RecyclerVIew-＞加速再减速的RecyclerVIew平滑对齐工具类SnapHelper

XML文件

• ItemView的XML文件R.layout.shape_item_view

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="100dp"android:layout_height="100dp"android:background="@drawable/shape_item_view"><TextViewandroid:id="@+id/textView"android:layout_width="wrap_content"android:layout_height="wrap_content"android:layout_gravity="center"android:textSize="16sp" />
</FrameLayout>

• 滑动到对齐ItemView的XML文件 R.drawable.shape_item_view_selected

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"><solid android:color="#FFFFFF" /><corners android:radius="8dp" /><stroke android:color="#FFFF00" android:width="5dp" />
</shape>

• 未滑动到对齐ItemView的XML文件 R.drawable.shape_item_view

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"><solid android:color="#FFFFFF" /><corners android:radius="8dp" /><stroke android:color="#000000" android:width="5dp" />
</shape>

• Activity的XML文件R.layout.activity_main

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="match_parent"android:layout_height="match_parent"android:gravity="center"><androidx.recyclerview.widget.RecyclerViewandroid:id="@+id/recyclerView"android:layout_width="match_parent"android:layout_height="100dp"android:layout_marginLeft="@dimen/edit_crop_frame_padding"android:layout_marginRight="@dimen/edit_crop_frame_padding"android:orientation="horizontal" />
</LinearLayout>

RecyclerView代码

• Adapter代码

class MyAdapter(private val numbers: List<Int>) : RecyclerView.Adapter<MyAdapter.MyViewHolder>() {// 记录选中位置private var selectedPosition = RecyclerView.NO_POSITIONoverride fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {val view = LayoutInflater.from(parent.context).inflate(R.layout.item_view, parent, false)return MyViewHolder(view)}override fun onBindViewHolder(holder: MyViewHolder, position: Int) {holder.textView.text = numbers[position].toString()if (selectedPosition == position) {holder.itemView.setBackgroundResource(R.drawable.shape_item_view_selected)} else {holder.itemView.setBackgroundResource(R.drawable.shape_item_view)}}override fun getItemCount() = numbers.size// 给外部工具类SnapHelper实现类使用，滚动到对齐位置，修改ItemView的轮廓fun setSelectedPosition(position: Int) {val oldPosition = selectedPositionselectedPosition = positionnotifyItemChanged(oldPosition)notifyItemChanged(selectedPosition)}class MyViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {val textView: TextView = itemView.findViewById(R.id.textView)}
}

• ItemDecoration代码

class SpaceItemDecoration(private val spaceSize: Int, private val itemSize : Int) : RecyclerView.ItemDecoration() {private val paint = Paint().apply {color = Color.REDstyle = Paint.Style.FILL}var spacePadding = 0override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {super.getItemOffsets(outRect, view, parent, state)// 正常Item的DecorationoutRect.left = spaceSizeoutRect.right = spaceSize// 第一个和最后一个Item的DecorationspacePadding = (parent.measuredWidth / 2 - itemSize / 2)val size = parent.adapter?.itemCount ?: 0val position = parent.getChildAdapterPosition(view)if (position == 0) {outRect.left = spacePadding} else if (position == size - 1) {outRect.right = spacePadding}}override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {super.onDraw(c, parent, state)val childCount = parent.childCountfor (i in 0 until childCount) {val child = parent.getChildAt(i)val position = parent.getChildAdapterPosition(child)val params = child.layoutParams as RecyclerView.LayoutParamsvar left : Intvar right : Intvar top : Intvar bottom : Intif (position == 0) {left = child.left - params.leftMargin - spacePaddingright = child.right + params.rightMargin + spaceSizetop = child.top - params.topMarginbottom = child.bottom + params.bottomMargin} else if (position == parent.adapter?.itemCount!! - 1) {left = child.left - params.leftMargin - spaceSizeright = child.right + params.rightMargin + spacePaddingtop = child.top - params.topMarginbottom = child.bottom + params.bottomMargin} else {// 绘制其他 Item 的装饰left = child.left - params.leftMargin - spaceSizeright = child.right + params.rightMargin + spaceSizetop = child.top - params.topMarginbottom = child.bottom + params.bottomMargin}c.drawRect(left.toFloat(), top.toFloat(), right.toFloat(), bottom.toFloat(), paint)}}
}

