Hello Compose enthusiasts :slightly_smiling_face: ...

# compose

[svenjacobs]

Hello Compose enthusiasts 🙂

If I have something like this (in a ViewModel for example)

private val _selected = mutableStateOf(false)

val isSelected: Boolean
    get() = _selected.value

and use isSelected in a composable, it gets recomposed when _selected changes although isSelected is a Boolean and not State<Boolean>. How does it work? What is the Compose compiler “magic” here? I would like to understand what’s going on.

[svenjacobs]

I have seen this pattern in state classes of Compose UI for example.

[shikasd]

Compose doesn't use compiler magic for states, it records the state as "read" whenever .value is executed.

[shikasd]

Practically, the state system has a global Snapshot variable with a readObserver field on it. Whenever you call State.value, it calls that observer provided by composition.

[svenjacobs]

And when a state is read the next restartable composable in this scope is registered to be recomposed when that state changes?

[Zach Klippenstein (he/him) [MOD]]

Every time a restartable composable is recomposed, it’s ran with a listener to learn what states are read when the composable function is run. Those state objects are associated with that composable. There’s a global listener for state writes (and applied snapshots) that triggers a new frame when something is written. Then on that frame, we use the set of changed states to figure out which composables need rerunning, and repeat the process.

[Zach Klippenstein (he/him) [MOD]]

This listener mechanism is the simplest part of snapshots actually. Every state object read/write literally just looks at a thread local to find the current snapshot, then calls the read/write listener stored with that snapshot. When you create a snapshot, you have the option of specifying read and/or write observers.

[Zach Klippenstein (he/him) [MOD]]

just threadlocals and callbacks

[svenjacobs]

Thanks @shikasd and @Zach Klippenstein (he/him) [MOD] for the explanation. I always thought that there is an explicit dependency between a State and a composable, meaning it needs to be a State (on the first layer) when reading from a composable. I wasn’t aware that there is an implicit dependency which works even when reading a State through a layer of abstractions.

[svenjacobs]

This and the succeeding article by Zach about states and snapshots really helped me in better understanding the concepts 🤯 Thanks!