Chaining Modifier based on certain conditions in Android Compose

Question

I want to apply modifier in such a way that if the width is provided, it should use the provided width, else use the max-width available.

I am applying the Modifier in the below fashion, but the result is not as expected. The view width is going haywire. Requesting guidance here.

val myModifier = Modifier.padding(
    start = 4.dp, end = 4.dp, top = 8.dp, bottom = 8.dp
)

if (viewWidth == null)
    myModifier.then(Modifier.fillParentMaxWidth(1f))
else
    myModifier.then(Modifier.width(viewWidth))

myModifier.then(
    Modifier.height(viewHeight ?: 100.dp)
        .clickable(onClick = { listener.onItemClick(item) })
)

Answer 1

Modifier has a then function to concatenate the current modifier with another modifier. This then function returns a new modifier that you have not used it. You have to re-initialize your myModifier variable with the returned modifier.

Check the below code:

var myModifier = Modifier.padding(
    start = 4.dp, end = 4.dp, top = 8.dp, bottom = 8.dp
)

if (viewWidth == null)
  myModifier = myModifier.then(Modifier.fillParentMaxWidth(1f))
else
  myModifier = myModifier.then(Modifier.width(viewWidth))

myModifier = myModifier.then(
  Modifier
    .height(viewHeight ?: 100.dp)
    .clickable(onClick = { listener.onItemClick(item) })
)

Answer 2

You can create a conditional modifier via an extension function:

fun Modifier.conditional(condition : Boolean, modifier : Modifier.() -> Modifier) : Modifier {
    return if (condition) {
        modifier.invoke(this)
    } else {
        this
    }
}

This lets you chain a modifier in a conditional block like this:

val applySpecialBackground : Boolean = [...]
Column(
    modifier = Modifier
        .fillMaxWidth()
        .conditional(applySpecialBackground) {
            background(Color.Red)
        }
        .padding(16.dp)

) { [...] }

It will only apply the conditional modifier when the condition is true .

Answer 3

You can also use Modifier.then in a more compact way:

val modifier = Modifier
            .padding(start = 4.dp, end = 4.dp, top = 8.dp, bottom = 8.dp)
            .then(if(viewWidth == null) Modifier.fillMaxWidth(1f) else Modifier.width(viewWidth))
            .height(viewHeight ?: 100.dp)
            .clickable(onClick = { listener.onItemClick(item) })

See: https://jetc.dev/slack/2020-12-13-conditional-modifiers.html

Answer 4

All the above answers are good enough but i would like to add my one cent. I found this snippet more readable and clean.

   ClickableText(
                modifier = Modifier
                    .let {
                        if (selectedPosition == index) {
                            return@let it
                                .background(
                                    Green200,
                                    shape = RoundedCornerShape(12.dp)
                                )

                        }
                        it
                    }
                    .padding(horizontal = 12.dp, vertical = 4.dp),
                text = AnnotatedString(categories[index]),
                style = TextStyle(
                    fontSize = 14.sp,
                    textAlign = TextAlign.Center
                ),
                onClick = {
                    selectedPosition = index
                }
            )