RecyclerView SnapHelper 无法显示第一个/最后一个项目
Posted
技术标签:
【中文标题】RecyclerView SnapHelper 无法显示第一个/最后一个项目【英文标题】：RecyclerView SnapHelper fails to show first/last items 【发布时间】：2019-03-12 08:09:35 【问题描述】：我有一个RecyclerView，它附加到LinearSnapHelper 以捕捉到中心项目。当我滚动到第一个或最后一个项目时，这些项目不再完全可见。此问题如下图所示。如何解决？
【问题讨论】：
【参考方案1】：当第一个/最后一个项目的中心更靠近容器的中心时，会发生此问题。因此，我们应该对捕捉功能进行一些更改以忽略这种情况。由于我们需要LinearSnapHelper类中的一些字段，我们可以复制其源代码并在findCenterView方法上进行如下修改：
MyLinearSnapHelper.kt
/*
* Copyright (C) 2016 The android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.aminography.view.component
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.OrientationHelper
import android.support.v7.widget.RecyclerView
import android.support.v7.widget.SnapHelper
import android.view.View
/**
* Implementation of the [SnapHelper] supporting snapping in either vertical or horizontal
* orientation.
*
*
* The implementation will snap the center of the target child view to the center of
* the attached [RecyclerView]. If you intend to change this behavior then override
* [SnapHelper.calculateDistanceToFinalSnap].
*/
class MyLinearSnapHelper : SnapHelper()
// Orientation helpers are lazily created per LayoutManager.
private var mVerticalHelper: OrientationHelper? = null
private var mHorizontalHelper: OrientationHelper? = null
override fun calculateDistanceToFinalSnap(
layoutManager: RecyclerView.LayoutManager, targetView: View): IntArray?
val out = IntArray(2)
if (layoutManager.canScrollHorizontally())
out[0] = distanceToCenter(layoutManager, targetView,
getHorizontalHelper(layoutManager))
else
out[0] = 0
if (layoutManager.canScrollVertically())
out[1] = distanceToCenter(layoutManager, targetView,
getVerticalHelper(layoutManager))
else
out[1] = 0
return out
override fun findTargetSnapPosition(layoutManager: RecyclerView.LayoutManager, velocityX: Int,
velocityY: Int): Int
if (layoutManager !is RecyclerView.SmoothScroller.ScrollVectorProvider)
return RecyclerView.NO_POSITION
val itemCount = layoutManager.itemCount
if (itemCount == 0)
return RecyclerView.NO_POSITION
val currentView = findSnapView(layoutManager) ?: return RecyclerView.NO_POSITION
val currentPosition = layoutManager.getPosition(currentView)
if (currentPosition == RecyclerView.NO_POSITION)
return RecyclerView.NO_POSITION
val vectorProvider = layoutManager as RecyclerView.SmoothScroller.ScrollVectorProvider
// deltaJumps sign comes from the velocity which may not match the order of children in
// the LayoutManager. To overcome this, we ask for a vector from the LayoutManager to
// get the direction.
val vectorForEnd = vectorProvider.computeScrollVectorForPosition(itemCount - 1)
?: // cannot get a vector for the given position.
return RecyclerView.NO_POSITION
var vDeltaJump: Int
var hDeltaJump: Int
if (layoutManager.canScrollHorizontally())
hDeltaJump = estimateNextPositionDiffForFling(layoutManager,
getHorizontalHelper(layoutManager), velocityX, 0)
if (vectorForEnd.x < 0)
hDeltaJump = -hDeltaJump
else
hDeltaJump = 0
if (layoutManager.canScrollVertically())
vDeltaJump = estimateNextPositionDiffForFling(layoutManager,
getVerticalHelper(layoutManager), 0, velocityY)
if (vectorForEnd.y < 0)
vDeltaJump = -vDeltaJump
else
vDeltaJump = 0
val deltaJump = if (layoutManager.canScrollVertically()) vDeltaJump else hDeltaJump
if (deltaJump == 0)
return RecyclerView.NO_POSITION
var targetPos = currentPosition + deltaJump
if (targetPos < 0)
targetPos = 0
if (targetPos >= itemCount)
targetPos = itemCount - 1
return targetPos
override fun findSnapView(layoutManager: RecyclerView.LayoutManager): View?
