A painter that wraps another painter to overwrite its color filter, alpha, and/or onDraw.

complex_forwarding_painter.kt

/**
 * Create and return a new [Painter] that wraps [painter] with its [alpha], [colorFilter], or [onDraw] overwritten.
 */
fun forwardingPainter(
    painter: Painter,
    alpha: Float = DefaultAlpha,
    colorFilter: ColorFilter? = null,
    onDraw: DrawScope.(ForwardingDrawInfo) -> Unit = DefaultOnDraw,
): Painter = ForwardingPainter(painter, alpha, colorFilter, onDraw)

data class ForwardingDrawInfo(
    val painter: Painter,
    val alpha: Float,
    val colorFilter: ColorFilter?,
)

private class ForwardingPainter(
    private val painter: Painter,
    private var alpha: Float,
    private var colorFilter: ColorFilter?,
    private val onDraw: DrawScope.(ForwardingDrawInfo) -> Unit,
) : Painter() {

    private var info = newInfo()

    override val intrinsicSize get() = painter.intrinsicSize

    override fun applyAlpha(alpha: Float): Boolean {
        if (alpha != DefaultAlpha) {
            this.alpha = alpha
            this.info = newInfo()
        }
        return true
    }

    override fun applyColorFilter(colorFilter: ColorFilter?): Boolean {
        if (colorFilter != null) {
            this.colorFilter = colorFilter
            this.info = newInfo()
        }
        return true
    }

    override fun DrawScope.onDraw() = onDraw(info)

    private fun newInfo() = ForwardingDrawInfo(painter, alpha, colorFilter)
}

private val DefaultOnDraw: DrawScope.(ForwardingDrawInfo) -> Unit = { info ->
    with(info.painter) {
        draw(size, info.alpha, info.colorFilter)
    }
}

As of revision 3, I notice that applyAlpha and applyColorFilter have conditionals that currently only apply the changes if the passed parameter is set to the default. I do not immediately see why a default checking guard is needed, but in any event I'm thinking that it was intended to be checking that the original value was still defaulted, so this.alpha == DefaultAlpha and this.colorFilter == null respectively. I wonder if the guard might be further improved by looking for a change instead, this.alpha != alpha and this.colorFilter != colorFilter? (Also the return values might be updated to reflect whether the value was applied.) Thanks for posting this.

    override fun applyAlpha(alpha: Float): Boolean {
        if (alpha == DefaultAlpha) {
            this.alpha = alpha
            this.info = newInfo()
        }
        return true
    }
    ...
override fun applyColorFilter(colorFilter: ColorFilter?): Boolean {
        if (colorFilter == null) {
            this.colorFilter = colorFilter
            this.info = newInfo()
        }
        return true
    }

May I ask, why is always setting null and the DefaultAlpha?

@desgraci That was a typo! Updated the gist.