android - Jetpack Compose - Order of Modifiers

Question

Documentation says that Modifiers are applied from the left. But from this example it looks like they are applied from the right: First border and then padding because there is no space between text and border

Text("Hi there!", Modifier.padding(10.dp).border(2.dp, Color.Magenta))

Answer 1

There’s Layouts in Jetpack Compose codelab containing Layout modifiers under the hood step which explains the modifier order, see "Order matters" section.

order matters when chaining modifiers as they're applied to the composable they modify from earlier to later, meaning that the measurement and layout of the modifiers on the left will affect the modifier on the right. The final size of the composable depends on all modifiers passed as a parameter. First, modifiers will update the constraints from left to right, and then, they return back the size from right to left.

To understand it better I'd recommend to figure out how layouts work in Compose. In short, padding() is a LayoutModifer, it takes in some constraints, measures its child size based on a projection of that constraints and places the child at some coordinates.

Let’s see an example:

Box(
  modifier = Modifier
    .border(1.dp, Color.Red)
    .size(32.dp)
    .padding(8.dp)
    .border(1.dp, Color.Blue)
)

And the result:

But let's swap the .size() and the .padding()

Box(
  modifier = Modifier
    .border(1.dp, Color.Red)
    .padding(8.dp)
    .size(32.dp)
    .border(1.dp, Color.Blue)
)

Now we have a different result:

I hope this sample helps you to figure out how the modifiers are applied.

One can expect that the red border should be the closest to the box since it was added first, so the order might seem reversed, but such an order has pros too. Let’s take a look at this composable:

@Composable
fun MyFancyButton(modifier: Modifier = Modifier) {
  Text(
    text = "Ok",
    modifier = modifier
      .clickable(onClick = { /*do something*/ })
      .background(Color.Blue, RoundedCornerShape(4.dp))
      .padding(8.dp)
  )
}

Just by moving the modifier to the arguments the composable allows its parents to add additional modifiers such as extra margin. Because the lastly added modifiers are the closest to the button, the border and the inner padding won’t be affected.