel object

Solution:

we need to create our custom ViewModelProvider class that can provide initialized viewModel object

For That We Will our class with ViewModelProvider.Factory class and override its methods


package com.example.androidarchitecture

import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider

class MainViewModelProvider(val counter : Int) : ViewModelProvider.Factory {

    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        return MainViewModel(counter) as T
    }
}

Factory will take Data and create and return initialized viewModel Object

How to Use View Model Provider Factory

Instead of creating viewModel object by Default Factory


viewModel = ViewModelProvider(this@MainActivity)[MainViewModel::class.java]

We need to specify Our Custom ViewModelProvider.Factory class object

It will take Parameter that will be used to create viewModel Object with Initial Value.


viewModel = ViewModelProvider(this@MainActivity,MainViewModelProvider(10))[MainViewModel::class.java]