Custom layout ignores size of composeable-CodePudding

I'm currently playing around with custom Layouts in Jetpack Compose. This is what i got so far just for testing:

@Preview(widthDp = 1000, heightDp = 1000)
@Composable
fun MyLayout() {
    Layout({ Box(Modifier.size(48.dp).background(Color.Blue)) }) { measurables, constraints ->
        val placeables = measurables.map { it.measure(constraints) }
        layout(constraints.maxWidth, constraints.maxHeight) {
            placeables.forEach { it.place(0, 0) }
        }
    }
}

My problem here is that the Box fills the whole layout and not only it's size of 48.dp. Can someone explain to me why this is? I read here about creating custom layouts but couldn't find anything usefull.

CodePudding user response：

Layout size is determined by the size passed into layout(width, height).

Your Layout doesn't have any modifiers, that's why it has maxWidth/maxHeight constraints equal to available spacing. Using layout(constraints.maxWidth, constraints.maxHeight) has the same effect as applying Modifier.fillMaxSize. Actually with Modifier.fillMaxSize your'll set both max and min constraints to the available size.

To solve this you have two options:

Apply Modifier.size to the Layout itself, instead of applying to the Box - in this case max/min constraints will be 48.dp
Calculate layout size depending on placeable. The actual code depend on what layout do you expect to get. Your code looks like Box analog, so you need maximum of all placeable sizes:
```
layout(placeables.maxOfOrNull { it.width } ?: 0, placeables.maxOfOrNull { it.height } ?: 0) {
```