使用自定义ItemDecoration的各种姿势

自定义ItemDecoration实现间距：

• 1.继承RecyclerView.ItemDecoration重写其getItemOffsets方法：

override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) 

其中第一个参数outRect为Decoration间距的距离4个方向的距离，在RecycleView布局时会加载itemView的padding上，可以通过以下代码设置距离

outRect.set(left,top,right,bottom)

• 2.针对不同布局管理器兼容间距
  完整代码：

class SpanItemDecoration(val left: Int = 0, val right: Int = 0, val top: Int = 0, val bottom: Int = 0) : RecyclerView.ItemDecoration() {
        //实现边距
        override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
            when (parent.layoutManager) {
                is GridLayoutManager -> getGridLayoutItemOffsets(outRect, view, parent, state, (parent.layoutManager as GridLayoutManager).spanCount)
                is StaggeredGridLayoutManager -> getStaggeredGridHorizontalItemOffsets(outRect, view, parent, state, (parent.layoutManager as StaggeredGridLayoutManager).spanCount, (parent.layoutManager as StaggeredGridLayoutManager).orientation)
                is LinearLayoutManager -> getLinearLayoutItemOffsets(outRect, view, parent, state, (parent.layoutManager as LinearLayoutManager).orientation)
            }
        }

        /***
         * 针对LinearLayoutManager的边距
         */
        private fun getLinearLayoutItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State, orientation: Int) {
            val itemCount = parent.adapter?.itemCount ?: 0
            val currentCount = parent.getChildAdapterPosition(view)
            when (orientation) {
                LinearLayoutManager.HORIZONTAL -> outRect.set(left.dp, top.dp, if (itemCount - 1 == currentCount) right.dp else 0, bottom.dp)
                LinearLayoutManager.VERTICAL -> outRect.set(left.dp, top.dp, right.dp, if (itemCount - 1 == currentCount) bottom.dp else 0)
            }
        }

        /***
         * 针对GridLayoutManager和StaggeredGridLayoutManager的VERTICAL模式的设置边距离
         */
        private fun getGridLayoutItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State, spanCount: Int) {
            val itemCount = parent.adapter?.itemCount ?: 0
            val currentCount = parent.getChildAdapterPosition(view)
            //每一行的最后一个View才需要添加right
            val lastOfLine = currentCount % spanCount == spanCount - 1 || currentCount == itemCount - 1
            outRect.set(left.dp, top.dp, if (lastOfLine) right.dp else 0, if (lastOfLine) bottom.dp else 0)
        }

        /***
         * 针对StaggeredGridLayoutManager的设置边距离
         */
        private fun getStaggeredGridHorizontalItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State, spanCount: Int, orientation: Int) {
            val params = view.layoutParams as StaggeredGridLayoutManager.LayoutParams
            val spanIndex = params.spanIndex StaggeredGridLayoutManager.LayoutParams
            val spanIndex = params.spanIndex
            val lastOfLine = spanIndex == spanCount - 1
            when (orientation) {
                RecyclerView.HORIZONTAL -> {
                    outRect.set(left.dp, top.dp, 0, if (lastOfLine) bottom.dp else 0)
                }
                RecyclerView.VERTICAL -> {
                    outRect.set(left.dp, top.dp, if (lastOfLine) right.dp else 0, 0)
                }
            }

        }


    }

其中可以采用parent.adapter.itemCount获得所有子View的数量，通过parent.getChildAdapterPosition获得该View在adapter中的位置(所有item)，通过parent.getChildCount获得所有可见的item的数量
针对瀑布流布局，需要使用StaggeredGridLayoutManager.LayoutParams中的spanIndex属性判断设置的第几列的边距。

自定义ItemDecoration实现分割线和悬浮头部动画：

自定义ItemDecoration实现分割线：

• 1.实现RecyclerView.ItemDecoration接口，重写getItemOffsets方法为分割线留出空隙。

override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
            if (parent.layoutManager !is LinearLayoutManager) return
            val position = parent.getChildAdapterPosition(view)
            //如果是最后一个item就不需要画分割线
            val needDivider = position != parent.adapter?.itemCount ?: 0 - 1
            outRect.bottom = if (needDivider) dividerHeight.dp else 0
        }

