Android 如何判断堆肥是否为空

如果希望具有可为空的内容，则需要将声明设置为

@Composable
fun BottomSectionWrapper(
    modifier: Modifier = Modifier,
    content: @Composable (() -> Unit)? = null,
    bottomSection: @Composable (() -> Unit)? = null,
) {
    Column(modifier = modifier) {
        content?.invoke()
        bottomSection?.invoke()
    }
}

您可以像在此演示中一样使用它

@Preview
@Composable
private fun BottomSectionWrapperTest() {

    Column {
        var isSet by remember {
            mutableStateOf(false)
        }

        Button(onClick = { isSet = isSet.not() }) {
            Text("isSet: $isSet")
        }
        

        val bottomSection: (@Composable () -> Unit)? = if (isSet) {
            {
                Box(modifier = Modifier.size(100.dp).background(Color.Green)) {
                    Text("Bottom Section")
                }
            }

        } else null


        BottomSectionWrapper(
            modifier = Modifier.border(2.dp, Color.Red),
            content = {
                Text("Content Section")
            },
            bottomSection = bottomSection
        )
    }
}

如果您希望检测Composable是空的，还是有0.dp大小或任何大小，您可以使用Layout，具有不同的方法、尺寸设置和放置选项.

第二个Modifier.layoutId用于默认的合成体，如TextField、Tab

@Composable
fun BottomSectionWrapperLayout(
    modifier: Modifier = Modifier,
    content: @Composable () -> Unit = {},
    bottomSection: @Composable () -> Unit = {},
) {

    Layout(
        modifier = modifier,
        contents = listOf(content, bottomSection)
    ) { measurables: List<List<Measurable>>, constraints: Constraints ->

        val looseConstraints = constraints.copy(minWidth = 0, minHeight = 0)

        val contentPlaceable: Placeable? =
            measurables.first().firstOrNull()?.measure(looseConstraints)

        val bottomSectionPlaceable: Placeable? =
            measurables.last().firstOrNull()?.measure(looseConstraints)

        println("content: $contentPlaceable, bottomSectionPlaceable: $bottomSectionPlaceable")

        // you can set width and height as required, i make it as a Column
        // this width and height determines whether it's a Column, Row or a Box

        val contentWidth = contentPlaceable?.width ?: 0
        val contentHeight = contentPlaceable?.height ?: 0

        val bottomSectionWidth = bottomSectionPlaceable?.width ?: 0
        val bottomSectionHeight = bottomSectionPlaceable?.height ?: 0

        val totalWidth = if (constraints.hasFixedWidth && constraints.hasBoundedWidth)
            constraints.maxWidth else contentWidth.coerceAtLeast(bottomSectionWidth)
            .coerceIn(constraints.minWidth, constraints.maxWidth)

        val totalHeight = if (constraints.hasFixedHeight && constraints.hasBoundedHeight)
            constraints.maxHeight else (contentHeight + bottomSectionHeight)
            .coerceIn(constraints.minHeight, constraints.maxHeight)

        layout(totalWidth, totalHeight) {
            // You can place them according to your logic here
            contentPlaceable?.placeRelative(0, 0)
            bottomSectionPlaceable?.placeRelative(0, contentHeight)
        }
    }
}

对于这Layout，您可以通过判断Placeable是否为空(这意味着传递空的lambda)来检测它们中是否有任何一个为空，如果不为空，则可以判断它的大小.

@Preview
@Composable
private fun LayoutTest() {
    Column(modifier = Modifier.fillMaxSize()) {
        BottomSectionWrapperLayout(
            modifier = Modifier.border(2.dp, Color.Red),
            content = {
                Text("Content Section")
            },
            bottomSection = {
                Text("Bottom Section")
            }
        )

        Spacer(Modifier.height(20.dp))
        BottomSectionWrapperLayout(
            modifier = Modifier.border(2.dp, Color.Red),
            bottomSection = {
                Text("Bottom Section")
            }
        )

        Spacer(Modifier.height(20.dp))
        BottomSectionWrapperLayout(
            modifier = Modifier.border(2.dp, Color.Red),
            content = {
                Text("Content Section")
            }
        )
   
    }
}

如果您查看println，您可以看到在每种情况下，您将能够看到哪些是空的，哪些不是.此外，您还可以获得可放置的尺寸.

另一个选项是创建此布局，就像Textfield在第495行中所做的那样.

https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/material/material/src/commonMain/kotlin/androidx/compose/material/TextField.kt;l=495?q=TextField&ss=androidx%2Fplatform%2Fframeworks%2Fsupport个

但是使用这种方法，您将无法判断是否传递了空lambda，因为当它不为空时，它被放在一个Box中.然而，这种方法经常被使用，因为您可以通过ID而不是判断列表来判断和比较内容.

@Composable
fun BottomSectionWrapperLayout2(
    modifier: Modifier = Modifier,
    content: @Composable (() -> Unit)? = null,
    bottomSection: @Composable (() -> Unit)? = null,
) {

    Layout(
        modifier = modifier,
        content = {

            if (content != null) {
                Box(
                    modifier = Modifier.layoutId("content")
                ) {
                    content()
                }
            }

            if (bottomSection != null) {
                Box(
                    modifier = Modifier.layoutId("bottomSection")
                ) {
                    bottomSection()
                }
            }
        }
    ) { measurables: List<Measurable>, constraints: Constraints ->

        val looseConstraints = constraints.copy(minWidth = 0, minHeight = 0)

        val contentPlaceable: Placeable? =
            measurables.find { it.layoutId == "content" }?.measure(looseConstraints)
        val bottomSectionPlaceable =
            measurables.find { it.layoutId == "bottomSection" }?.measure(looseConstraints)

        // you can set width and height as required, i make it as a Column
        // this width and height determines whether it's a Column, Row or a Box

        val contentWidth = contentPlaceable?.width ?: 0
        val bottomSectionWidth = bottomSectionPlaceable?.width ?: 0

        val contentHeight = contentPlaceable?.height ?: 0
        val bottomSectionHeight = bottomSectionPlaceable?.height ?: 0

        val totalWidth = if (constraints.hasFixedWidth && constraints.hasBoundedWidth)
            constraints.maxWidth else contentWidth.coerceAtLeast(bottomSectionWidth)
            .coerceIn(constraints.minWidth, constraints.maxWidth)

        val totalHeight = if (constraints.hasFixedHeight && constraints.hasBoundedHeight)
            constraints.maxHeight else (contentHeight + bottomSectionHeight)
            .coerceIn(constraints.minHeight, constraints.maxHeight)


        layout(totalWidth, totalHeight) {
            contentPlaceable?.placeRelative(0, 0)
            bottomSectionPlaceable?.placeRelative(0, contentHeight)
        }
    }
}

用法与其他布局相同.