RecyclerView对齐工具类SnapHelper实现类代码

• ① attachToRecyclerView()方法：将RecyclerView对齐操作交给SnapHelper实现类

override fun attachToRecyclerView(recyclerView: RecyclerView?) {Log.i(TAG, "attachToRecyclerView")mRecyclerView = recyclerViewsuper.attachToRecyclerView(recyclerView)
}

• ② createScroller()方法：创建惯性滑动的Scroller
  • onTargetFound()回调方法：找到对齐位置之后回调，计算目标位置到对齐位置需要滚动的距离和时间
  • calculateSpeedPerPixel()回调方法：滚动一英寸所需时间除以屏幕密度，得到滚动一像素所需的时间

override fun createScroller(layoutManager: RecyclerView.LayoutManager?): RecyclerView.SmoothScroller? {if (layoutManager !is RecyclerView.SmoothScroller.ScrollVectorProvider) {return null}Log.i(TAG, "createScroller")return object : LinearSmoothScroller(mRecyclerView?.context) {override fun onTargetFound(targetView: View, state: RecyclerView.State, action: Action) {if (mRecyclerView == null) returnLog.i(TAG, "onTargetFound")// 计算当前位置到目标位置的距离val snapDistances : IntArray? = calculateDistanceToFinalSnap(mRecyclerView?.layoutManager!!, targetView)val dx = snapDistances?.get(0) ?: 0val dy = snapDistances?.get(1) ?: 0// 这里增加滑动时间，可以使得滑动速度变慢val time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy))) * 10if (time > 0) {// 这里传入的是LinearOutSlowInInterpolator(), 也就是先加速再减速的插值器// 相比LinearSnapHelper中的DecelerateInterpolator, 这个插值器更符合自然滑动的效果action.update(dx, dy, time, mInterpolator)}}override fun calculateSpeedPerPixel(displayMetrics: DisplayMetrics?): Float {Log.i(TAG, "calculateSpeedPerPixel")// 计算滑动一个像素的时间return MILLISECONDS_PER_INCH / displayMetrics?.densityDpi!!}}
}

• ③ findTargetSnapPosition()方法：找到需要对齐的ItemView的位置
  • RecyclerView.SmoothScroller.ScrollVectorProvider.computeScrollVectorForPosition()：计算从0的位置滚动到ItemCount-1的位置需要滚动的方向，vectorForEnd.x表示水平方向(>0向右，<0向左)，vectorForEnd.y表示竖直方向(>0向下，<0向上)，正负值由结束位置和开始位置的差值得出

