Hi, I try to render a blue rectangle with fixed wi...

# compose

[Joseph Cheng]

Hi, I try to render a blue rectangle with fixed width and height, but it always fill the parent.

setContent {
            MaterialTheme {
                ColoredRect(color = Color.Blue, width = 100.dp, height = 50.dp)
            }
        }

But once I put a Container as the parent, it works as expected. May I know why?

setContent {
            MaterialTheme {
                Container {
                    ColoredRect(color = Color.Blue, width = 100.dp, height = 50.dp)
                }
            }
        }

[Klaas Kabini]

From what I observed is that if DrawShape or DrawImage is wrapped inside a Container that is the root of a composable function then compose will assume that the shape or image is a background and make it fill ignoring the dimensions parameters you specified. This looks like it was by design and not a bug.

[Mihai Popa]

Yep, the root of the layout hierarchy will need to fill all available space. When you add the Container this will fill the screen and the ColoredRect will be able to use its desired size

[Joseph Cheng]

May I know where is the source code defining this behavior? I try to trace the source code but not found.

[Mihai Popa]

This behavior comes from the View system - this will onMeasure the AndroidComposeView with MeasureSpec.EXACTLY which we then translate to fixed constraints in the Compose hierarchy, determining the root of the layout hierarchy to be forced due to constraints to fill the whole screen

[Mihai Popa]

Then you can see in Container.kt that it will looseMin the constraints before measuring its children, so its children will not be forced anymore to fill the whole screen

[Joseph Cheng]

Nice, thanks for your explanation.