Does `remember` only work with `MutableState` variables?

Question

I was just doing some testing for one of my Composables when I found this very bizarre behaviour.

This code:-

{ //Composable-Scope
 var fired = remember { false }
 if(!fired) { Fire(); fired = true }
}

calls the methof Fire() on every recomposition.

On the other hand, however,

{ //Composable Scope
 var fired by remember { mutableStateOf(false) }
 if(!fired) { Fire(); fired = true }
}

calls Fire() only upon the first Composition.

If this is the intended behaviour, is there a good explanation for why remember is not made universal? If it is not the intended behavior, I'll probably just file a bug.

Seems like only states are remembered. Can't find any good explanation. Kindly add the link to the bug here, in case you raise one. — Abhimanyu, Nov 06 '21 at 20:09

score 3 · Accepted Answer · answered Nov 06 '21 at 20:54

3

This is intended behaviour.

During the first composition value of remember lambda is calculated and cached. On each next recomposition calling remember { ... } will return the cached value.

In your first code block you're creating a variable, which will get the same initial value on each recomposition. You can modify it during the composition code, but on recomposition it'll be reset, as if you were not using remember at all.

With mutable state remember does the same thing: it remembers the content of the lambda. But in this case the result is a wrapper class instance. This instance is the same between recompositions, but you can modify the value property of this object, that's why it lives thought recompositions.

answered Nov 06 '21 at 20:54

Phil Dukhov

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Wait why is the wrapper class instance same between recompositions? – Richard Onslow Roper Nov 06 '21 at 21:00
@MARSK because it's cached by `remember`. If you don't specify a different key, and the configuration isn't changed, (which may trigger remember values reset), your `remember` lambda won't be re-called, so the object will be the same. You can see it by removing the delegation(`by`) and adding logs to see the variable address. – Phil Dukhov Nov 06 '21 at 21:15
Ok so why is the block 1 var not cached? – Richard Onslow Roper Nov 06 '21 at 21:26
I specified no key, and neither a configuration change occurred – Richard Onslow Roper Nov 06 '21 at 21:26
1

@MARSK it's cached. The cached value is `false`. When you're modifying the variable later, it has nothing to do with `remember`, so `remember` can't know it was modified. – Phil Dukhov Nov 06 '21 at 22:01
Oh. How stupid. Can I delete this question so I do not appear dumb, please? – Richard Onslow Roper Nov 06 '21 at 22:03

Does `remember` only work with `MutableState` variables?

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