ViewPager2的源码分析
一 ViewPager2新特性
基于RecyclerView实现。这意味着RecyclerView的优点将会被ViewPager2所继承。
2.支持竖直滑动。只需要一个参数就可以改变滑动方向。
3.支持关闭用户输入。通过setUserInputEnabled来设置是否禁止用户滑动页面。
4.支持通过编程方式滚动。通过fakeDragBy(offsetPx)代码模拟用户滑动页面。
5.CompositePageTransformer 支持同时添加多个PageTransformer。
6.支持DiffUtil ，可以添加数据集合改变的item动画。
7.支持RTL (right-to-left)布局。
二 基本使用
- 引入viewpager2库
// module的build.gradle文件
implementation("androidx.viewpager2:viewpager2:1.0.0")2 在布局中添加viewpager2
<androidx.viewpager2.widget.ViewPager2android:id="@+id/viewPager"android:layout_width="match_parent"android:layout_height="match_parent" />3 继承FragmentStateAdapter创建adapter
class DemoAapdater(activity: FragmentActivity) : FragmentStateAdapter(activity) {override fun getItemCount(): Int {return 6}override fun createFragment(position: Int): Fragment {val demoFragment = DemoFragment()demoFragment.arguments = Bundle().apply {putInt("TEXT", position)}return demoFragment}
}4 为viewpager2设置adapter
demoAapdater = DemoAapdater(this)
viewPager2.adapter = demoAapdater三 ViewPager2源码分析
ViewPager初始化
private void initialize(Context context, AttributeSet attrs) {// .... 省约部分// 初始化RecyclerViewmRecyclerView = new RecyclerViewImpl(context);mRecyclerView.setId(ViewCompat.generateViewId());// 初始化LayoutManagermLayoutManager = new LinearLayoutManagerImpl(context);mRecyclerView.setLayoutManager(mLayoutManager);setOrientation(context, attrs);mRecyclerView.setLayoutParams(new ViewGroup.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));mRecyclerView.addOnChildAttachStateChangeListener(enforceChildFillListener());// 创建滑动事件转换器的对象mScrollEventAdapter = new ScrollEventAdapter(mLayoutManager);// 创建模拟拖动事件的对象mFakeDragger = new FakeDrag(this, mScrollEventAdapter, mRecyclerView);// 创建PagerSnapHelper对象，用来实现页面切换的基本效果mPagerSnapHelper = new PagerSnapHelperImpl();mPagerSnapHelper.attachToRecyclerView(mRecyclerView);mRecyclerView.addOnScrollListener(mScrollEventAdapter);// ······}1.其中mRecyclerView = new RecyclerViewImpl(context);和mLayoutManager = new LinearLayoutManagerImpl(context); 属于重新封装的RecyclerView和LinearLayoutManager， 是分别为了处理事件拦截和ViewPager缓存页面的问题。
2.给RecyclerView设置了滑动监听事件，涉及到的组件是ScrollEventAdapter，后面的基本功能都需要这个组件的支持； (2) 设置了PagerSnapHelper,目的是实现切面切换的效果
1.PagerSnapHelper分析
这个类主要继承了SnapHelper 然后实现了里面3个重要的方法
calculateDistanceToFinalSnap()
findSnapView()
findTargetSnapPosition()这三个方法在RecyclerView 滑动时调用
1.手指在快速滑动一个RecyclerView，在手指离开屏幕之前，如上的三个方法都不会被调用。
2.如果手指如果手指离开了屏幕，接下来就是Fling事件来滑动RecyclerView，在Fling事件触发之际，findTargetSnapPosition方法会被调用，此方法的作用就是用来计算Fling事件能滑动到位置。
@Overridepublic boolean onFling(int velocityX, int velocityY) {RecyclerView.LayoutManager layoutManager = mRecyclerView.getLayoutManager();if (layoutManager == null) {return false;}RecyclerView.Adapter adapter = mRecyclerView.getAdapter();if (adapter == null) {return false;}int minFlingVelocity = mRecyclerView.getMinFlingVelocity();return (Math.abs(velocityY) > minFlingVelocity || Math.abs(velocityX) > minFlingVelocity)1&& snapFromFling(layoutManager, velocityX, velocityY);}private boolean snapFromFling(@NonNull RecyclerView.LayoutManager layoutManager, int velocityX,int velocityY) {if (!(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {return false;}RecyclerView.SmoothScroller smoothScroller = createScroller(layoutManager);if (smoothScroller == null) {return false;}int targetPosition = findTargetSnapPosition(layoutManager, velocityX, velocityY);if (targetPosition == RecyclerView.NO_POSITION) {return false;}smoothScroller.setTargetPosition(targetPosition);layoutManager.startSmoothScroll(smoothScroller);return true;}3.当Fling事件结束之际，RecyclerView会回调SnapHelper内部OnScrollListener接口的onScrollStateChanged方法。
