Android Compose实现伸缩ToolBar的思路详解

// 应用栏高度
private val toolBarHeight = 56.dp
// 导航图标大小
private val navigationIconSize = 50.dp

//整体布局实现
Box(modifier = Modifier
        .height(scrollableAppBarHeight) //scrollableAppBarHeight 为高度参数,为外部获取
        .fillMaxWidth()
) {
    Image(painter = painterResource(id = backgroundImageId), contentDescription = "background", contentScale = ContentScale.FillBounds)

    // 自定义应用栏
    Row(
        modifier = modifier
            .height(toolBarHeight) //设置高度为toolBarHeight
            .fillMaxWidth(),
        verticalAlignment = Alignment.CenterVertically //设置垂直方向为居中对齐
    ) {
        // 导航图标
        Box(modifier = Modifier.size(navigationIconSize),contentAlignment = Alignment.Center) {
            navigationIcon()
        }
    }

    // title定义
    Box(
        modifier = Modifier
            .height(toolBarHeight) //和ToolBar同高
            .fillMaxWidth()
            .align(Alignment.BottomStart),
        contentAlignment = Alignment.CenterStart
    ) {
        Text(text = title,
            color = Color.White,
            modifier = Modifier.padding(start = 20.dp).matchParentSize(), // 使用 matchParentSize 修饰符保证不影响父 Box尺寸
            fontSize = 20.sp
        )
    }
}

// title定义
Box(
    modifier = Modifier
        .height(toolBarHeight) //和ToolBar同高
        .fillMaxWidth()
        .align(Alignment.BottomStart),
    contentAlignment = Alignment.CenterStart
) {
    Text(text = title,
            color = Color.White,
            modifier = Modifier.padding(start = 20.dp).matchParentSize(), // 使用 matchParentSize 修饰符保证不影响父 Box尺寸
            fontSize = 20.sp
        )
}

modifier = Modifier
        .height(toolBarHeight) //和ToolBar同高
        .fillMaxWidth()

modifier = Modifier
        .height(toolBarHeight) //和ToolBar同高
        .fillMaxWidth()
        .align(Alignment.BottomStart)

contentAlignment = Alignment.CenterStart

// 应用栏最大向上偏移量
val maxOffsetHeightPx = with(LocalDensity.current) { scrollableAppBarHeight.roundToPx().toFloat() - toolBarHeight.roundToPx().toFloat() }

// Title 偏移量参考值
val titleOffsetWidthReferenceValue = with(LocalDensity.current) { navigationIconSize.roundToPx().toFloat() }

val toolbarOffsetHeightPx: MutableState<Float> //向上偏移量

@Composable
fun ScrollableAppBar(
    modifier: Modifier = Modifier,
    title: String = stringResource(id = R.string.app_name), //默认为应用名
    navigationIcon: @Composable () -> Unit, //导航图标
    @DrawableRes backgroundImageId: Int, // 背景图片
    scrollableAppBarHeight: Dp, //定义的ScrollableAppBar高度
    toolbarOffsetHeightPx: MutableState<Float> //向上偏移量
) {
    Box(modifier = Modifier
        .height(scrollableAppBarHeight)
        .offset {
            IntOffset(
              x = 0,
              y = toolbarOffsetHeightPx.value.roundToInt() //设置偏移量
            )
        }
        .fillMaxWidth()
    ) {
        .... // 背景图等内容
    }
}

// 自定义应用栏
Row(
    modifier = modifier
        .offset {
            IntOffset(
                x = 0,
                y = -toolbarOffsetHeightPx.value.roundToInt() //保证应用栏是始终不动的
            )
        }
        .height(toolBarHeight)
        .fillMaxWidth(),
    verticalAlignment = Alignment.CenterVertically
) {
    ... // 导航图标
}

x = -((toolbarOffsetHeightPx.value / maxOffsetHeightPx) * titleOffsetWidthReferenceValue).roundToInt()

// title部分
Box(
    modifier = Modifier
        .height(toolBarHeight) //和ToolBar同高
        .fillMaxWidth()
        .align(Alignment.BottomStart)
        .offset {
            IntOffset(
                x = -((toolbarOffsetHeightPx.value / maxOffsetHeightPx) * titleOffsetWidthReferenceValue).roundToInt(), //水平方向位移
                y = 0
            )
        },
    contentAlignment = Alignment.CenterStart
) {
    ... //title部分
}