override fun findTargetSnapPosition(layoutManager: RecyclerView.LayoutManager?, velocityX: Int, velocityY: Int): Int {// 判断layoutManager是否实现了RecyclerView.SmoothScroller.ScrollVectorProvider这个接口if (layoutManager !is RecyclerView.SmoothScroller.ScrollVectorProvider) return RecyclerView.NO_POSITION// 判断ItemView个数是否小于等于0val itemCount = layoutManager.itemCountif (itemCount == 0) return RecyclerView.NO_POSITION// 找到需要对齐的ItemViewval currentView = findSnapView(layoutManager) ?: return RecyclerView.NO_POSITION// 获取需要对齐ItemView的位置val currentPosition = layoutManager.getPosition(currentView)if (currentPosition == RecyclerView.NO_POSITION) return RecyclerView.NO_POSITION// 判断layoutManager的布局方向val vectorProvider = layoutManager as RecyclerView.SmoothScroller.ScrollVectorProviderval vectorForEnd = vectorProvider.computeScrollVectorForPosition(itemCount - 1) ?: return RecyclerView.NO_POSITIONLog.i(TAG, "findTargetSnapPosition")// 计算水平, 垂直方向最多能惯性滑动的ItemView个数，在当前ItemView的位置上，进行加减Position操作var maxHorizontalItemViewCount: Intvar maxVerticalItemViewCount: Intif (layoutManager.canScrollHorizontally()) {maxHorizontalItemViewCount = estimateNextPositionDiffForFling(layoutManager, getHorizontalHelper(layoutManager), velocityX, 0)var sign = Math.signum(velocityX.toFloat())if (sign == 0f) sign = 1f// 限制最多能惯性滑动的ItemView个数，限制滑动的 ItemView 的个数在 1 到 2 之间maxHorizontalItemViewCount = (sign * Math.min(Math.max(Math.abs(maxHorizontalItemViewCount), 0), 2)).toInt()if (vectorForEnd.x < 0) {maxHorizontalItemViewCount = - maxHorizontalItemViewCount}}else{maxHorizontalItemViewCount = 0}if (layoutManager.canScrollVertically()) {maxVerticalItemViewCount = estimateNextPositionDiffForFling(layoutManager, getVerticalHelper(layoutManager), 0, velocityY)var sign = Math.signum(velocityY.toFloat())if (sign == 0f) sign = 1f// 限制最多能惯性滑动的ItemView个数，限制滑动的 ItemView 的个数在 1 到 2 之间maxVerticalItemViewCount = (sign * Math.min(Math.max(Math.abs(maxVerticalItemViewCount), 0), 2)).toInt()if (vectorForEnd.y < 0) {maxVerticalItemViewCount = - maxVerticalItemViewCount}}else{maxVerticalItemViewCount = 0}// 根据滑动的方向，计算出最终的 ItemView 个数val finalItemCount = if(layoutManager.canScrollHorizontally()){maxHorizontalItemViewCount}else{maxVerticalItemViewCount}if (finalItemCount == 0) return RecyclerView.NO_POSITION// 确定最终的对齐位置，并做边界处理var targetPosition = currentPosition + finalItemCountif (targetPosition < 0) targetPosition = 0if (targetPosition >= layoutManager.itemCount) targetPosition = layoutManager.itemCount - 1return targetPosition
}

• ④ findSnapView()方法：调用findCenterView()找到最接近中心点的ItemView
  • findCenterView()方法：拿到每个ItemView的left加上自身宽度的一半和RecyclerView的中心点进行比较，找到最接近中心点的ItemView

override fun findSnapView(layoutManager: RecyclerView.LayoutManager?): View? {Log.i(TAG, "findSnapView")if (layoutManager!!.canScrollVertically()) {return findCenterView(layoutManager, getVerticalHelper(layoutManager))} else if (layoutManager.canScrollHorizontally()) {return findCenterView(layoutManager, getHorizontalHelper(layoutManager))}return null
}

private fun findCenterView(layoutManager: RecyclerView.LayoutManager, helper: OrientationHelper): View? {Log.i(TAG, "findCenterView")val childCount = layoutManager.childCountif (childCount == 0) return null// 最接近RecyclerView中心的ItemViewvar closestItemView: View? = null// RecyclerView的中心点val center = helper.startAfterPadding + helper.totalSpace / 2var absClosest = Int.MAX_VALUEfor (i in 0 until childCount) {val child = layoutManager.getChildAt(i)// ItemView的中心点, 这里用Left是因为有ItemDecoration的存在，val childCenter = child?.left!! + helper.getDecoratedMeasurement(child) / 2val childDistance = Math.abs(childCenter - center)// 找到最靠近RecyclerView中心的ItemViewif (childDistance < absClosest) {absClosest = childDistanceclosestItemView = child}}return closestItemView
}