private final RecyclerView.OnScrollListener mScrollListener =new RecyclerView.OnScrollListener() {boolean mScrolled = false;@Overridepublic void onScrollStateChanged(RecyclerView recyclerView, int newState) {super.onScrollStateChanged(recyclerView, newState);if (newState == RecyclerView.SCROLL_STATE_IDLE && mScrolled) {mScrolled = false;1 此时RecyclerView的滑动状态为 RecyclerView.SCROLL_STATE_IDLE,所以就会分别调用findSnapView方法来找到需要显示在RecyclerView的最前面的View。找到目标View之后，就会调用calculateDistanceToFinalSnap方法来计算需要滑动的距离，然后调动RecyclerView相关方法进行滑动。snapToTargetExistingView();}}@Overridepublic void onScrolled(RecyclerView recyclerView, int dx, int dy) {if (dx != 0 || dy != 0) {mScrolled = true;}}};接着分析
snapToTargetExistingView
当RecyclerView在Fling时，如果想要不去拦截Fling时间,想让RecyclerView开心的Fling，可以直接在findTargetSnapPosition方法返回RecyclerView.NO_POSITION即可,或者我们可以在findTargetSnapPosition方法来计算滑动的最终位置，然后通过SmoothScroller来实现滑动。。
/* Snaps to a target view which currently exists in the attached {@link RecyclerView}. This* method is used to snap the view when the {@link RecyclerView} is first attached; when* snapping was triggered by a scroll and when the fling is at its final stages.*/void snapToTargetExistingView() {if (mRecyclerView == null) {return;}RecyclerView.LayoutManager layoutManager = mRecyclerView.getLayoutManager();if (layoutManager == null) {return;}注释1 View snapView = findSnapView(layoutManager);if (snapView == null) {return;}注释2 int[] snapDistance = calculateDistanceToFinalSnap(layoutManager, snapView);if (snapDistance[0] != 0 || snapDistance[1] != 0) {mRecyclerView.smoothScrollBy(snapDistance[0], snapDistance[1]);}}因为viewPager2是不支持快速滑动Filing事件的，所以在PagerSnapHelper中的findTargetSnapPosition()方法中做了处理下面看看
@Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX,int velocityY) {// ······// 1 找到与当前View相邻的View，包括左相邻和右响铃，并且计算滑动的距离for (int i = 0; i < childCount; i++) {final View child = layoutManager.getChildAt(i);if (child == null) {continue;}final int distance = distanceToCenter(layoutManager, child, orientationHelper);if (distance <= 0 && distance > distanceBefore) {// Child is before the center and closer then the previous bestdistanceBefore = distance;closestChildBeforeCenter = child;}if (distance >= 0 && distance < distanceAfter) {// Child is after the center and closer then the previous bestdistanceAfter = distance;closestChildAfterCenter = child;}}// 2 根据滑动的方向来返回的相应位置final boolean forwardDirection = isForwardFling(layoutManager, velocityX, velocityY);if (forwardDirection && closestChildAfterCenter != null) {return layoutManager.getPosition(closestChildAfterCenter);} else if (!forwardDirection && closestChildBeforeCenter != null) {return layoutManager.getPosition(closestChildBeforeCenter);}// 3 最后兜底计算View visibleView = forwardDirection ? closestChildBeforeCenter : closestChildAfterCenter;if (visibleView == null) {return RecyclerView.NO_POSITION;}int visiblePosition = layoutManager.getPosition(visibleView);int snapToPosition = visiblePosition+ (isReverseLayout(layoutManager) == forwardDirection ? -1 : +1);if (snapToPosition < 0 || snapToPosition >= itemCount) {return RecyclerView.NO_POSITION;}return snapToPosition;
