Is it reasonable to have a `@Composable` function ...

# compose

[chao]

Is it reasonable to have a @Composable function with return type? I am trying to evaluate the feasibility to write JVM unit tests for them:

@Composable
fun spanStyleRange(textAttribute: TextAttribute): AnnotatedString.Range<SpanStyle> {
    val spanStyle: SpanStyle = when (textAttribute.type) {
        TextAttributeType.HASHTAG ->
            SpanStyle(color = MaterialTheme.colors.primary, fontWeight = FontWeight.Bold)
        else -> SpanStyle()
    }
    return AnnotatedString.Range(
        item = spanStyle,
        start = textAttribute.start,
        end = textAttribute.start + textAttribute.length
    )
}

In thie example, spanStyleRange need to be annotated with @Composable because it is using MaterialTheme.colors

[Daniel]

Absolutely. If you couldn't do this, you wouldn't be able to refactor out code that doesn't show something on the screen. The guidelines say that composables that return a value (like say remember ) should follow standard naming conventions.
https://github.com/androidx/androidx/blob/androidx-main/compose/docs/compose-api-guidelines.md#naming-composable-functions-that-return-values

[chao]

Thanks. I remember reading this coding guideline a few months back but forgot about this completely. Now my next question would be is there an easy way to write unit test for this

[Daniel]

Probably not? Depends on what you mean by unit test

[Daniel]

This is like a method on an activity, it requires a mocked or real whole Android device

[Daniel]

So you'll need to test this in an integration test. The integration test can just call only that function though

[chao]

What I meant was not an Android instrumentation test. The unit test I am looking forward is to run on Jvm.

[Adam Powell]

you can do this but you'll probably want a non-@Composable overload that accepts a MaterialColors as a parameter and does the heavy lifting. That way you can more easily use and test the non-composable version with all of the logic, but the composable version without the parameter can be used as a convenience.

[Adam Powell]

as for easy ways to write unit tests for these, we've got a number of utilities in the runtime for doing non-UI composable tests host-side like this one: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/[…]pose/runtime/CompositionTests.kt;l=3017?q=localRecomposerTest

[Adam Powell]

not public in the test artifacts, but pretty short/simple to copy and use if you find them useful

[chao]

Thanks for the great information. I will look into this

[chao]

I had another case uses ImageBitmap.Companion.imageResource which is a top-level extension. It feels quite awkward to extract a non-@Composable function to accept (@DrawableRes id) -> Unit as a parameter.