I’m very new in compose, please say why it happens: I look into `isSystemInDarkTheme()` sources [1]:...

# compose

[iamthevoid]

I’m very new in compose, please say why it happens:
I look into isSystemInDarkTheme() sources [1]:
but when i try to change uiMode [2]
i get nothing [3]
If i open Configuration sources i see that uiMode just public int so (theoretically) it must changes and reads as expected but it doesn’t

[Zach Klippenstein (he/him) [MOD]]

For future reference, it’s courteous to post longer code snippets inside a thread instead of posting in the channel directly, to make the channel more scrollable.

[iamthevoid]

Sorry, i’ll fix it quickly

[Zach Klippenstein (he/him) [MOD]]

The apply {} function doesn’t actually do anything, it’s just syntactic sugar. Calling something.apply {} is an expression that just returns something – it doesn’t have any side effects.

[iamthevoid]

[1]

@Composable
@ReadOnlyComposable
fun isSystemInDarkTheme(): Boolean {
    val uiMode = LocalConfiguration.current.uiMode
    return (uiMode and Configuration.UI_MODE_NIGHT_MASK) == Configuration.UI_MODE_NIGHT_YES
}

[iamthevoid]

[2]

@Composable
private fun setDarkTheme() {
    LocalConfiguration.current.apply {
        uiMode = uiMode or Configuration.UI_MODE_NIGHT_MASK
    }
}

@Composable
private fun setLightTheme() {
    LocalConfiguration.current.apply {
        uiMode = uiMode and Configuration.UI_MODE_NIGHT_MASK.inv()
    }
}

[iamthevoid]

[3]

class TypographyTest {

    @get:Rule
    val composeTestRule = createAndroidComposeRule<ComponentActivity>()

    @Test
    fun testTypography() {
        composeTestRule.setContent {

            setLightTheme()
            Assert.assertEquals(false, isSystemInDarkTheme())

            setDarkTheme()
            Assert.assertEquals(true, isSystemInDarkTheme()) // Assertion error
.........

[cb]

You need to use CompositionLocalProvider to change a local value: https://developer.android.com/reference/kotlin/androidx/compose/runtime/package-summary#CompositionLocalProvider(kotlin.Array,kotlin.Function0)

[cb]

Also, I recommend looking at Jetchat for how to test in both light/dark, and in a portable way: https://github.com/android/compose-samples/blob/main/Jetchat/app/src/androidTest/java/com/example/compose/jetchat/ConversationTest.kt

[iamthevoid]

@cb thx, but jetchat example not applicable in my case. I want to “mock” call isSystemInDarkTheme() directly if it possible, so i’ll try look to CompositionLocalProvider

[iamthevoid]

@Zach Klippenstein (he/him) [MOD] I understand what actually apply do and try to change receiver’s (LocalConfiguration.current) field in my snippet. It should work.

[Zach Klippenstein (he/him) [MOD]]

That’s the same as doing a.value = 34 - apply doesn’t actually do anything there except shift syntax around

[Zach Klippenstein (he/him) [MOD]]

So what your code is doing is just mutating a field inside the Configuration, but because Configuration isn’t aware of compose, changing its fields won’t trigger recomposition at all.

[Zach Klippenstein (he/him) [MOD]]

What you’d need to do is create a copy of the configuration, mutate the field on the copy, and then pass the copy down using CompositionLocalProvider like Chris suggested. But that’s a lot of unnecessary work when you could just create a more explicit mechanism for selecting between light and dark themes.

[cb]

Yeah, you no longer need to mock system calls like this for unit testing. IMO, your tests should not rely on any Android specific APIs now. If they are, then they're probably assuming too much.

If you wanted to test the system API, write a separate test which changes the uiMode (say via AppCompatDelegate.setDefaultNightMode) and assert that the resolved theme is light/dark.

[cb]

@ppvi probably has opinions here too

[iamthevoid]

Let me to explain.

In my app i should use design system of my org. I develop android sdk for design system too. Core of all is colors. Colors module just returns colors set depends on isSystemInDarkTheme() . Tests placed in another (Typography) module. Typography uses colors and doesn’t aware about day-night theme. I try to check typography changes when system theme changes. There is no more simpler way to check it than mocking isSystemInDarkTheme call (i quess)

[iamthevoid]

I don’t want to expose any parts of colors module for simplify testing, because they should be internal for color module, but mocking typography module logic is not enough for proper testing

[cb]

Sure, but I think there's a split here:
• Most of tests should verify that the components + styling updates when it is told to use light /dark theme. This can be written to be platform agnostic.
• Then the second type verifies that the (system) toggle for light/dark works. This could look at, and modify the Android configuration.
Up to you what you do of course, but mocking system APIs usually ends up in lots of pain.