When writing a composable that accepts a mandatory...

# compose

[Mark]

When writing a composable that accepts a mandatory click handler, I’d prefer to put that as the last argument, but this violates the recommendation to put a Modifier arg after all args with no defaults. What’s the reasoning here, and am I just resigned to shifting the click handler arg to be before the modifier arg?

[curioustechizen]

I prefer to reserve the trailing lambda argument for a Composable lambda. A click handler is not Composable so I place it before the modifier.

[Mark]

In my particular use case, I have a composable for a Dialog Button, and there really isn’t much to configure other than the label and the click handler. It’s the same for any leaf node composable, I think.

[Mark]

Another example is a composable (emitting content) which uses the builder pattern. So the builderAction would be a mandatory arg but must go before Modifier and therefore the caller cannot use a trailing lambda for the builder arg.

[mattinger]

If you're asking why that's recommended, it's because of the way kotlin resolves parameters. If you have default values for parameters and you skip one, everything after it has to be explicitly called out as a named argument. So the example below can't compile until you put the modifier parameter after the text.

@Composable
@Preview
fun SomeComposablePreview() {
    SomeComposable("foobar")
}

@Composable
fun SomeComposable(
    modifier: Modifier = Modifier,
    text: String,
) {
    
}

[mattinger]

Also as others have said, trailing lambas in compose are best suited for composable content. If you have a function as your last parameter, Android Studio will instinctively autocomplete your function into the trailing lambda syntax, which gets annoying. Especially if you're like me and prefer named arguments for most things.

[mattinger]

And the parameter thing is a general kotlin recommendation, not specific to compose really.

[Mark]

I have no issue with your example above, because there is no reason to place a non-lambda arg after the optional modifier arg. I don’t really understand your last point. AS auto-completing a mandatory click handler, is a good thing IMO, just like for when you have a last arg composable lambda.

It looks way better to have:

fun FooButton("foo") {
    doSomethingOnClick(...)
}

So I understand the argument when the composable is accepting a composable content arg, but I’m talking about ones that don’t.

[mattinger]

Except that when people read compose code, they expect trailing lamba blocks of a composable function to be composable code. It's about conventions.

[Mark]

Okay, that’s a stronger argument

[mattinger]

That said, there are times where i don't have nested composable content, and have multiple callback functions. And i just live with it.

[ephemient]

androidx.compose.material.Button has onClick: () -> Unit as the first parameter

[Mark]

But that also has a content arg at the end

[Mark]

So I guess the content arg is an exception to general rule of arg ordering

[ephemient]

https://android.googlesource.com/platform/frameworks/support/+/androidx-main/compose/docs/compose-api-guidelines.md

Layout functions SHOULD place their primary or most common @Composable function parameter in the last parameter position to permit the use of Kotlin's trailing lambda syntax for that parameter.

yes

[ephemient]

one where that isn't the case: androidx.compose.ui.layout.Layout has the content: @Composable () -> Unit in non-trailing position, but that's more consistent with the other overloads taking contents: List<@Composable () -> Unit> parameters in the same argument position, and a MeasurePolicy ends up in the final position instead, usually written with a SAM-converted trailing lambda

[ephemient]

but I can't think of other composable layouts with trailing non-composable lambdas

[Mark]

But I’m really only talking about composable non-layouts (leaf nodes?). Whatever the terminology is.

[ephemient]

any @Composable function returning Unit is called a layout, because its only purpose to be called is for the side-effect of emitting UI

[ephemient]

hmm. that's my thinking but I guess that disagrees with the doc

[ephemient]

those are called Elements, according to it

[Mark]

BTW, compose-lints doesn’t complain if I put a single non-null mandatory click handler () -> Unit as the last argument. And sure enough fits the message we get when there is an issue:

Parameters in a composable function should be ordered following this pattern: params without defaults, modifiers, params with defaults and optionally, a trailing function that might not have a default param.

[Filip Wiesner]

I agree with Matthew

Except that when people read compose code, they expect trailing lamba blocks of a composable function to be composable code. It's about conventions.

That is the main argument for me. Doesn't matter if it's leaf composable or not. I expect UI elements with trailing lambda to be "composable". I don't want to check the documentation to know if it is or isn't composable and the annotation is not visible when typing parameters.

[Mark]

Makes sense but it’s interesting that compose-lints allows for any trailing lambda, not just composable ones.

[Filip Wiesner]

Maybe it's hard to write lint check for annotation of function parameter 😄 I don't know