}从上面看出为了拦截Filing事件，PagerSnapHelper中的findTargetSnapPosition()方法直接返回当前ItemView的上一个ItemView或者下一个ItemView的位置,只要不返回RecyclerView.NO_POSITION就不会有Fling效果! ——> 怎么阻止的Fling事件的触发呢？看看原因:(1)会调用父类(SnapHelper)里面的snapFromFling方法,只要findTargetSnapPosition方法返回不为RecyclerView.NO_POSITION,就会直接走startSmotthiScroll(),所以RecyclerView最终滑到的位置为当前位置的上一个或者下一个，不会产生Fling的效果。
接着分析findSnapView
public View findSnapView(RecyclerView.LayoutManager layoutManager) {if (layoutManager.canScrollVertically()) {return findCenterView(layoutManager, getVerticalHelper(layoutManager));} else if (layoutManager.canScrollHorizontally()) {return findCenterView(layoutManager, getHorizontalHelper(layoutManager));}return null;} private View findCenterView(RecyclerView.LayoutManager layoutManager,OrientationHelper helper) {int childCount = layoutManager.getChildCount();if (childCount == 0) {return null;}View closestChild = null;final int center = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;int absClosest = Integer.MAX_VALUE;for (int i = 0; i < childCount; i++) {final View child = layoutManager.getChildAt(i);int childCenter = helper.getDecoratedStart(child)+ (helper.getDecoratedMeasurement(child) / 2);int absDistance = Math.abs(childCenter - center);//注释说了很清楚,滑动中间时相邻的那个最近，就确定距离屏幕中心最近的页面/* if child center is closer than previous closest, set it as closest */if (absDistance < absClosest) {absClosest = absDistance;closestChild = child;}}return closestChild;}1 当recyclerView滑动完之后，就会调用findSnapView这个方法,来确认最终位置的ItemView
2 在findSnapView内部，调用findCenterView方法，找到当前中心距离屏幕中心最近的ItemView
距离屏幕中心最近的页面。findCenterView方法的作用便是如此。
接着分析 snapToTargetExistingView 中注释2处 calculateDistanceToFinalSnap 方法
public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager,@NonNull View targetView) {int[] out = new int[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;
} private int distanceToCenter(@NonNull RecyclerView.LayoutManager layoutManager,@NonNull View targetView, OrientationHelper helper) {final int childCenter = helper.getDecoratedStart(targetView)+ (helper.getDecoratedMeasurement(targetView) / 2);final int containerCenter = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;return childCenter - containerCenter;}由上面的代码可知
calculateDistanceToFinalSnap()方法就是计算RecyclerView需要滑动的距离，主要通过distanceToCenter方法来计算,确定最终要展示的positon
PagerSnapHelper在最后总结 PagerSnapHelper可以实现页面切换的效果
(1)首先阻止RecyclerView的Fling事件，阻止的方式就是重写findTargetSnapPosition方法，当RecyclerView触发了Fling事件之后，直接滑动到下一个或者上一个。
(2)如果RecyclerView没有触发Fling事件，或者Fling阶段未能滑动到正确位置，此时需要findSnapView方法和calculateDistanceToFinalSnap来保证滑动到正确的页面。
2.ScrollEventAdapter分析
//表示当前ViewPager2处于停止状态private static final int STATE_IDLE = 0;//表示当前ViewPager2处于手指拖动状态private static final int STATE_IN_PROGRESS_MANUAL_DRAG = 1;//表示当前ViewPager2处于缓慢滑动的状态。这个状态只在调用了ViewPager2的setCurrentItem方法才有可能出现。private static final int STATE_IN_PROGRESS_SMOOTH_SCROLL = 2;//表示当前ViewPager2处于迅速滑动的状态。这个状态只在调用了ViewPager2的setCurrentItem方法才有可能出现。private static final int STATE_IN_PROGRESS_IMMEDIATE_SCROLL = 3;//表示当前ViewPager2未使用手指滑动，而是通过FakerDrag实现的。private static final int STATE_IN_PROGRESS_FAKE_DRAG = 4;ScrollEventAdapter实现了recyclerView的OnScrollListener事件,我们只需要看onScrollStateChanged()和onScolled方法
