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private void initRecyclerView() {
LinearLayoutManager manager = new LinearLayoutManager(this);
manager.setOrientation(LinearLayoutManager.HORIZONTAL);
mRecyclerView.setLayoutManager(manager);
LinearSnapHelper snapHelper = new LinearSnapHelper();
snapHelper.attachToRecyclerView(mRecyclerView);
SnapAdapter adapter = new SnapAdapter(this);
mRecyclerView.setAdapter(adapter);
adapter.addAll(getData());
}private void initRecyclerView() {
LinearLayoutManager manager = new LinearLayoutManager(this);
manager.setOrientation(LinearLayoutManager.HORIZONTAL);
mRecyclerView.setLayoutManager(manager);
PagerSnapHelper snapHelper = new PagerSnapHelper();
snapHelper.attachToRecyclerView(mRecyclerView);
SnapAdapter adapter = new SnapAdapter(this);
mRecyclerView.setAdapter(adapter);
adapter.addAll(getData());
}public void attachToRecyclerView(@Nullable RecyclerView recyclerView)
throws IllegalStateException {
if (mRecyclerView == recyclerView) {
return; // nothing to do
}
if (mRecyclerView != null) {
destroyCallbacks();
}
mRecyclerView = recyclerView;
if (mRecyclerView != null) {
setupCallbacks();
mGravityScroller = new Scroller(mRecyclerView.getContext(),
new DecelerateInterpolator());
snapToTargetExistingView();
}
}private void setupCallbacks() throws IllegalStateException {
if (mRecyclerView.getOnFlingListener() != null) {
throw new IllegalStateException("An instance of OnFlingListener already set.");
}
mRecyclerView.addOnScrollListener(mScrollListener);
mRecyclerView.setOnFlingListener(this);
}void snapToTargetExistingView() {
if (mRecyclerView == null) {
return;
}
LayoutManager layoutManager = mRecyclerView.getLayoutManager();
if (layoutManager == null) {
return;
}
View snapView = findSnapView(layoutManager);
if (snapView == null) {
return;
}
int[] snapDistance = calculateDistanceToFinalSnap(layoutManager, snapView);
if (snapDistance[0] != 0 || snapDistance[1] != 0) {
mRecyclerView.smoothScrollBy(snapDistance[0], snapDistance[1]);
}
}然后来看一下mScrollListener监听里面做了什么
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mScrolled为true表示之前进行过滚动，newState为SCROLL_STATE_IDLE状态表示滚动结束停下来
private final RecyclerView.OnScrollListener mScrollListener =
new RecyclerView.OnScrollListener() {
boolean mScrolled = false;
@Override
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
if (newState == RecyclerView.SCROLL_STATE_IDLE && mScrolled) {
mScrolled = false;
snapToTargetExistingView();
}
}
@Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
if (dx != 0 || dy != 0) {
mScrolled = true;
}
}
};@SuppressWarnings("WeakerAccess")
@Nullable
public abstract int[] calculateDistanceToFinalSnap(@NonNull LayoutManager layoutManager,
@NonNull View targetView);@SuppressWarnings("WeakerAccess")
@Nullable
public abstract View findSnapView(LayoutManager layoutManager);public abstract int findTargetSnapPosition(LayoutManager layoutManager, int velocityX,
int velocityY);@Override
public boolean onFling(int velocityX, int velocityY) {
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)
&& snapFromFling(layoutManager, velocityX, velocityY);
}接着看看snapFromFling方法源代码，就是通过该方法实现平滑滚动并使得在滚动停止时itemView对齐到目的坐标位置
最终通过 SmoothScroller 来滑动到指定位置
private boolean snapFromFling(@NonNull LayoutManager layoutManager, int velocityX,
