How can I reset certain states when some other states change? I don't know if that's the best way t...

# compose

[Md. Nazmun Sadat Khan]

How can I reset certain states when some other states change?

I don't know if that's the best way to state my problem, so let me elaborate.

I have a composable function that displays an image (well a box with an image inside), that supports double tap, pinch-zoom, panning and glide. All of the states like current scale level and offset are internal to the composable. It takes a ImageBitmap as its input.

Now, in the main activity, there's the option to display different images. Now whenever I display a different image, I want to reset current zoom level and pan. Of course, when the ImageBitmap passed to my composable, only the displayed image is changed and any previous zoom and pan are retained.

How do I reset these internal zoom and pan states, whenever ImageBitmap is changed, without exposing the internal states to the main activity?

[Oleksandr Balan]

I guess you can use derivedStateOf
See: https://developer.android.com/reference/kotlin/androidx/compose/runtime/package-summary#derivedStateOf(kotlin.Function0)

[Md. Nazmun Sadat Khan]

I think that might work, but wouldn't it look inelegant? I don't really need to use the changed value of ImageBitmap, only that its value has changed.

var scale by remember {
        derivedStateOf { 
              imageBitmap
              0f
        }
}

Would the code look something like this? Would it reset to 0 whenever imageBitmap changes?

[Oleksandr Balan]

True, looks weird.

Did you try to use image as a key for remember?
https://developer.android.com/reference/kotlin/androidx/compose/runtime/package-summary#remember(kotlin.Any,kotlin.Function0)

[Md. Nazmun Sadat Khan]

No, I didn't know remember had a key argument. I think that's what I am looking for.

So the code should look this, right?

var scale by remember(imageBitmap) { 0f }

One other thing, it's a minor question but why are all the key arguments named key1? key doesn't seem to be a keyword in Kotlin.

[Oleksandr Balan]

I think you want scale to be a mutable state, so:

var scale by remember(imageBitmap) { mutableStateOf(0f) }

There are also key2 and key3 , so that’s why they choose key1 for name of the first key 🤷

[Md. Nazmun Sadat Khan]

Oh right, I forgot to use mutableStateOf

Thanks for all your help! 😊