1.onScrollStateChanged()
public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {// 1. 开始拖动 会调用startDrag方法表示拖动开始if ((mAdapterState != STATE_IN_PROGRESS_MANUAL_DRAG|| mScrollState != SCROLL_STATE_DRAGGING)&& newState == RecyclerView.SCROLL_STATE_DRAGGING) {startDrag(false);return;}// 2. 拖动手势的释放，此时ViewPager2会准备滑动到正确的位置if (isInAnyDraggingState() && newState == RecyclerView.SCROLL_STATE_SETTLING) {// Only go through the settling phase if the drag actually moved the pageif (mScrollHappened) {dispatchStateChanged(SCROLL_STATE_SETTLING);mDispatchSelected = true;}return;}//3.滑动结束 此时ScrollEventAdapter会调用相关的方法更新状态// Drag is finished (dragging || settling -> idle)if (isInAnyDraggingState() && newState == RecyclerView.SCROLL_STATE_IDLE) {boolean dispatchIdle = false;updateScrollEventValues();if (!mScrollHappened) {if (mScrollValues.mPosition != RecyclerView.NO_POSITION) {dispatchScrolled(mScrollValues.mPosition, 0f, 0);}dispatchIdle = true;} else if (mScrollValues.mOffsetPx == 0) {dispatchIdle = true;if (mDragStartPosition != mScrollValues.mPosition) {dispatchSelected(mScrollValues.mPosition);}}if (dispatchIdle) {dispatchStateChanged(SCROLL_STATE_IDLE);resetState();}}if (mAdapterState == STATE_IN_PROGRESS_SMOOTH_SCROLL&& newState == RecyclerView.SCROLL_STATE_IDLE && mDataSetChangeHappened) {updateScrollEventValues();if (mScrollValues.mOffsetPx == 0) {if (mTarget != mScrollValues.mPosition) {dispatchSelected(mScrollValues.mPosition == NO_POSITION ? 0 : mScrollValues.mPosition);}dispatchStateChanged(SCROLL_STATE_IDLE);resetState();}}}2.onScrolled()
public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {mScrollHappened = true;// 1 updateScrollEventValues();if (mDispatchSelected) {// 2 mDispatchSelected = false;boolean scrollingForward = dy > 0 || (dy == 0 && dx < 0 == isLayoutRTL());mTarget = scrollingForward && mScrollValues.mOffsetPx != 0? mScrollValues.mPosition + 1 : mScrollValues.mPosition;if (mDragStartPosition != mTarget) {dispatchSelected(mTarget);}} else if (mAdapterState == STATE_IDLE) {// 调用了setAdapter方法dispatchSelected(mScrollValues.mPosition);}dispatchScrolled(mScrollValues.mPosition, mScrollValues.mOffset, mScrollValues.mOffsetPx);// 3if ((mScrollValues.mPosition == mTarget || mTarget == NO_POSITION)&& mScrollValues.mOffsetPx == 0 && !(mScrollState == SCROLL_STATE_DRAGGING)) {dispatchStateChanged(SCROLL_STATE_IDLE);resetState();}
}1 处新且计算当前position和offset的值
2 拖动手势释放，ViewPager2正在滑动到正确的位置
3 因为调用了setCurrentItem(x, false)不会触发IDLE状态的产生
onScrolled()方法中主要是调用updateScrollEventValues()方法更新状态ScrollEventValues里面3个变量，(2)调用相关dispatch...方法更新状态和分发状态下去
private static final class ScrollEventValues {int mPosition;//从开始滑动到滑动结束，一直记录着当前滑动到的位置。float mOffset;//从一个页面滑动到另一个页面，记录着滑动的百分比。int mOffsetPx;//记录着从开始滑动的页面与当前状态的滑动。每次滑动结束之后，会被重置。}ScrollEventAdapter总结:
(1)当调用ViewPager2的setAdapter方法时,此时应该回调一次dispatchSelected方法。
(2)当调用setCurrentItem(x, false)方法，不会调用onScrollStateChanged方法，于是不会产生idle状态，
所以，咱们须要在onScrolled方法特殊处理(onScrolled方法会被调用)。
(3)正常的拖动和释放，就是onScrollStateChanged方法和onScrolled方法的正常回调。
3.PageTransformerAdapter
