Android 获取 Compose 的布局宽度和高度答案

【问题标题】：Android getting layout width & height for ComposeAndroid 获取 Compose 的布局宽度和高度
【发布时间】：2021-08-31 21:50:41
【问题描述】：

如何使用 compose 获取实际大小值？如果在将数据提供给视图之前必须完成布局大小的操作？我不喜欢检查。当每次画布重绘时

    Box {
        var count by rememberSaveable {
            mutableStateOf(0)
        }
        var states = remember { mutableListOf<Rain>() }
        Canvas(modifier = Modifier.fillMaxSize()) {
            // i need the canvas' size so i initialize the object inside this scope, and check it every time when canvas redraws
            Log.e("canvas", "repainted")
            if(states.isEmpty()) states.addAll(MutableList(300) {
                Rain(size.width, size.height)
            })
            drawRect(color = Color.Black)
            repeat(300) {
                val c = count
                val x1 = states[it].x1
                val x2 = states[it].x2
                val y1 = states[it].y1
                val y2 = states[it].y2
                val color = states[it].color
                drawLine(start = Offset(x1, y1), end = Offset(x2, y2), color = color)
                states[it]()
            }
        }
        LaunchedEffect(isPaused) {
            while (!isPaused) {
                delay(40)
                count++
            }
        }
    }

【问题讨论】：

标签： android kotlin android-jetpack-compose

【解决方案1】：

您可以将canvasSize 保存到状态变量中，然后将derivedStateOf 用于您的states：只有在更改内部使用的任何可变状态时才会重新计算此块的内容，在这种情况下它将是canvasSize

Box {
    var count by rememberSaveable {
        mutableStateOf(0)
    }
    var canvasSize by remember { mutableStateOf(Size.Unspecified) }
    val states by remember(canvasSize) {
        derivedStateOf {
            List(300) {
                Rain(canvasSize.width, canvasSize.height)
            }
        }
    }
    Canvas(
        modifier = Modifier
            .fillMaxSize()
    ) {
        canvasSize = size
        drawRect(color = Color.Blue)
        repeat(300) {
            val x1 = states[it].x1
            val x2 = states[it].x2
            val y1 = states[it].y1
            val y2 = states[it].y2
            drawLine(start = Offset(x1, y1), end = Offset(x2, y2), color = color)
        }
        states[it]()
    }
    LaunchedEffect(isPaused) {
        while (!isPaused) {
            delay(40)
            count++
        }
    }
}

【讨论】：

对不起，我不知道如何使用记住在 drawScope 中创建状态，撰写调用只能在可组合函数的上下文中发生，它是这么说的。我必须实现我自己的？
你的意思是 Canvas 内部的可变状态本身就是一种记忆吗？
@user16799920 我误解了你的问题，检查更新
谢谢，它确实有效。我的语言不好理解是错误的。

【解决方案2】：

DrawScope 里面有一些属性，其中之一是size，它本身有 width、height、minDimension 和 maxDimension 属性。

Canvas {
       size.width
       size.height
}

其他人

@DrawScopeMarker
interface DrawScope : Density {

    /**
     * The current [DrawContext] that contains the dependencies
     * needed to create the drawing environment
     */
    val drawContext: DrawContext

    /**
     * Center of the current bounds of the drawing environment
     */
    val center: Offset
        get() = drawContext.size.center

    /**
     * Provides the dimensions of the current drawing environment
     */
    val size: Size
        get() = drawContext.size

    /**
     * The layout direction of the layout being drawn in.
     */
    val layoutDirection: LayoutDirection
...
}

【讨论】：