int velocityY) {
if (!(layoutManager instanceof ScrollVectorProvider)) {
return false;
}
RecyclerView.SmoothScroller smoothScroller = createSnapScroller(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;
}接着看下createSnapScroller这个方法源码
@Nullable
protected LinearSmoothScroller createSnapScroller(LayoutManager layoutManager) {
if (!(layoutManager instanceof ScrollVectorProvider)) {
return null;
}
return new LinearSmoothScroller(mRecyclerView.getContext()) {
@Override
protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
int[] snapDistances = calculateDistanceToFinalSnap(mRecyclerView.getLayoutManager(),
targetView);
final int dx = snapDistances[0];
final int dy = snapDistances[1];
final int time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy)));
if (time > 0) {
action.update(dx, dy, time, mDecelerateInterpolator);
}
}
@Override
protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
}
};
}calculateDistanceToFinalSnap源码如下所示
distanceToCenter方法主要作用是：计算水平或者竖直方向需要移动的距离
@Override
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;
if (layoutManager.getClipToPadding()) {
containerCenter = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;
} else {
containerCenter = helper.getEnd() / 2;
}
return childCenter - containerCenter;
}@Override
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;
}接着看看findCenterView方法源代码
循环LayoutManager的所有子元素，计算每个 childView的中点距离Parent 的中点，找到距离最近的一个，就是需要居中对齐的目标View
@Nullable
private View findCenterView(RecyclerView.LayoutManager layoutManager,
OrientationHelper helper) {
int childCount = layoutManager.getChildCount();
if (childCount == 0) {
return null;
}
View closestChild = null;
final int center;
if (layoutManager.getClipToPadding()) {
center = helper.getStartAfterPadding() + helper.getTotalSpace() / 2;
} else {
center = helper.getEnd() / 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);
/** if child center is closer than previous closest, set it as closest **/
if (absDistance < absClosest) {
absClosest = absDistance;
closestChild = child;
}
}
return closestChild;
}LinearSnapHelper实现了SnapHelper，来看一下在findTargetSnapPosition操作了什么
这个方法在计算targetPosition的时候把布局方式和布局方向都考虑进去了。布局方式可以通过layoutManager.canScrollHorizontally()/layoutManager.canScrollVertically()来判断，布局方向就通过RecyclerView.SmoothScroller.ScrollVectorProvider这个接口中的computeScrollVectorForPosition()方法来判断。
@Override
public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager, int velocityX,
int velocityY) {
if (!(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {
return RecyclerView.NO_POSITION;
}
final int itemCount = layoutManager.getItemCount();
if (itemCount == 0) {
return RecyclerView.NO_POSITION;
}
final View currentView = findSnapView(layoutManager);
if (currentView == null) {
return RecyclerView.NO_POSITION;
}
final int currentPosition = layoutManager.getPosition(currentView);
if (currentPosition == RecyclerView.NO_POSITION) {
return RecyclerView.NO_POSITION;
}
RecyclerView.SmoothScroller.ScrollVectorProvider vectorProvider =
(RecyclerView.SmoothScroller.ScrollVectorProvider) layoutManager;
// 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.
PointF vectorForEnd = vectorProvider.computeScrollVectorForPosition(itemCount - 1);
if (vectorForEnd == null) {
// cannot get a vector for the given position.