• ⑤ estimateNextPositionDiffForFling()方法：计算当前位置到目标对齐位置还差了几个ItemView的个数
  • calculateScrollDistance()：计算RecyclerView的滚动距离
  • computeDistancePerChild()：计算每个ItemView的滚动距离

private fun estimateNextPositionDiffForFling(layoutManager: RecyclerView.LayoutManager, helper: OrientationHelper, velocityX: Int, velocityY: Int): Int {Log.i(TAG, "estimateNextPositionDiffForFling")val distances = calculateScrollDistance(velocityX, velocityY)val distancePerChild = computeDistancePerChild(layoutManager, helper)if (distancePerChild <= 0) return 0val distance = if (Math.abs(distances[0]) > Math.abs(distances[1])) distances[0] else distances[1]return Math.round(distance / distancePerChild)
}

override fun calculateScrollDistance(velocityX: Int, velocityY: Int): IntArray {Log.i(TAG, "calculateScrollDistance")return super.calculateScrollDistance(velocityX, velocityY)
}

private fun computeDistancePerChild(layoutManager: RecyclerView.LayoutManager, helper: OrientationHelper): Float {Log.i(TAG, "computeDistancePerChild")var minPositionView : View ?= nullvar maxPositionView : View ?= nullvar minPosition = Integer.MAX_VALUEvar maxPosition = Integer.MIN_VALUEval itemViewCount = layoutManager.childCountif (itemViewCount == 0) return INVALID_DISTANCE// 遍历所有ItemView, 找到最小位置和最大位置的ItemView，记录Positionfor (i in 0 until itemViewCount) {val child = layoutManager.getChildAt(i) ?: continueval position = layoutManager.getPosition(child)if (position == RecyclerView.NO_POSITION) continueif (position < minPosition) {minPosition = positionminPositionView = child}if (position > maxPosition) {maxPosition = positionmaxPositionView = child}}if (minPositionView == null || maxPositionView == null) return INVALID_DISTANCE// 计算最小位置和最大位置的ItemView离RecyclerView左边的距离val start = Math.min(helper.getDecoratedStart(minPositionView), helper.getDecoratedStart(maxPositionView))// 计算最小位置和最大位置的ItemView离RecyclerViewj右边的距离val end = Math.max(helper.getDecoratedEnd(minPositionView), helper.getDecoratedEnd(maxPositionView))// 计算最小位置和最大位置的ItemView的宽度val distance = end - startif (distance <= 0) return INVALID_DISTANCEreturn 1f * distance / (maxPosition - minPosition + 1)
}

• ⑥ onTargetFound()方法：找对对齐ItemView位置后回调
  • calculateDistanceToFinalSnap()：计算最终需要滚动到对齐ItemView位置的距离
  • calculateTimeForDeceleration()：计算最终需要滚动到对齐ItemView位置所花时间

override fun onTargetFound(targetView: View, state: RecyclerView.State, action: Action) {if (mRecyclerView == null) returnLog.i(TAG, "onTargetFound")// 计算当前位置到目标位置的距离val snapDistances : IntArray? = calculateDistanceToFinalSnap(mRecyclerView?.layoutManager!!, targetView)val dx = snapDistances?.get(0) ?: 0val dy = snapDistances?.get(1) ?: 0// 这里增加滑动时间，可以使得滑动速度变慢val time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy))) * 10if (time > 0) {// 这里传入的是LinearOutSlowInInterpolator(), 也就是先加速再减速的插值器// 相比LinearSnapHelper中的DecelerateInterpolator, 这个插值器更符合自然滑动的效果action.update(dx, dy, time, mInterpolator)}
}

override fun calculateDistanceToFinalSnap(layoutManager: RecyclerView.LayoutManager, targetView: View): IntArray? {Log.i(TAG, "calculateDistanceToFinalSnap")// 计算当前位置到目标位置的距离val out = IntArray(2)if (layoutManager.canScrollHorizontally()) {out[0] = distanceToCenter(targetView, getHorizontalHelper(layoutManager)!!)} else {out[0] = 0}if (layoutManager.canScrollVertically()) {out[1] = distanceToCenter(targetView, getVerticalHelper(layoutManager)!!)} else {out[1] = 0}return out
}

