Android Flow vs StateFlow

Question

I recently started using Flows in Android. I read that Flows are cold StateFlows are hot, then why should we prefer using StateFlows for Android over Flows? Won't it be better to use Flows as they will stop the producer when the app goes to the background? Is there a scenario in Android development where Flows should be used over Stateflow?

Answer 1

The Advantage of the StateFlow over a regular Flow is that the former will stop wasting resources if the State is not the desired one.

If you look at the official StateFlow documentation you'll notice in the example the key part of this, and I quote:

// Trigger the flow and start listening for values.
// Note that this happens when lifecycle is STARTED and stops
// collecting when the lifecycle is STOPPED

This last bit is important. A Hot flow that will be collecting values so as long as the scope in which it was started is Started, and will stop when stopped. This is not what a normal Flow/Livedata would do thus wasting potential resources keeping a series of components working while, for example, the UI is stopped.

It's important to understand that LiveData and StateFlow are similar with two important distinctions (related with the above, and explained in the linked documentation):

• StateFlow requires an initial state to be passed in to the constructor, while LiveData does not.

• LiveData.observe() automatically unregisters the consumer when the view goes to the STOPPED state, whereas collecting from a StateFlow or any other flow does not stop collecting automatically. To achieve the same behavior, you need to collect the flow from a Lifecycle.repeatOnLifecycle block.

In other words, you could say that StateFlow is a potentially more efficient LiveData that gives you the ability to say: If the state is XYZ, then stop collecting and using resources because I have my reasons (e.g.: my UI is gone, so i don't need to listen to stuff I cannot process).