Im working with a Canvas, but once it drawn, it re...

# compose

[Lisandro Di Meo]

Im working with a Canvas, but once it drawn, it redraws every composition, causing non desirable effects. Anyone know how to avoid this kind of situation?

[nlindberg]

Put inside a composable that only receives data that should cause a redraw.

[Lisandro Di Meo]

🤔 the case is, I only want it to draw once

fun Background(){
    val path = Path()
    val paint = Paint()
    path.fillType = PathFillType.EvenOdd
    paint.apply {
        strokeWidth = 10f
        color = BlueRibbon
        style = PaintingStyle.Fill
        isAntiAlias = true
    }
    Canvas(modifier = Modifier
        .fillMaxSize() , onDraw = {
        val canvasWidth = size.width
        val canvasHeight = size.height/4
        val offsetForVertex = 300f
        println("$canvasHeight $canvasWidth")
        paint.shader = LinearGradientShader(from = Offset(canvasWidth, 0f), to = Offset(0f, canvasHeight), listOf(SkyBlue, BlueRibbon, Idk))
        drawIntoCanvas {
            path.moveTo(0f,canvasHeight)
            path.cubicTo(0f, canvasHeight, canvasWidth/2, canvasHeight + offsetForVertex, canvasWidth, canvasHeight)
            path.lineTo(canvasWidth, 0f)
            path.lineTo(0f, 0f)
            path.lineTo(0f, canvasHeight)
            path.close()
            it.drawPath(path, paint)
        }
    })
}

[Lisandro Di Meo]

And it actually doesnt receive nothing

[Lisandro Di Meo]

Oh, im dumb. Actually I had to move the path & paint init inside onDrawn 🤣

[Loney Chou]

It's better to use drawWithCache, cache them inside the block, then use them in onDraw.

[Loney Chou]

Spacer(
    modifier = Modifier
        .fillMaxSize()
        .drawWithCache {
            val path = Path()
            // ...
            onDraw {
                drawPath(path, ...)
            }
        }
)

[Lisandro Di Meo]

So, I shouldn’t be using a Canvas? 🤔

[Loney Chou]

Canvas is just a wrapper of Modifier.drawBehind, and inside drawBehind you get DrawScope as this, meaning this is where your stuff gets drawn each time it needs to be drawn. If you create Path and stuff like that it will end up creating them each time it draws, which may affect the performance. In contrast, drawWithCache will give you a CacheDrawScope, but it's not a DrawScope, instead it's where you build your draw params, and you use them inside the final onDraw or other callback builders which is like drawBehind. By using drawWithCache it means stuff like Path will only gets recreated when it needs like a state inside the cache builder changes.

[Lisandro Di Meo]

Thanks for the explanation, will switch to drawWithCache 😄

[Lisandro Di Meo]

I'd like to know more about drawing and such, if you know any extra resources about, it will be appreciated 🙌