void setPageTransformer(@Nullable PageTransformer transformer) {// TODO: add support for reverseDrawingOrder: b/112892792// TODO: add support for pageLayerType: b/112893074mPageTransformer = transformer; }@Overridepublic void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {if (mPageTransformer == null) {return;}float transformOffset = -positionOffset;for (int i = 0; i < mLayoutManager.getChildCount(); i++) {View view = mLayoutManager.getChildAt(i);if (view == null) {throw new IllegalStateException(String.format(Locale.US,"LayoutManager returned a null child at pos %d/%d while transforming pages",i, mLayoutManager.getChildCount()));}int currPos = mLayoutManager.getPosition(view);float viewOffset = transformOffset + (currPos - position);mPageTransformer.transformPage(view, viewOffset);}}PageTransformerAdapter实现也非常简单,PageTransformerAdapter实现于OnPageChangeCallback接口，监听的是ScrollEventAdapter的页面滑动事件，然后将页面滑动事件转换成为上面特殊的事件
4.FragmentStateAdapter
//(1)key为itemId，value为Fragment。表示position与所放Fragment的对应关系(itemId与position有对应关系)final LongSparseArray<Fragment> mFragments = new LongSparseArray<>();//(2)key为itemId，value为Fragment的状态private final LongSparseArray<Fragment.SavedState> mSavedStates = new LongSparseArray<>();//(3)key为itemId, value为ItemView的id。private final LongSparseArray<Integer> mItemIdToViewHolder = new LongSparseArray<>();private FragmentMaxLifecycleEnforcer mFragmentMaxLifecycleEnforcer;boolean mIsInGracePeriod = false;private boolean mHasStaleFragments = false;1 onCreateViewHolder
public final class FragmentViewHolder extends ViewHolder {private FragmentViewHolder(@NonNull FrameLayout container) {super(container);}@NonNull static FragmentViewHolder create(@NonNull ViewGroup parent) {FrameLayout container = new FrameLayout(parent.getContext());container.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,ViewGroup.LayoutParams.MATCH_PARENT));container.setId(ViewCompat.generateViewId());container.setSaveEnabled(false);return new FragmentViewHolder(container);}@NonNull FrameLayout getContainer() {return (FrameLayout) itemView;}
}创建FrameLayout视图
2 onBindViewHolder()
public final void onBindViewHolder(final @NonNull FragmentViewHolder holder, int position) {final long itemId = holder.getItemId();final int viewHolderId = holder.getContainer().getId();final Long boundItemId = itemForViewHolder(viewHolderId); // 1 如果当前ItemView已经加载了Fragment，并且不是同一个Fragment// 那么就移除if (boundItemId != null && boundItemId != itemId) {removeFragment(boundItemId);mItemIdToViewHolder.remove(boundItemId);}mItemIdToViewHolder.put(itemId, viewHolderId); // this might overwrite an existing entry// 2 保证对应位置的Fragment已经初始化，并且放在mFragments中-->确定相应的Fragment，ensureFragment(position);final FrameLayout container = holder.getContainer();// 3 特殊情况处理，当RecyclerView让ItemView保持在Window，但是不在视图树中。if (ViewCompat.isAttachedToWindow(container)) {if (container.getParent() != null) {throw new IllegalStateException("Design assumption violated.");}container.addOnLayoutChangeListener(new View.OnLayoutChangeListener() {@Overridepublic void onLayoutChange(View v, int left, int top, int right, int bottom,int oldLeft, int oldTop, int oldRight, int oldBottom) {if (container.getParent() != null) {container.removeOnLayoutChangeListener(this);placeFragmentInViewHolder(holder);}}});}gcFragments();}private void ensureFragment(int position) {long itemId = getItemId(position);if (!mFragments.containsKey(itemId)) {// TODO(133419201): check if a Fragment provided here is a new FragmentFragment newFragment = createFragment(position);newFragment.setInitialSavedState(mSavedStates.get(itemId));mFragments.put(itemId, newFragment);}}3 onViewAttachedToWindow (ViewHolder与Fragment进行绑定)