if (layoutManager.canScrollVertically())
return findCenterView(layoutManager, getVerticalHelper(layoutManager))
else if (layoutManager.canScrollHorizontally())
return findCenterView(layoutManager, getHorizontalHelper(layoutManager))
return null
private fun distanceToCenter(layoutManager: RecyclerView.LayoutManager,
targetView: View, helper: OrientationHelper): Int
val childCenter = helper.getDecoratedStart(targetView) + helper.getDecoratedMeasurement(targetView) / 2
val containerCenter: Int = if (layoutManager.clipToPadding)
helper.startAfterPadding + helper.totalSpace / 2
else
helper.end / 2
return childCenter - containerCenter
/**
* Estimates a position to which SnapHelper will try to scroll to in response to a fling.
*
* @param layoutManager The [RecyclerView.LayoutManager] associated with the attached
* [RecyclerView].
* @param helper The [OrientationHelper] that is created from the LayoutManager.
* @param velocityX The velocity on the x axis.
* @param velocityY The velocity on the y axis.
*
* @return The diff between the target scroll position and the current position.
*/
private fun estimateNextPositionDiffForFling(layoutManager: RecyclerView.LayoutManager,
helper: OrientationHelper, velocityX: Int, velocityY: Int): Int
val distances = calculateScrollDistance(velocityX, velocityY)
val distancePerChild = computeDistancePerChild(layoutManager, helper)
if (distancePerChild <= 0)
return 0
val distance = if (Math.abs(distances[0]) > Math.abs(distances[1])) distances[0] else distances[1]
return Math.round(distance / distancePerChild)
/**
* Return the child view that is currently closest to the center of this parent.
*
* @param layoutManager The [RecyclerView.LayoutManager] associated with the attached
* [RecyclerView].
* @param helper The relevant [OrientationHelper] for the attached [RecyclerView].
*
* @return the child view that is currently closest to the center of this parent.
*/
private fun findCenterView(layoutManager: RecyclerView.LayoutManager,
helper: OrientationHelper): View?
// ----- Added by aminography
if (layoutManager is LinearLayoutManager)
if (layoutManager.findFirstCompletelyVisibleItemPosition() == 0)
return layoutManager.getChildAt(0)
else if (layoutManager.findLastCompletelyVisibleItemPosition() == layoutManager.itemCount - 1)
return layoutManager.getChildAt(layoutManager.itemCount - 1)
// -----
val childCount = layoutManager.childCount
if (childCount == 0)
return null
var closestChild: View? = null
val center: Int = if (layoutManager.clipToPadding)
helper.startAfterPadding + helper.totalSpace / 2
else
helper.end / 2
var absClosest = Integer.MAX_VALUE
for (i in 0 until childCount)
val child = layoutManager.getChildAt(i)
val childCenter = helper.getDecoratedStart(child) + helper.getDecoratedMeasurement(child) / 2
val absDistance = Math.abs(childCenter - center)
/** if child center is closer than previous closest, set it as closest */
if (absDistance < absClosest)
absClosest = absDistance
closestChild = child
return closestChild
/**
* Computes an average pixel value to pass a single child.
*
*
* Returns a negative value if it cannot be calculated.
*
* @param layoutManager The [RecyclerView.LayoutManager] associated with the attached
* [RecyclerView].
* @param helper The relevant [OrientationHelper] for the attached
* [RecyclerView.LayoutManager].
*
* @return A float value that is the average number of pixels needed to scroll by one view in
* the relevant direction.
*/
private fun computeDistancePerChild(layoutManager: RecyclerView.LayoutManager,
helper: OrientationHelper): Float
var minPosView: View? = null
var maxPosView: View? = null
var minPos = Integer.MAX_VALUE
var maxPos = Integer.MIN_VALUE
val childCount = layoutManager.childCount
if (childCount == 0)
return INVALID_DISTANCE
for (i in 0 until childCount)
val child = layoutManager.getChildAt(i)
val pos = layoutManager.getPosition(child!!)
