如何在 Jetpack Compose 中显示带按钮 onclick 的小吃店

Question

I want to show snackbar with a button onclick in Jetpack Compose
I tried this

Button(onClick = {
    Snackbar(action = {}) {
        Text("hello")
    }
} 

But AS said "@Composable invocations can only happen from the context of a @Composable function"
Shall you give me a nice program.
I wish it can run in Button.onclick()

Answer 1

You'll need two things:

1. SnackbarHostState - which will manage the state of your Snackbar ( you would usually get that from the ScaffoldState )
2. CoroutineScope - which will be responsible for showing your Snackbar

---

@Composable
fun SnackbarDemo() {
  val scaffoldState = rememberScaffoldState() // this contains the `SnackbarHostState`
  val coroutineScope = rememberCoroutineScope()

  Scaffold(
        modifier = Modifier,
        scaffoldState = scaffoldState // attaching `scaffoldState` to the `Scaffold`
    ) {
        Button(
            onClick = {
                coroutineScope.launch { // using the `coroutineScope` to `launch` showing the snackbar
                    // taking the `snackbarHostState` from the attached `scaffoldState`
                    val snackbarResult = scaffoldState.snackbarHostState.showSnackbar(
                        message = "This is your message",
                        actionLabel = "Do something."
                    )
                    when (snackbarResult) {
                        Dismissed -> Log.d("SnackbarDemo", "Dismissed")
                        ActionPerformed -> Log.d("SnackbarDemo", "Snackbar's button clicked")
                    }
                }
            }
        ) {
            Text(text = "A button that shows a Snackbar")
        }
    }
}

Answer 2

Building on Bartek's answer you can also use Compose's side-effects . Then you don't have to manage the CoroutineScope itself.

Since Composables itself should be side-effect free , it is recommended to make use of Compose's Effect APIs so that those side effects are executed in a predictable manner.

In your specific case you can use the LaunchedEffect API to run suspend functions in the scope of a composable. The example code would look like the following:

@Composable
fun SnackbarDemo() {
    val scaffoldState = rememberScaffoldState() // this contains the `SnackbarHostState`
    val (showSnackBar, setShowSnackBar) = remember {
        mutableStateOf(false)
    }
    if (showSnackBar) {
        LaunchedEffect(scaffoldState.snackbarHostState) {
            // Show snackbar using a coroutine, when the coroutine is cancelled the
            // snackbar will automatically dismiss. This coroutine will cancel whenever
            // `showSnackBar` is false, and only start when `showSnackBar` is true
            // (due to the above if-check), or if `scaffoldState.snackbarHostState` changes.
            val result = scaffoldState.snackbarHostState.showSnackbar(
                message = "Error message",
                actionLabel = "Retry message"
            )
            when (result) {
                SnackbarResult.Dismissed -> {
                    setShowSnackBar(false)
                }
                SnackbarResult.ActionPerformed -> {
                    setShowSnackBar(false)
                    // perform action here
                }
            }
        }
    }
    Scaffold(
        modifier = Modifier,
        scaffoldState = scaffoldState // attaching `scaffoldState` to the `Scaffold`
    ) {
        Button(
            onClick = {
                setShowSnackBar(true)
            }
        ) {
            Text(text = "A button that shows a Snackbar")
        }
    }
}

Answer 3

If you use new Material3 there is new field in Scaffold: "snackbarHost"

val snackbarHostState = remember { SnackbarHostState() }
val scope = rememberCoroutineScope()

Scaffold(
  snackbarHost = { SnackbarHost(snackbarHostState) },
  ...

            // for showing snackbar in onClick for example:
            scope.launch {
                snackbarHostState.showSnackbar(
                    "Snackbar Test"
                )
            }

Answer 4

Just in case you need a snackbar as a separate function to call in other screens with custom parameters, and without using scaffold, I use this below.

The parameter openSnackbar: (Boolean) -> Unit is used to reset the opening condition var openMySnackbar by remember { mutableStateOf(false) } and allow open snackbar other times.