@Override
public final void onViewAttachedToWindow(@NonNull final FragmentViewHolder holder) {placeFragmentInViewHolder(holder);gcFragments();
}调用placeFragmentInViewHolder方法加载Fragment
4.onViewRecycled (ViewHolder与Fragment解除绑定)
@Overridepublic final void onViewRecycled(@NonNull FragmentViewHolder holder) {final int viewHolderId = holder.getContainer().getId();final Long boundItemId = itemForViewHolder(viewHolderId); // item currently bound to the VHif (boundItemId != null) {removeFragment(boundItemId);mItemIdToViewHolder.remove(boundItemId);}}当ViewHolder被回收到回收池中，onViewRecycled方法会被调用 。
有在onViewDetachedFromWindow中去删除Fragment,而是在onViewRecycled中回收。因为当onViewRecycled方法被调用，表示当前ViewHolder已经彻底没有用了，被放入回收池，等待后面被复用，此时存在的情况可能有：(1).当前ItemView手动移除掉了；(2).当前位置对应的视图已经彻底不在屏幕中，被当前屏幕中某些位置复用了。所以在onViewRecycled方法里面移除Fragment比较合适 2.不在onViewDetachedFromWindow中删除的原因呢? 因为每当一个页面被滑走，都会调用这个方法，如果对其Fragment进行卸载，此时用户又滑回来，又要重新加载一次，这性能就下降了很多
5 .placeFragmentInViewHolder
void placeFragmentInViewHolder(@NonNull final FragmentViewHolder holder) {// ······// 1.Fragment未添加到ItemView中，但是View已经创建// 非法状态if (!fragment.isAdded() && view != null) {throw new IllegalStateException("Design assumption violated.");}// 2.Fragment添加到ItemView中，但是View未创建// 先等待View创建完成，然后将View添加到Container。if (fragment.isAdded() && view == null) {scheduleViewAttach(fragment, container);return;}// 3.Fragment添加到ItemView中，同时View已经创建完成并且添加到Container中// 需要保证View添加到正确的Container中。if (fragment.isAdded() && view.getParent() != null) {if (view.getParent() != container) {addViewToContainer(view, container);}return;}// 4.Fragment添加到ItemView中，同时View已经创建完成但是未添加到Container中// 需要将View添加到Container中。if (fragment.isAdded()) {addViewToContainer(view, container);return;}// 5.Fragment未创建，View未创建、未添加if (!shouldDelayFragmentTransactions()) {scheduleViewAttach(fragment, container);mFragmentManager.beginTransaction().add(fragment, "f" + holder.getItemId()).setMaxLifecycle(fragment, STARTED) //懒加载.commitNow();mFragmentMaxLifecycleEnforcer.updateFragmentMaxLifecycle(false);} else {// 调用了第5步，但是Fragment还未真正创建if (mFragmentManager.isDestroyed()) {return; // nothing we can do}mLifecycle.addObserver(new GenericLifecycleObserver() {@Overridepublic void onStateChanged(@NonNull LifecycleOwner source,@NonNull Lifecycle.Event event) {if (shouldDelayFragmentTransactions()) {return;}source.getLifecycle().removeObserver(this);if (ViewCompat.isAttachedToWindow(holder.getContainer())) {placeFragmentInViewHolder(holder);}}});}}最后总结
1 ViewPager2本身是一个ViewGroup，没有特殊作用，只是用来装一个RecyclerView。
2 PagerSnapHelper实现页面切换效果的原因是calculateDistanceToFinalSnap阻止RecyclerView的Fling事件，直接让它滑动相邻页面；findSnapView方法和findTargetSnapPosition用来辅助滑动到正确的位置。
3 ScrollEventAdapter的作用将RecyclerView的滑动事件转换成为ViewPager2的页面滑动事件。
4 PageTransformerAdapter的作用将普通的页面滑动事件转换为特殊事件。
5 FragmentStateAdapter完美实现了使用Adapter加载Fragment。在FragmentStateAdapter中，完美地考虑到ViewHolder的复用，Fragment加载和卸载。