if (pos == RecyclerView.NO_POSITION)
continue
if (pos < minPos)
minPos = pos
minPosView = child
if (pos > maxPos)
maxPos = pos
maxPosView = child
if (minPosView == null || maxPosView == null)
return INVALID_DISTANCE
val start = Math.min(helper.getDecoratedStart(minPosView),
helper.getDecoratedStart(maxPosView))
val end = Math.max(helper.getDecoratedEnd(minPosView),
helper.getDecoratedEnd(maxPosView))
val distance = end - start
return if (distance == 0)
INVALID_DISTANCE
else 1f * distance / (maxPos - minPos + 1)
private fun getVerticalHelper(layoutManager: RecyclerView.LayoutManager): OrientationHelper
if (mVerticalHelper == null || mVerticalHelper!!.layoutManager !== layoutManager)
mVerticalHelper = OrientationHelper.createVerticalHelper(layoutManager)
return mVerticalHelper!!
private fun getHorizontalHelper(
layoutManager: RecyclerView.LayoutManager): OrientationHelper
if (mHorizontalHelper == null || mHorizontalHelper!!.layoutManager !== layoutManager)
mHorizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager)
return mHorizontalHelper!!
companion object
private const val INVALID_DISTANCE = 1f
【讨论】：
【参考方案2】：我知道我迟到了，但我想建议一个用 Java 代码编写的简单解决方案：
创建CustomSnapHelper 类：
public class CustomSnapHelper extends LinearSnapHelper
@Override
public View findSnapView(RecyclerView.LayoutManager layoutManager)
if(layoutManager instanceof LinearLayoutManager)
LinearLayoutManager linearLayoutManager = (LinearLayoutManager) layoutManager;
if(needToDoSnap(linearLayoutManager)==false)
return null;
return super.findSnapView(layoutManager);
public boolean needToDoSnap(LinearLayoutManager linearLayoutManager)
return linearLayoutManager.findFirstCompletelyVisibleItemPosition()!=0&&linearLayoutManager.findLastCompletelyVisibleItemPosition()!=linearLayoutManager.getItemCount()-1;
附加CustomSnapHelper 的对象以供回收站查看：
CustomSnapHelper mSnapHelper = new CustomSnapHelper();
mSnapHelper.attachToRecyclerView(mRecyclerView);
【讨论】：
谢谢！我希望它会帮助某人:)【参考方案3】：我尝试实施一个简单的解决方案。基本上我检查了第一个/最后一个项目是否完全可见。如果是这样，我们不需要执行快照。请参阅下面的解决方案：
class CarouselSnapHelper : LinearSnapHelper()
override fun findSnapView(layoutManager: RecyclerView.LayoutManager): View?
val linearLayoutManager = layoutManager as? LinearLayoutManager
?: return super.findSnapView(layoutManager)
return linearLayoutManager
.takeIf isValidSnap(it)
?.run super.findSnapView(layoutManager)
private fun isValidSnap(linearLayoutManager: LinearLayoutManager) =
linearLayoutManager.findFirstCompletelyVisibleItemPosition() != 0 &&
linearLayoutManager.findLastCompletelyVisibleItemPosition() != linearLayoutManager.itemCount - 1
【讨论】：
对不起，我现在无法检查这个答案，但我希望这可以帮助其他用户:)【参考方案4】：我找到了一个侵入性较小的答案：
private class PagerSelectSnapHelper : LinearSnapHelper()
override fun findSnapView(layoutManager: RecyclerView.LayoutManager): View?
// Use existing LinearSnapHelper but override when the itemDecoration calculations are off
val snapView = super.findSnapView(layoutManager)
return if (!snapView.isViewInCenterOfParent(layoutManager.width))
val endView = layoutManager.findViewByPosition(layoutManager.itemCount - 1)
val startView = layoutManager.findViewByPosition(0)
when
endView.isViewInCenterOfParent(layoutManager.width) -> endView
startView.isViewInCenterOfParent(layoutManager.width) -> startView
else -> snapView
else
snapView
private fun View?.isViewInCenterOfParent(parentWidth: Int): Boolean
if (this == null || width == 0)
return false
val parentCenter = parentWidth / 2
return left < parentCenter && parentCenter < right
【讨论】：
谢谢！我试试看：）以上是关于RecyclerView SnapHelper 无法显示第一个/最后一个项目的主要内容，如果未能解决你的问题，请参考以下文章
全面解析：SnapHelper用法、原理，含演示效果对比+实现代码