• 2.重写onDraw方法对当前布局内的所有View绘制分割线

 override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
            val left: Int = parent.paddingLeft
            val right: Int = parent.width - parent.paddingRight
            //获取屏幕可见的view数量
            val childCount: Int = parent.childCount
            for (i in 0 until childCount) {
                //计算分割线起始位置
                val child: View = parent.getChildAt(i)
                val params = child
                        .layoutParams as RecyclerView.LayoutParams
                // divider的top 应该是 item的bottom 加上 marginBottom 再加上 Y方向上的位移  -->计算起始y坐标
                val top = child.bottom + params.bottomMargin +
                        child.translationY.roundToInt()
                // divider的bottom就是top加上divider的高度了
                val bottom = (top + dividerHeight.dp)         //计算结束y坐标
                c.drawRect(left.toFloat(), top.toFloat(), right.toFloat(), bottom.toFloat(), paint) //画笔自定义
            }
            super.onDraw(c, parent, state)

        }

实现悬浮头部：

• 1.实现悬浮原理：由于悬停头部需要在ItemView之上显示，所以绘制要重写onDrawOver来绘制。简单的图形可以使用Canvas绘制，复杂的布局需要使用自定义的View再将其layout后再使用Canvas绘制出来。由于如果使用onDrawOver，绘制的view会覆盖在itemView之上，这个时候需要跟随RecycleView滚动的话，可以采用Canvas的translate方法将画布移动再绘制

        //仍然需要在getItemOffsets为其留出空间，否则会覆盖在ItemView之上
        override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
            super.onDrawOver(c, parent, state)
            //获得布局中第一个View的位置
            val child: View = parent.getChildAt(0)
            if (parent.getChildAdapterPosition(child) != 0) {
                //获取是否是adapter第一个View -->这里可以控制只在第一个Vie时显示或者分组的时候显示
                // 如果第一个item已经被回收,没有显示在recycler view上,则不需要draw header
                return
            }
            val params = child
                    .layoutParams as RecyclerView.LayoutParams
            // divider的top 应该是 item的bottom 加上 marginBottom 再加上 Y方向上的位移
            val top = child.top + params.topMargin -
                    child.translationY.roundToInt()
            //移动画布达到绘制的效果随布局移动的效果 -->每次滚动都会调用重绘，从而每次的位置值可以通过itemView的顶部偏移算出画布的移动
            c.translate(0.toFloat(), top - dividerHeight.dp.toFloat())
            //模拟一个自定义的View
            val view = TextView(parent.context).apply {
                text = "1231412412412"
                height = dividerHeight.dp
            }
            //一定要先layout自定义的View
            view.layout(0, 0, 200.dp, dividerHeight.dp)
            view.draw(c)
            c.translate(0.toFloat(), -(top - dividerHeight.dp.toFloat()))
        }

实现粘性头部：在onDraw中绘制每个分组的头部，在onDrawOver中绘制常驻的粘性头部并且根据分组的最后一个距离顶上的距离移动画布实现在分组交换时对头部的过渡，其中头部可自定义

• 使用：

rv.addItemDecoration(StickHeaderItemDecoration(R.layout.stick_view, 80).apply {
            setTransfer(object : StickHeadTransfer() {
                override fun isFirst(position: Int): Boolean {
                    //判定位置是否为分组的第一个 可以根据自己的数据类型传递
                    return adapter.getData()[position].isFirst
                }

                override fun isLast(position: Int): Boolean {
                    //判定位置是否为分组的最后一个
                    return adapter.getData()[position].isLast
                }

                override fun bindData(stickView: View, position: Int) {
                    //绑定视图
                    val data = adapter.getData()[position]
                    stickView.findViewById<TextView>(R.id.tvTitle).text = data.description
                }

            })
        })    

