A ViewModelScope is an extension property defined for each ViewModel in the app. Any coroutine launched in this scope is automatically canceled if the ViewModel is cleared.

On Android, you can use a scope to cancel all running coroutines when, for example, the user navigates away from an Activity or Fragment.

Scopes also allow you to specify a default dispatcher. A dispatcher controls which thread runs a coroutine.

For coroutines started by the UI, it is typically correct to start them on Dispatchers.Main which is the main thread on Android. They won't block the main thread while suspended. This saves extra thread switches, but it's possible to switch to another dispatcher if you need to do something fairly complex out of the main thread (e.g. JSON parsing).

Example ​

The AndroidX lifecycle-viewmodel-ktx library adds a viewModelScope as an extension function to ViewModels. This scope is bound to Dispatchers.Main and will get cancelled once the ViewModel is cleared.

implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:x.x.x"

Switching from threads to coroutines ​

private fun updateTaps() {
	// Using Threads:
	//		tapCount++
	//		BACKGROUND.submit {
	//			Thread.sleep(1_000)
	//			_taps.postValue("${tapCount} taps")
	//		}

	// using ViewModelScope:
	viewModelScope.launch {
		tapCount++
		delay(1000) // suspends the coroutine for 1s
		_taps.postValue("$tapCount taps")
	}
}

BACKGROUND ExecutorService is an examplary implememnation of running in a background thread, not important here.

In the viewModelScope based code the default dispatcher of Dispathers.Main is used. The delay function is a suspend function.