CODEWITHSUNDEEP
androidkotlinandroid-jetpack-compose
Kostya Dzyuba
Kostya Dzyuba

Reputation: 521

Manually observe SnapshotStateList

How to manually observe SnapshotStateList or MutableState? It said that mutableStateListOf returns list that can be observed, but I can't find such method. Is it true that only Compose can observe changes?

Upvotes: 3

Views: 5395

Answers (1)

chuckj
chuckj

Reputation: 29615

The easiest way to observe of a state object, such as the result of mutableStateOf or mutableStateListOf(), is by using snapshotFlow.

snapshotFlow's block parameter will be called when any state object it uses has been changed and the result of block will be emitted to the flow such as snapshotFlow { snapshotList.toList() }. Note the toList() is required here otherwise the snapshotList itself is used which the snapshotFlow doesn't consider changed (it is always the same instance) so it will only emit once. In compose runtime version 1.4, toList() was changed so that calling toList() on a SnapshotStateList does not perform a copy.

Alternately, you can use SnapshotStateObserver which enables multiplexing a single apply observer to observe a multitude of lambdas. This is what Compose UI layout and draw use, for example, to track when to recalculate the layout and when to replay drawing commands.

State objects do not have an observe or subscribe equivalent because Snapshots was designed and optimized to observe an open, dynamic set of objects read by a function and any function it calls (such as Compose needs); and no fast path for a single object (which Compose does not need) was implemented and is therefore expensive. Having an observe method of a state object would imply it is fast and cheap, which it is not.

Upvotes: 4

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