• 自定义的ItemDecoration：

    abstract class StickHeadTransfer {
        abstract fun isFirst(position: Int): Boolean
        abstract fun isLast(position: Int): Boolean
        abstract fun bindData(stickView: View, position: Int)
    }
    
    
    class StickHeaderItemDecoration(private val layoutId: Int, private val stickViewHeight: Int = 0) : RecyclerView.ItemDecoration() {
        private var transfer: StickHeadTransfer? = null
        private lateinit var stickHeadView: View
    
        fun <T : StickHeadTransfer> setTransfer(transfer: T) {
            this.transfer = transfer
        }
    
        override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
            super.getItemOffsets(outRect, view, parent, state)
            //获取的View的位置
            initStickView(parent)
            if (transfer?.isFirst(parent.getChildAdapterPosition(view)) == true) {
                //头部添加宽度
                outRect.top = stickViewHeight.dp
            }
        }
    
        override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
            super.onDraw(c, parent, state)
            initStickView(parent)
            val left: Int = parent.paddingLeft
            val right: Int = parent.width - parent.paddingRight
            for (position in 0 until parent.childCount) {
                val child = parent.getChildAt(position)
                val adapterPosition = parent.getChildAdapterPosition(child)
                if (transfer?.isFirst(adapterPosition) == true) {
                    //计算ItemView的实际top
                    val bottom = getViewRealTop(child)
                    val top = bottom - stickViewHeight.dp
                    val view = LayoutInflater.from(parent.context).inflate(layoutId, parent, false)
                    //先绑定数据再测量
                    transfer?.bindData(view, adapterPosition)
                    view.measure(child.measuredWidth, child.measuredHeight)
                    view.layout(left, 0, right, stickViewHeight.dp)
                    c.translate(0.toFloat(), top.toFloat())
                    view.draw(c)
                    c.translate(0.toFloat(), -top.toFloat())
                }
            }
        }
    
        override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
            super.onDrawOver(c, parent, state)
            if (parent.childCount <= 0) return
            initStickView(parent)
            /***
             * 1、不是第一个需要添加一个悬浮的View。
             * 2、是最后一个需要添加一个随着其的Top逐渐变化的View
             */
            val left: Int = parent.paddingLeft
            val right: Int = parent.width - parent.paddingRight
            val child = parent.getChildAt(0)
            val adapterPosition = parent.getChildAdapterPosition(child)
            //先绑定数据再测量
            transfer?.bindData(stickHeadView, adapterPosition)
            stickHeadView.measure(child.measuredWidth, child.measuredHeight)
            stickHeadView.layout(left, 0, right, stickViewHeight.dp)
            if (!beginTransfer(c, parent)) stickHeadView.draw(c)
    
        }
    
    
        private fun beginTransfer(c: Canvas, parent: RecyclerView): Boolean {
            //判断分组的最后一个距离头部的距离判断是否开始移动
            for (position in 0 until parent.childCount) {
                val view = parent.getChildAt(position)
                if (transfer?.isLast(parent.getChildAdapterPosition(view)) == true) {
                    val top = getViewRealTop(view)
                    val distance = top + view.height
                    if (distance > stickViewHeight.dp) return false
                    //由于悬停布局需要覆盖到最后一个Item全部所以多移动一个的height高度
                    c.translate(0.toFloat(), (top - view.height).toFloat())
                    stickHeadView.draw(c)
                    c.translate(0.toFloat(), -(top - view.height).toFloat())
                    return true
                }
            }
            return false
        }
    
        private fun getViewRealTop(child: View): Int {
            val params = child
                    .layoutParams as RecyclerView.LayoutParams
            return child.top + params.topMargin +
                    child.translationY.roundToInt()
        }
    
        private fun initStickView(parent: RecyclerView) {
            if (this::stickHeadView.isInitialized) return
            else stickHeadView = LayoutInflater.from(parent.context).inflate(layoutId, parent, false)
        }
    }

本文地址：https://blog.csdn.net/qq_39650441/article/details/107140718