深入解析Android View体系

// 自定义View测量示例
override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
    val minWidth = 100.dpToPx() // 最小宽度
    val width = when (MeasureSpec.getMode(widthMeasureSpec)) {
        MeasureSpec.EXACTLY -> MeasureSpec.getSize(widthMeasureSpec)
        MeasureSpec.AT_MOST -> minOf(minWidth, MeasureSpec.getSize(widthMeasureSpec))
        else -> minWidth // UNSPECIFIED
    }
    setMeasuredDimension(width, resolveHeight(heightMeasureSpec))
}

// 自定义ViewGroup布局实现
override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {
    var topPos = paddingTop
    for (i in 0 until childCount) {
        val child = getChildAt(i)
        if (child.visibility != GONE) {
            val childHeight = child.measuredHeight
            child.layout(
                paddingLeft, 
                topPos,
                paddingLeft + child.measuredWidth,
                topPos + childHeight
            )
            topPos += childHeight + spacing
        }
    }
}

public void draw(Canvas canvas) {
    // Step 1. 绘制背景
    drawBackground(canvas);
    
    // Step 2. 保存图层（可选）
    if (!dirtyOpaque) onDraw(canvas);
    
    // Step 3. 绘制子View
    dispatchDraw(canvas);
    
    // Step 4. 绘制装饰（滚动条等）
    onDrawForeground(canvas);
}