How can I set the Paint style when I use drawLine in Android Jetpack Compose?

Question

I'm learning Jetpack Compose Canvas. I know that drawIntoCanvas() — Provides access to draw directly with the underlying Canvas. This is helpful for situations to re-use alternative drawing logic in combination with DrawScope.

I want to use different style line for different goal, such as:

Style 1:  color = Color.Blue , strokeWidth = 3f，...   
Style 2:  color = Color.Red ,  strokeWidth = 4f, ...   

I can re_use thses styles with Code B. I hope to I can re_use these style with Code A, how can I do?

Code A

@Composable
fun setCanvas() {
    Canvas(
        modifier = Modifier
            .fillMaxSize()
    ) {
        val canvasWidth = size.width
        val canvasHeight = size.height

        drawLine(
            start = Offset(x = 0f, y = canvasHeight),
            end = Offset(x = canvasWidth, y = 0f),
            color = Color.Blue,
            strokeWidth = 3f      
        )
    }

}

Code B

@Composable
fun setCanvas() {
    val linePaint1 = Paint()
    linePaint1.isAntiAlias = true
    linePaint1.style = PaintingStyle.Stroke
    linePaint1.color = Color.Blue


    val linePaint2 = Paint()
    linePaint2.isAntiAlias = true
    linePaint2.style = PaintingStyle.Stroke
    linePaint2.color = Color.Red


    Canvas(
        modifier = Modifier
            .fillMaxSize()
    ) {
        val canvasWidth = size.width
        val canvasHeight = size.height

        drawIntoCanvas {
            it.drawLine(
               Offset(x = 0f, y = canvasHeight),
               Offset(x = canvasWidth, y = 0f),
               linePaint1
            )

            it.drawLine(
               Offset(x = 10f, y = canvasHeight-50),
               Offset(x = canvasWidth-10, y = 10f),
               linePaint2
            )

        }

    }

}

Answer 1

There is no reason not to use drawInCanvas, and the DrawScope you are calling in A is meant to be stateless and does not depens on androids Paint class. If you must use paint objects, you could create an extension like so:

fun DrawScope.drawLine(
    start: Offset?,
    end: Offset?,
    paint: Paint,
    blendMode: BlendMode? = DefaultBlendMode
) = drawLine(
    start,
    end,
    paint.strokeWidth,
    paint.strokeCap.toStrokeCap(),
    paint.pathEffect.toComposePathEffect(),
    paint.alpha.toFloat() / 255f,
    paint.colorFilter.asComposeColorFilter(),
    blendMode
)

fun Paint.Cap.toStrokeCap() = when(this) {
    Paint.Cap.BUTT -> StrokeCap.Butt
    Paint.Cap.SQUARE -> StrokeCap.Square
    Paint.Cap.ROUND -> StrokeCap.Round
}

Since there is only a difference in color, I encourage you not to use this method and just call the existing drawLine function. If you must use paints, use this.