• ⑦ distanceToCenter()方法：计算目标对齐ItemView距离RecyclerView中心点的距离

private fun distanceToCenter(targetView: View, helper: OrientationHelper): Int {Log.i(TAG, "distanceToCenter")// 计算目标ItemView的中心点(ItemView包含ItemDecoration的部分 + ItemView的宽度/高度的一半)的val childCenter = helper.getDecoratedStart(targetView) + helper.getDecoratedMeasurement(targetView) / 2// 计算RecyclerView的中心点(RecyclerView减去Padding的部分 + RecyclerView的宽度/高度的一半)val containerCenter = helper.startAfterPadding + helper.totalSpace / 2return childCenter - containerCenter
}

• 完整加减速的对齐工具类SnapHelper的代码

open class MySmoothSnapHelper : SnapHelper() {private val INVALID_DISTANCE = 1f // 无法计算有效对齐距离，返回这个值private val MILLISECONDS_PER_INCH = 25f // 滑动速度，每英寸25毫秒// 通过LayoutManager创建方向工具类，其中包含了RecyclerView的布局参数，包括padding，margin等private var mVerticalHelper : OrientationHelper ?= nullprivate var mHorizontalHelper : OrientationHelper ?= nullprivate var mRecyclerView : RecyclerView ?= null// 加速->减速插值器private val mInterpolator = LinearOutSlowInInterpolator()// 将RecyclerView交给SnapHelper, 计算惯性滑动后需要对齐的位置override fun attachToRecyclerView(recyclerView: RecyclerView?) {Log.i(TAG, "attachToRecyclerView")mRecyclerView = recyclerViewsuper.attachToRecyclerView(recyclerView)}// 创建惯性滑动的Scrolleroverride fun createScroller(layoutManager: RecyclerView.LayoutManager?): RecyclerView.SmoothScroller? {Log.i(TAG, "createScroller")if (layoutManager !is RecyclerView.SmoothScroller.ScrollVectorProvider) {return null}return object : LinearSmoothScroller(mRecyclerView?.context) {override fun onTargetFound(targetView: View, state: RecyclerView.State, action: Action) {if (mRecyclerView == null) returnLog.i(TAG, "onTargetFound")// 计算当前位置到目标位置的距离val snapDistances : IntArray? = calculateDistanceToFinalSnap(mRecyclerView?.layoutManager!!, targetView)val dx = snapDistances?.get(0) ?: 0val dy = snapDistances?.get(1) ?: 0// 这里增加滑动时间，可以使得滑动速度变慢val time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy))) * 10if (time > 0) {// 这里传入的是LinearOutSlowInInterpolator(), 也就是先加速再减速的插值器// 相比LinearSnapHelper中的DecelerateInterpolator, 这个插值器更符合自然滑动的效果action.update(dx, dy, time, mInterpolator)}}override fun calculateSpeedPerPixel(displayMetrics: DisplayMetrics?): Float {Log.i(TAG, "calculateSpeedPerPixel")// 计算滑动一个像素的时间return MILLISECONDS_PER_INCH / displayMetrics?.densityDpi!!}}}override fun calculateDistanceToFinalSnap(layoutManager: RecyclerView.LayoutManager, targetView: View): IntArray? {Log.i(TAG, "calculateDistanceToFinalSnap")// 计算当前位置到目标位置的距离val out = IntArray(2)if (layoutManager.canScrollHorizontally()) {out[0] = distanceToCenter(targetView, getHorizontalHelper(layoutManager)!!)} else {out[0] = 0}if (layoutManager.canScrollVertically()) {out[1] = distanceToCenter(targetView, getVerticalHelper(layoutManager)!!)