return RecyclerView.NO_POSITION;
}
int vDeltaJump, hDeltaJump;
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;
}
int deltaJump = layoutManager.canScrollVertically() ? vDeltaJump : hDeltaJump;
if (deltaJump == 0) {
return RecyclerView.NO_POSITION;
}
int targetPos = currentPosition + deltaJump;
if (targetPos < 0) {
targetPos = 0;
}
if (targetPos >= itemCount) {
targetPos = itemCount - 1;
}
return targetPos;
}如何创建OrientationHelper对象呢？如下所示
@NonNull
private OrientationHelper getVerticalHelper(@NonNull RecyclerView.LayoutManager layoutManager) {
if (mVerticalHelper == null || mVerticalHelper.mLayoutManager != layoutManager) {
mVerticalHelper = OrientationHelper.createVerticalHelper(layoutManager);
}
return mVerticalHelper;
}@NonNull
private OrientationHelper getHorizontalHelper(
@NonNull RecyclerView.LayoutManager layoutManager) {
if (mHorizontalHelper == null || mHorizontalHelper.mLayoutManager != layoutManager) {
mHorizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager);
}
return mHorizontalHelper;
}
private int estimateNextPositionDiffForFling(RecyclerView.LayoutManager layoutManager,
OrientationHelper helper, int velocityX, int velocityY) {
int[] distances = calculateScrollDistance(velocityX, velocityY);
float distancePerChild = computeDistancePerChild(layoutManager, helper);
if (distancePerChild <= 0) {
return 0;
}
int distance =
Math.abs(distances[0]) > Math.abs(distances[1]) ? distances[0] : distances[1];
return (int) Math.round(distance / distancePerChild);
}重写calculateDistanceToFinalSnap方法
@Nullable
@Override
public int[] calculateDistanceToFinalSnap(RecyclerView.LayoutManager layoutManager, View targetView) {
int[] out = new int[2];
if (layoutManager.canScrollHorizontally()) {
out[0] = distanceToStart(targetView, getHorizontalHelper(layoutManager));
} else {
out[0] = 0;
}
if (layoutManager.canScrollVertically()) {
out[1] = distanceToStart(targetView, getVerticalHelper(layoutManager));
} else {
out[1] = 0;
}
return out;
}private int distanceToStart(View targetView, OrientationHelper helper) {
return helper.getDecoratedStart(targetView) - helper.getStartAfterPadding();
}
写findSnapView方法，找到当前时刻的SnapView
@Nullable
@Override
public View findSnapView(RecyclerView.LayoutManager layoutManager) {
if (layoutManager instanceof LinearLayoutManager) {
if (layoutManager.canScrollHorizontally()) {
return findStartView(layoutManager, getHorizontalHelper(layoutManager));
} else {
return findStartView(layoutManager, getVerticalHelper(layoutManager));
}
}
return super.findSnapView(layoutManager);
}
private View findStartView(RecyclerView.LayoutManager layoutManager, OrientationHelper helper) {
if (layoutManager instanceof LinearLayoutManager) {
int firstChild = ((LinearLayoutManager) layoutManager).findFirstVisibleItemPosition();
//需要判断是否是最后一个Item，如果是最后一个则不让对齐，以免出现最后一个显示不完全。
boolean isLastItem = ((LinearLayoutManager) layoutManager).findLastCompletelyVisibleItemPosition()
== layoutManager.getItemCount() - 1;
if (firstChild == RecyclerView.NO_POSITION || isLastItem) {
return null;
}
View child = layoutManager.findViewByPosition(firstChild);
if (helper.getDecoratedEnd(child) >= helper.getDecoratedMeasurement(child) / 2
&& helper.getDecoratedEnd(child) > 0) {
return child;
} else {
if (((LinearLayoutManager) layoutManager).findLastCompletelyVisibleItemPosition()
== layoutManager.getItemCount() - 1) {
return null;
} else {
return layoutManager.findViewByPosition(firstChild + 1);
}
}
}
return super.findSnapView(layoutManager);
}修改滚动速率
@Nullable
protected LinearSmoothScroller createSnapScroller(final RecyclerView.LayoutManager layoutManager) {
if (!(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {
return null;
}
return new LinearSmoothScroller(mRecyclerView.getContext()) {
@Override
protected void onTargetFound(View targetView, RecyclerView.State state, RecyclerView.SmoothScroller.Action action) {
int[] snapDistances = calculateDistanceToFinalSnap(mRecyclerView.getLayoutManager(), targetView);
final int dx;
final int dy;
if (snapDistances != null) {
dx = snapDistances[0];
dy = snapDistances[1];
final int time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy)));
if (time > 0) {
action.update(dx, dy, time, mDecelerateInterpolator);
}
}
}
@Override
protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
//这个地方可以自己设置
return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
}
};
}
