自定义瀑布式流布局

class WaterFlowLayout constructor(context: Context, attrs: AttributeSet) : ViewGroup(context, attrs) {

    override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {
        //定义子view的初始位置为parent传入的初始位置
        var left: Int
        var right: Int
        var top: Int
        var bottom: Int
        //开始遍历
        //当前行高
        var currentHeight = 0
        list.forEachIndexed { index, arrayList ->

            var currentWidth = 0
            arrayList.forEach {
                val marginLayoutParams = it.layoutParams as MarginLayoutParams
                top = t + currentHeight + marginLayoutParams.topMargin
                left = l + currentWidth + marginLayoutParams.leftMargin
                right = left + it.layoutParams.width + marginLayoutParams.rightMargin
                bottom = top + it.layoutParams.height + marginLayoutParams.bottomMargin
                currentWidth = right
                it.layout(left, top, right,bottom)
            }
            currentHeight += heightList.get(index)
        }
    }

    override fun generateLayoutParams(attrs: AttributeSet?): LayoutParams {
        return MarginLayoutParams(context, attrs)
    }

    /**
     * 因为onLayout时需要遍历各个子view确定他们的位置
     * 所以我们需要一个list来保存各个子view
     * list的每个元素表示一行
     */
    private var list = arrayListOf<ArrayList<View>>()

    /**
     * 同样为了onLayout时确定每一行的行高
     * 我们再加一个List保存行高信息
     */
    private var heightList = arrayListOf<Int>()

    /**
     * 通过源码我们知道  这里传进来的时parent的约束信息
     */
    override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        //因为onMeasure方法会调用两次  所以需要clear
        list.clear()
        heightList.clear()
        //根据传进来的参数，获取父容器的宽高
        val parentWidth = MeasureSpec.getSize(widthMeasureSpec)
        val parentHeight = MeasureSpec.getSize(heightMeasureSpec)

        //定义变量记录最终测量出的宽高信息
        var measureWidth = 0
        var measureHeight = 0
        //获取父容器传进来的Mode信息
        val widthMode = MeasureSpec.getMode(widthMeasureSpec)
        val heightMode = MeasureSpec.getMode(heightMeasureSpec)
        //如果Mode是match_parent, 那么直接返回父容器的宽高
        if (widthMode == MeasureSpec.EXACTLY && heightMode == MeasureSpec.EXACTLY){
            measureWidth = parentWidth
            measureHeight = parentHeight
        }else{
            //如果不是EXACTLY，就需要计算宽高

            //定义当前行宽，高
            var currentWidth = 0
            var currentHeight = 0

            //定义一个list保存当前行里的child
            var childList = arrayListOf<View>()
            //遍历childs
            children.forEach {
                val marginLayoutParams = it.layoutParams as MarginLayoutParams
                //每个元素的宽等于它本身的宽加左右Margin
                val childWidth = it.layoutParams.width + marginLayoutParams.leftMargin + marginLayoutParams.rightMargin
                //每个元素的高等于它本身的高加上下Margin
                val childHeight =  it.layoutParams.height +marginLayoutParams.topMargin + marginLayoutParams.bottomMargin

                //如果当前已有行宽+该子view行宽已经大于父容器给定的行宽， 则需要换行
                if (currentWidth + childWidth > parentWidth){
                    //换行
                    //保存上一行的行高
                    heightList.add(currentHeight)

                    //换行后，新行宽度就等于子View的宽度
                    currentWidth = childWidth
                    //新行高度等于子View的高度
                    currentHeight = childHeight
                    //最终测量出来的宽度为各行的最大宽度
                    measureWidth = Math.max(currentWidth, measureWidth)
                    //最终测量出来的高度为各行高度累加
                    measureHeight += currentHeight

                    //换行的话, 先把原本的当前行加到list中
                    list.add(childList)

                    //再把childlist置空，放入新的childView
                    childList = arrayListOf(it)

                }else {
                    //否则， 将该子view放在当前行
                    currentWidth +=childWidth
                    //高度取已有行高和当前子view高度之间的最大值
                    currentHeight = Math.max(currentHeight, childHeight)
                    //最终测量出来的宽度为各行的最大宽度
                    measureWidth = Math.max(currentWidth, measureWidth)

                    //将这个子view放在当前行
                    childList.add(it)
                }
            }
            //for循环执行完之后，将最后一个当前行加入到list
            Log.d("-----", "end child list size:"+childList.size )
            list.add(childList)
            measureHeight += currentHeight
            heightList.add(currentHeight)
        }
        //必须set, 否则会抛出IllegalStateException
        setMeasuredDimension(measureWidth, measureHeight)
    }
}