How can I create new Modifiers that depend on some other Modifiers' values? Say, for the sake of an example, I want to make the child components's height to be half of the one that is passed in from the parent plus some constant (thus not being able to use fillMaxHeight(fraction: Float)).

@Composable
fun View() {
   MyComposable(modifier = Modifier.size(40.dp))
}

@Composable
fun MyComposable(
  modifier: Modifier          // Assuming this modifier contains a size
) {
  Box(modifier) {             // Setting the the size from the passed in modifier
    val childHeightModifier = Modifier.height(???) // <-- Wanted to be some specific value depending on parent size
    
    Box(childHeightModifier) {
       ...
    }
  }
}

CodePudding user response：

You can use BoxWithConstraints for this particular usecase, it provides constraints imposed by parent to its children:

@Composable
fun View() {
   MyComposable(modifier = Modifier.size(40.dp))
}

@Composable
fun MyComposable(modifier: Modifier) {
  BoxWithConstraints(modifier) { // this: BoxWithConstraintsScope
    val childHeightModifier = Modifier.height(maxHeight * 0.5f)
    Box(childHeightModifier) {
       ...
    }
  }
}

As for communication between Modifiers, there's a ModifierLocal system in the works, which will allow doing exactly that.

CodePudding user response：

Generally there's no way to do that, you can pass this value as an argument.

But in this particular case you can set your view size depending on parent using:

1. .fillMaxHeight(fraction = 0.5f) where fraction is part of the parent view, e.g. equivalent of .height(heightDp * 0.5f)
2. .padding(left = 10.dp)(or right) to descries the size by a static value.

Changing order of fillMaxHeight and padding will give you different effect too(.height(heightDp * 0.5f - 10.dp) vs (heightDp - 10.dp) * 0.5f), check out more about modifiers order in this answer