Why A works and B doesn t Isn t `remember` just to keep the

Question

Why A works and B doesn t Isn t `remember` just to keep the value across recompositions `mutableStateOf` is not enough to trigger recompositions ``` Composable Preview fun Test var a by remember mutableStateOf false var b by mutableStateOf false val c by derivedStateOf a amp amp b Column Text text = A $a Text text = B $b Text text = C $c Button onClick = a = a Text A Button onClick = b = b Text B ```

mikehearn · Answer

Every time the Test function is recomposed re executed `b` will be recreated from scratch

mikehearn · Answer

You need `remember ` to stop that happening and ensure the object persists even across re executions of the function Yeah it s a bit weird Think of the function as if it s a class and `remember ` as if it s a property if that helps Then you can see that `b` will be just a normal local variable

Tgo1014 · Answer

But `b` is a object that keep the value no So if I do `b = b` why it goes back to false I don t think the entire screen is being recreated no

krzysztof · Answer

Calling `b = b` will recompose `Test ` again This is because the `Column` is inlined so that the recomposition scope goes up to `Test` itself And calling `Test` itself will create a new member variable `b` with `false` value

mikehearn · Answer

Composition is a fancy term for re execution Every time anything changes in your UI conceptually the entire code from `application ` or `setContent ` on down is re executed fresh The only way to preserve data is either use global variables or use `remember`

mikehearn · Answer

`b` is not an object that keeps its value A `MutableState` is just an observable variable with fancy extras It can trigger event handlers when it changes but it has nothing to say about how long it lasts in the heap It s no different to allocating any other object as a local variable

Tgo1014 · Answer

I thought when you changed `b` it would just recompose what s using the value of `b` thinking face

krzysztof · Answer

Because that s true gt the entire code from `application ` or `setContent ` on down is re executed fresh is wrong statement in that regards and would be super duper inefficient to have in any ui framework

Tgo1014 · Answer

Ok so it s as I imagined the `Test ` is not recomposed so why would the value be lost That s the bit I m missing

krzysztof · Answer

In your example like I said your Test would be recomposed as whole because the the `b` value is read in a scope of it

krzysztof · Answer

If you do this ``` Column Text text = A $a Text text = B $b Text text = C $c Button onClick = a = a Text A Button onClick = b = b Text B $ b note here ``` Then your `b` will keep the value until `a` is changed

krzysztof · Answer

that s because of recomposition scope where `b` is no longer read within `Test` scope but rather in Button s `content` scope `a` is still read within `Test` scope so if that changes compose calls `Test` function again

Tgo1014 · Answer

So basically anytime a `State` changes Compose will retrigger everything from the earliest access point of the `State`

krzysztof · Answer

Yes exactly The point to know is that Compose function creates recomposition scope so if any Snapshot value which is data holder created by `mutableStateOf` is read within that scope compose will re run that composable if that value changes

krzysztof · Answer

I can suggest this article by Vinay which is great in explaining this part <https www jetpackcompose app articles donut hole skipping in jetpack compose>

Tgo1014 · Answer

Thanks < krzysztof> super helpful I ll read the article smile

krzysztof · Answer

you re welcome good luck

mikehearn · Answer

Yes that s why I said conceptually It ll only re execute parts but that s an optimization