} else {out[1] = 0}return out}private fun distanceToCenter(targetView: View, helper: OrientationHelper): Int {Log.i(TAG, "distanceToCenter")// 计算目标ItemView的中心点(ItemView包含ItemDecoration的部分 + ItemView的宽度/高度的一半)的val childCenter = helper.getDecoratedStart(targetView) + helper.getDecoratedMeasurement(targetView) / 2// 计算RecyclerView的中心点(RecyclerView减去Padding的部分 + RecyclerView的宽度/高度的一半)val containerCenter = helper.startAfterPadding + helper.totalSpace / 2return childCenter - containerCenter}override fun findSnapView(layoutManager: RecyclerView.LayoutManager?): View? {Log.i(TAG, "findSnapView")if (layoutManager!!.canScrollVertically()) {return findCenterView(layoutManager, getVerticalHelper(layoutManager))} else if (layoutManager.canScrollHorizontally()) {return findCenterView(layoutManager, getHorizontalHelper(layoutManager))}return null}private fun findCenterView(layoutManager: RecyclerView.LayoutManager, helper: OrientationHelper): View? {Log.i(TAG, "findCenterView")val childCount = layoutManager.childCountif (childCount == 0) return null// 最接近RecyclerView中心的ItemViewvar closestItemView: View? = null// RecyclerView的中心点val center = helper.startAfterPadding + helper.totalSpace / 2var absClosest = Int.MAX_VALUEfor (i in 0 until childCount) {val child = layoutManager.getChildAt(i)// ItemView的中心点, 这里用Left是因为有ItemDecoration的存在，val childCenter = child?.left!! + helper.getDecoratedMeasurement(child) / 2val childDistance = Math.abs(childCenter - center)// 找到最靠近RecyclerView中心的ItemViewif (childDistance < absClosest) {absClosest = childDistanceclosestItemView = child}}return closestItemView}override fun findTargetSnapPosition(layoutManager: RecyclerView.LayoutManager?, velocityX: Int, velocityY: Int): Int {Log.i(TAG, "findTargetSnapPosition")// 判断layoutManager是否实现了RecyclerView.SmoothScroller.ScrollVectorProvider这个接口if (layoutManager !is RecyclerView.SmoothScroller.ScrollVectorProvider) return RecyclerView.NO_POSITION// 判断ItemView个数是否小于等于0val itemCount = layoutManager.itemCountif (itemCount == 0) return RecyclerView.NO_POSITION// 找到需要对齐的ItemViewval currentView = findSnapView(layoutManager) ?: return RecyclerView.NO_POSITION// 获取需要对齐ItemView的位置val currentPosition = layoutManager.getPosition(currentView)if (currentPosition == RecyclerView.NO_POSITION) return RecyclerView.NO_POSITION// 判断layoutManager的布局方向val vectorProvider = layoutManager as RecyclerView.SmoothScroller.ScrollVectorProviderval vectorForEnd = vectorProvider.computeScrollVectorForPosition(itemCount - 1) ?: return RecyclerView.NO_POSITION// 计算水平, 垂直方向最多能惯性滑动的ItemView个数，在当前ItemView的位置上，进行加减Position操作var maxHorizontalItemViewCount: Intvar maxVerticalItemViewCount: Intif (layoutManager.canScrollHorizontally()) {maxHorizontalItemViewCount = estimateNextPositionDiffForFling(layoutManager, getHorizontalHelper(layoutManager), velocityX, 0)var sign = Math.signum(velocityX.toFloat())if (sign == 0f) sign = 1f// 限制最多能惯性滑动的ItemView个数，限制滑动的 ItemView 的个数在 1 到 2 之间maxHorizontalItemViewCount = (sign * Math.min(Math.max(Math.abs(maxHorizontalItemViewCount), 0), 2)).toInt()if (vectorForEnd.x < 0) {maxHorizontalItemViewCount = - maxHorizontalItemViewCount}}else{maxHorizontalItemViewCount = 0}if (layoutManager.canScrollVertically()) {maxVerticalItemViewCount = estimateNextPositionDiffForFling(layoutManager, getVerticalHelper(layoutManager), 0, velocityY)var sign = Math.signum(velocityY.toFloat())if (sign == 0f) sign = 1f// 限制最多能惯性滑动的ItemView个数，限制滑动的 ItemView 的个数在 1 到 2 之间maxVerticalItemViewCount = (sign * Math.min(Math.max(Math.abs(maxVerticalItemViewCount), 