What is the difference between ViewModel and OnSavedInstanceState in Android?

ViewModel survives configuration changes (such as orientation or language change), preserving data. The lifecycle of ViewModel is tied to that of an Activity or Fragment, but it survives their recreation.

OnSavedInstanceState is part of the lifecycle of an Activity or Fragment. It is used to save a small amount of data before the component is destroyed (for example, due to low memory or configuration change). The data is stored in a Bundle.

Main differences:

• Lifecycle: ViewModel survives configuration changes, OnSavedInstanceState does not.
• Data volume: ViewModel is intended for storing data needed throughout the UI controller's lifecycle. OnSavedInstanceState is for saving a small state.
• Storage location: ViewModel is stored in a special storage provided by Architecture Components. OnSavedInstanceState saves data in a Bundle, which is restored upon UI controller creation.
• Purpose: ViewModel is meant for UI logic and data that survive configuration changes. OnSavedInstanceState is for saving user interface state at the moment of component destruction.

Criterion	ViewModel	OnSavedInstanceState
Survives configuration changes?	Yes	No
Typical data volume	Sufficiently large	Small
Storage mechanism	Special storage provided by Architecture Components	Bundle
Main purpose	UI logic and data	Saving user interface state

Example of using ViewModel:

// MyViewModel.kt
class MyViewModel : ViewModel() {
    val data: MutableLiveData<String> by lazy {
        MutableLiveData<String>()
    }

    init {
        // Load data
        data.value = "Initial data"
    }
}

Example of using OnSavedInstanceState:

// MyActivity.kt
override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    outState.putString("savedText", textView.text.toString())
}

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    if (savedInstanceState != null) {
        val savedText = savedInstanceState.getString("savedText")
        textView.text = savedText
    }
}