ShaderBrush#createShader state reads do not invalidate Text composable

[halilozercan]

Describe the bug
Brush instances created using the abstract ShaderBrush class, does not animate when applied on Text.

Affected platforms
Only tested on;

• Desktop (Windows, Linux, macOS)

Versions

• Libraries:
  • Compose Multiplatform version: 1.6.10

To Reproduce
Steps to reproduce the behavior:

1. Run this code snippet:

https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/foundation/foundation/integration-tests/foundation-demos/src/main/java/androidx/compose/foundation/demos/text/BrushDemo.kt;l=204-233

fun AnimatedBrush() {
    val infiniteTransition = rememberInfiniteTransition()
    val radius by infiniteTransition.animateFloat(
        initialValue = 100f,
        targetValue = 300f,
        animationSpec = infiniteRepeatable(
            animation = tween(durationMillis = 1000, easing = LinearEasing),
            repeatMode = RepeatMode.Reverse
        )
    )
    val brush = remember {
        // postpone the state read to shader creation time which happens during draw.
        ShaderBrush { size ->
            RadialGradientShader(
                center = size.center,
                radius = radius,
                colors = RainbowColors,
                colorStops = RainbowStops,
                tileMode = TileMode.Mirror
            )
        }
    }
    Text(
        text = loremIpsum(wordCount = 29),
        style = TextStyle(
            brush = brush,
            fontSize = 30.sp
        )
    )
}

Expected behavior
Expecting text to have an animated brush coloring.

Additional context

Android handles this by creating the shader in a derivedStateOf. This way any state read that happens in createShader invalidates the derived state and only re-runs the draw phase. Otherwise devs need to change the brush instance for every animation frame which can get very costly due to unnecessary recomposition.

https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/ui/ui-text/src/androidMain/kotlin/androidx/compose/ui/text/platform/AndroidTextPaint.android.kt;l=111-143

[m-sasha]

Looks like we need to apply (androidx/androidx@1631196) to SkiaTextPaint (where it says // Copied from AndroidTextPaint.) as part of merging in 1.7 (that change is in AndroidTextPaint.android.kt of the integration branch).

@igordmn

[MatkovIvan]

Unfortunately the fix from mentioned commit won't work for CMP because unlike Android we do not control Paint state during paragraph redrawing - it's fulling inside C++/skia. So to apply such changes, currently it requires full re-layout/re-creation of skia's paragraph. Even with a few layers of caching that we currently have, I doubt that it will animate smoothly. To properly do this, global changes in text rendering/layout are required.

[rock3r]

😭

[MatkovIvan]

Adopted it with re-layouting in JetBrains/compose-multiplatform-core#1395

[okushnikov]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.