    @Composable
    fun SnackbarWithoutScaffold(
        message: String,
        showSb: Boolean,
        openSnackbar: (Boolean) -> Unit
    ) {
    
        val snackState = remember { SnackbarHostState() }
        val snackScope = rememberCoroutineScope()
    
        SnackbarHost(
            modifier = Modifier,
            hostState = snackState
        ){
            Snackbar(
                snackbarData = it,
                backgroundColor = Color.White,
                contentColor = Color.Blue
            )
        }
    
    
        if (showSb){
            LaunchedEffect(Unit) {
                snackScope.launch { snackState.showSnackbar(message) }
                openSnackbar(false)
            }
   
        }
    
    
    }

And used like this:

@Composable fun MyScreen() {

var openMySnackbar by remember { mutableStateOf(false)  }
var snackBarMessage by remember { mutableStateOf("") }

Column() {

    Button(
        shape = RoundedCornerShape(50.dp),
        onClick = {
            openMySnackbar = true
            snackBarMessage = "Your message"
        },
    ) {
        Text(text = "Open Snackbar")
    }

    SnackbarWithoutScaffold(snackBarMessage, openMySnackbar, , {openMySnackbar = it})
}

}

Answer 5

You can use this

@Composable
fun MainScreen() {
    val coroutineScope = rememberCoroutineScope()
    val showSnackBar: (
        message: String?,
        actionLabel: String,
        actionPerformed: () -> Unit,
        dismissed: () -> Unit
    ) -> Unit = { message, actionLabel, actionPerformed, dismissed ->
        coroutineScope.launch {
            val snackBarResult = scaffoldState.snackbarHostState.showSnackbar(
                message = message.toString(),
                actionLabel = actionLabel
            )
            when (snackBarResult) {
                SnackbarResult.ActionPerformed -> actionPerformed.invoke()
                SnackbarResult.Dismissed -> dismissed.invoke()
            }
        }
    }


    showSnackBar.invoke(
        "YOUR_MESSAGE",
        "ACTION_LABEL",
        {
         //TODO ON ACTION PERFORMED
        },
        {
         //TODO ON DISMISSED
        }
    )
}

Answer 6

Let me add an other answer, i was not using scaffold and was using state hoisting

val loginState by viewModel.uiState.collectAsState() // STATE HOISTING
val snackbarHostState = remember { SnackbarHostState() }
val message = stringResource(id = R.string.error_login) 
if (loginState.snackbarVisible) { // When passing true, show snackbar
    LaunchedEffect(snackbarHostState) {
        snackbarHostState.showSnackbar(
            message = message,
            actionLabel = "Do something."
        )
    }
}


Box(
    modifier = Modifier.fillMaxSize()
) {
    // Your screen content
    // ...
    SnackbarHost(
        hostState = snackbarHostState,
        modifier = Modifier.align(Alignment.BottomCenter)
    ) // Snackbar location
}

Answer 7

First of all you need a SnackbarHostState, you can pass this state down to your composable where you want to trigger a snackbar message. or if you use a scaffold use that one scaffoldState.snackbarHostState

val snackbarHostState = remember { SnackbarHostState() }

Showing a snackbar is a side effect and should be wrapped in a LaunchEffect Composable

@Composable
fun MyScreen(
    scaffoldState: ScaffoldState = rememberScaffoldState()
) {
   Scaffold(scaffoldState = scaffoldState) {    

     Button(onClick = {
      LaunchedEffect(scaffoldState.snackbarHostState) {
         scaffoldState.snackbarHostState.showSnackbar(
                  message = "hello world",
                  actionLabel = "Retry"
         )
      }

    
     // ...
    }
}

See more information here https://developer.android.com/topic/architecture/ui-layer/events#consuming-trigger-updates

and https://developer.android.com/jetpack/compose/side-effects

Answer 8

You can show snackBar without Scaffold using SnackBar Composable either.

Box(modifier = Modifier.fillMaxSize()) {
    var showSnackbar by remember {
        mutableStateOf(false)
    }

    LaunchedEffect(key1 = showSnackbar) {
        if (showSnackbar) {
            delay(2000)
            showSnackbar = false
        }
    }


    Column {

        Button(onClick = {
            showSnackbar = !showSnackbar
        }) {
            Text("Show Snackbar")
        }
    }

    if (showSnackbar) {
        androidx.compose.material.Snackbar(modifier = Modifier.align(Alignment.BottomStart),
            action = {
                Text(text = "Action",
                    color = Color(0xffCE93D8),
                    modifier = Modifier.clickable {
                        showSnackbar = false
                    }
                )
            }) {
            androidx.compose.material.Text("Message")
        }
    }
}

LaunchedEffect is to remove it from composition. You can use a slide animation if you want to.