ViewModel in Android

Overview

ViewModel is lifecycle aware. It is a Model for your views like Activities and Fragments. Data require for your screen is stored at one place i.e. ViewModel. The data may involve the information to format a data, or it could be an accumulated data such as scores earned by a user or it could be a business logic that you display onto the screen. ViewModel is used only to store data, it doesn't care how you are using this data.

Code without ViewModel

 class MainActivity : AppCompatActivity() {
    private var counter: Int = 0;
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setContent()
    }

    /* Do not declare the fun as private because this function is declared in the xml file of the activity*/
    fun increment(view: View?) {
        counter++
        setContent()
    }

    private fun setContent() {
        tvCounter.text = counter.toString()
    }

 }

Steps to Follow

• Add the required dependency in build.gradle (Module) file.

 def lifecycle_version = "2.3.0"
 def arch_version = "2.1.0"
 // ViewModel
 implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
 // LiveData
 implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
 // Lifecycles only (without ViewModel or LiveData)
 implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"
 // Saved state module for ViewModel
 implementation "androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version"
 // Annotation processor
 annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"
 // alternately - if using Java8, use the following instead of lifecycle-compiler
 implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"
 // optional - helpers for implementing LifecycleOwner in a Service
 implementation "androidx.lifecycle:lifecycle-service:$lifecycle_version"
 // optional - ProcessLifecycleOwner provides a lifecycle for the whole application process
 implementation "androidx.lifecycle:lifecycle-process:$lifecycle_version"
 // optional - ReactiveStreams support for LiveData
 implementation "androidx.lifecycle:lifecycle-reactivestreams:$lifecycle_version"
 // optional - Test helpers for LiveData
 testImplementation "androidx.arch.core:core-testing:$arch_version"

• Create a class and extend it with ViewModel() class. Write the logic in this class.

 class MainViewModel : ViewModel() {
    /* Do not make the variable private because you have to use this variable in MainActivity */
    var counter: Int = 0;

    /* Do not make the function private because you have to use this function in MainActivity */
    fun increment() {
        counter++
    }
 }

• Create an instance of ViewModel in MainActvity (or the Activity where you want to display the data) and use its instance to run the method defined in ViewModel class and / or use its instance to display the properties defined in ViewModel

 class MainActivity : AppCompatActivity() {
    private lateinit var mainViewModel: MainViewModel
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        mainViewModel = ViewModelProvider(this).get(MainViewModel::class.java)
        setContent()
    }

    /* Do not declare the fun as private because this function is declared in the xml file of MainActivity*/
    fun increment(view: View?) {
        mainViewModel.increment()
        setContent()
    }

    private fun setContent() {
        tvCounter.text = mainViewModel.counter.toString()
    }

 }

Do's and Dont's

• Do not hold a reference of View in ViewModel because such reference could improve the risk of memory leak.

Reference

1. https://github.com/iamraajkanchan/ViewModel_Android