如何在 Jetpack Compose 中使用 Coil 显示自定义可组合占位符？

Question

I need to show a custom placeholder in Jetpack Compose using Coil, but that placeholder is not a drawable, it is a composable function that I customized. Is it possible to do this with the Coil? This is the code snippet where I use the Coil:

Image(
    modifier = Modifier
        .size(120.dp)
        .align(Alignment.CenterHorizontally),
    painter = rememberImagePainter(
        data = entry.imageUrl,
        builder = {
            crossfade(true)
            MyPlaceholder(resourceId = R.drawable.ic_video)
        },
    ),
    contentDescription = entry.pokemonName
)

This is my custom placeholder compose function:

@Composable
fun MyPlaceholder(@DrawableRes resourceId: Int) {
    Surface(
        modifier = Modifier.fillMaxSize(),
        color = Color(0xFFE0E0E0)
    ) {
        Box(
            modifier = Modifier.fillMaxSize(),
            contentAlignment = Alignment.Center,
        ) {
            Surface(
                modifier = Modifier.size(30.dp),
                shape = CircleShape,
                color = Color.White
            ) {
                Image(
                    modifier = Modifier
                        .padding(
                            PaddingValues(
                                start = 11.25.dp,
                                top = 9.25.dp,
                                end = 9.25.dp,
                                bottom = 9.25.dp
                            )
                        )
                        .fillMaxSize(),
                    painter = painterResource(id = resourceId),
                    contentDescription = null
                )
            }
        }
    }
}

My gradle (Coil):

// Coil
implementation 'io.coil-kt:coil-compose:1.4.0'

Answer 1

Coil has no built-in support for composable placeholders.

You can put your composable inside Box and display the placeholder over Image depending on the state.

In my example I display it if the state is Loading or Error . You can add another view parameter for the Error case and use Crossfade instead of AnimatedVisibility .

Also I add Modifier.matchParentSize() to the Image to follow parent size calculated on the modifier parameter. You can't pass modifier parameter directly to Image , because modifiers like align only work for direct children, that's why you alway have to pass it to the container view.

@Composable
fun Image(
    painter: ImagePainter,
    placeholder: @Composable () -> Unit,
    contentDescription: String?,
    modifier: Modifier = Modifier,
    alignment: Alignment = Alignment.Center,
    contentScale: ContentScale = ContentScale.Fit,
    alpha: Float = DefaultAlpha,
    colorFilter: ColorFilter? = null,
) {
    Box(modifier) {
        Image(
            painter = painter,
            contentDescription = contentDescription,
            alignment = alignment,
            contentScale = contentScale,
            alpha = alpha,
            colorFilter = colorFilter,
            modifier = Modifier.matchParentSize()
        )

        AnimatedVisibility(
            visible = when (painter.state) {
                is ImagePainter.State.Empty,
                is ImagePainter.State.Success,
                -> false
                is ImagePainter.State.Loading,
                is ImagePainter.State.Error,
                -> true
            }
        ) {
            placeholder()
        }
    }
}

Usage:

Image(
    painter = rememberImagePainter(imageUrl),
    placeholder = {
        CustomComposableView(...)
    },
    contentDescription = "...",
    modifier = Modifier
        ...
)