0), 2)).toInt()if (vectorForEnd.y < 0) {maxVerticalItemViewCount = - maxVerticalItemViewCount}}else{maxVerticalItemViewCount = 0}// 根据滑动的方向，计算出最终的 ItemView 个数val finalItemCount = if(layoutManager.canScrollHorizontally()){maxHorizontalItemViewCount}else{maxVerticalItemViewCount}if (finalItemCount == 0) return RecyclerView.NO_POSITION// 确定最终的对齐位置，并做边界处理var targetPosition = currentPosition + finalItemCountif (targetPosition < 0) targetPosition = 0if (targetPosition >= layoutManager.itemCount) targetPosition = layoutManager.itemCount - 1return targetPosition}override fun calculateScrollDistance(velocityX: Int, velocityY: Int): IntArray {Log.i(TAG, "calculateScrollDistance")return super.calculateScrollDistance(velocityX, velocityY)}private fun estimateNextPositionDiffForFling(layoutManager: RecyclerView.LayoutManager, helper: OrientationHelper, velocityX: Int, velocityY: Int): Int {Log.i(TAG, "estimateNextPositionDiffForFling")val distances = calculateScrollDistance(velocityX, velocityY)val distancePerChild = computeDistancePerChild(layoutManager, helper)if (distancePerChild <= 0) return 0val distance = if (Math.abs(distances[0]) > Math.abs(distances[1])) distances[0] else distances[1]return Math.round(distance / distancePerChild)}private fun computeDistancePerChild(layoutManager: RecyclerView.LayoutManager, helper: OrientationHelper): Float {Log.i(TAG, "computeDistancePerChild")var minPositionView : View ?= nullvar maxPositionView : View ?= nullvar minPosition = Integer.MAX_VALUEvar maxPosition = Integer.MIN_VALUEval itemViewCount = layoutManager.childCountif (itemViewCount == 0) return INVALID_DISTANCE// 遍历所有ItemView, 找到最小位置和最大位置的ItemView，记录Positionfor (i in 0 until itemViewCount) {val child = layoutManager.getChildAt(i) ?: continueval position = layoutManager.getPosition(child)if (position == RecyclerView.NO_POSITION) continueif (position < minPosition) {minPosition = positionminPositionView = child}if (position > maxPosition) {maxPosition = positionmaxPositionView = child}}if (minPositionView == null || maxPositionView == null) return INVALID_DISTANCE// 计算最小位置和最大位置的ItemView离RecyclerView左边的距离val start = Math.min(helper.getDecoratedStart(minPositionView), helper.getDecoratedStart(maxPositionView))// 计算最小位置和最大位置的ItemView离RecyclerViewj右边的距离val end = Math.max(helper.getDecoratedEnd(minPositionView), helper.getDecoratedEnd(maxPositionView))// 计算最小位置和最大位置的ItemView的宽度val distance = end - startif (distance <= 0) return INVALID_DISTANCEreturn 1f * distance / (maxPosition - minPosition + 1)}private fun getVerticalHelper(layoutManager: RecyclerView.LayoutManager): OrientationHelper {Log.i(TAG, "getVerticalHelper")if (mVerticalHelper == null || mVerticalHelper?.layoutManager != layoutManager) {mVerticalHelper = OrientationHelper.createVerticalHelper(layoutManager)}return mVerticalHelper!!}private fun getHorizontalHelper(layoutManager: RecyclerView.LayoutManager): OrientationHelper {Log.i(TAG, "getHorizontalHelper")if (mHorizontalHelper == null || mHorizontalHelper!!.layoutManager != layoutManager) {mHorizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager)}return mHorizontalHelper!!}
}

Activity代码

• 第一次findSnapView：正常滑动停止后触发，需要找到对齐的View
• 第二次findSnapView：惯性滑动停止后触发，需要找到对齐的View

const val TAG = "Yang"
class MainActivity : AppCompatActivity() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)val numberList = List(10){it}val layoutManager = LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false)val mRv = findViewById<RecyclerView>(R.id.recyclerView)val mAdapter = MyAdapter(numberList)// 添加 ItemDecorationmRv.addItemDecoration(SpaceItemDecoration(dpToPx(this, 25f), dpToPx(this, 100f)))// 添加 LinearSnapHelperval linearSnapHelper = object : MySmoothSnapHelper() {override fun findSnapView(layoutManager: RecyclerView.LayoutManager?): View? {val snapView = super.findSnapView(layoutManager)val snapPosition = snapView?.let {mRv.getChildAdapterPosition(it) }snapPosition?.let {if (snapPosition != RecyclerView.NO_POSITION) {mAdapter.setSelectedPosition(snapPosition)}}return snapView}}linearSnapHelper.attachToRecyclerView(mRv)mRv?.layoutManager = layoutManagermRv?.adapter = mAdapter}fun dpToPx(context: Context, dp: Float): Int {val metrics = context.resources.displayMetricsreturn TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp, metrics).toInt()}
}// log
2024-06-21 01:18:42.794 17860-17860 Yang                    I  attachToRecyclerView
2024-06-21 01:18:45.412 17860-17860 Yang                    I  createScroller
2024-06-21 01:18:45.413 17860-17860 Yang                    I  findTargetSnapPosition
2024-06-21 01:18:45.413 17860-17860 Yang                    I  findSnapView
2024-06-21 01:18:45.413 17860-17860 Yang                    I  getHorizontalHelper
2024-06-21 01:18:45.413 17860-17860 Yang                    I  findCenterView
2024-06-21 01:18:45.413 17860-17860 Yang                    I  getHorizontalHelper
2024-06-21 01:18:45.413 17860-17860 Yang                    I  estimateNextPositionDiffForFling
2024-06-21 01:18:45.413 17860-17860 Yang                    I  calculateScrollDistance
2024-06-21 01:18:45.413 17860-17860 Yang                    I  computeDistancePerChild
2024-06-21 01:18:45.430 17860-17860 Yang                    I  onTargetFound
2024-06-21 01:18:45.430 17860-17860 Yang                    I  calculateDistanceToFinalSnap
2024-06-21 01:18:45.430 17860-17860 Yang                    I  getHorizontalHelper
2024-06-21 01:18:45.430 17860-17860 Yang                    I  distanceToCenter
2024-06-21 01:18:45.430 17860-17860 Yang                    I  calculateSpeedPerPixel
2024-06-21 01:18:46.400 17860-17860 Yang                    I  findSnapView
2024-06-21 01:18:46.400 17860-17860 Yang                    I  getHorizontalHelper
2024-06-21 01:18:46.400 17860-17860 Yang                    I  findCenterView
2024-06-21 01:18:46.400 17860-17860 Yang                    I  calculateDistanceToFinalSnap
2024-06-21 01:18:46.400 17860-17860 Yang                    I  getHorizontalHelper
2024-06-21 01:18:46.400 17860-17860 Yang                    I  distanceToCenter

效果图