我已使用以下代码将PagerSnapHelper附加到带有Horizontal LinearLayoutManager的RecyclerView。
SnapHelper snapHelper = new PagerSnapHelper();
snapHelper.attachToRecyclerView(myRecyclerView);
然而，当我执行滚动时，捕捉仅在一个方向上发生。即它会在右侧滑动时捕捉到下一个项目，但不会在左侧滑动时捕捉到上一个项目。
这是默认行为吗？我是否可以覆盖它以在左侧滑动时启用捕捉到上一个项目？
这里有任何指示都会有很大帮助。谢谢！
答案 0 :(得分：1)
func handle(_ backgroundTasks: Set<WKRefreshBackgroundTask>) {
for backgroundTask in backgroundTasks {
switch backgroundTask {
case let connectivityTask as WKWatchConnectivityRefreshBackgroundTask:
connectivityTask.setTaskCompleted()
case let refreshTask as WKApplicationRefreshBackgroundTask:
MainInterfaceController.scheduleTask()
refreshTask.setTaskCompleted()
case let snapshotTask as WKSnapshotRefreshBackgroundTask:
snapshotTask.setTaskCompleted()
default:
backgroundTask.setTaskCompleted()
}
}
DataProcessor.sharedInstance.dataProcessing()
public static func scheduleTask() {
let fireDate = Date(timeIntervalSinceNow: 10.0)
WKExtension.shared().scheduleBackgroundRefresh(withPreferredDate: fireDate, userInfo: nil) { (error) in
if (error == nil) {
print("Background task scheduled")
}
}
}
在某些情况下有时会出现问题，例如项目计数等。
您可以使用从SnapHelper扩展的 GravitySnapHelper 类。
LinearSnapHelper用法很简单：
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.LinearSnapHelper;
import android.support.v7.widget.OrientationHelper;
import android.support.v7.widget.RecyclerView;
import android.view.Gravity;
import android.view.View;
public class GravitySnapHelper extends LinearSnapHelper {
private OrientationHelper mVerticalHelper;
private OrientationHelper mHorizontalHelper;
private int mGravity;
private boolean mIsRtlHorizontal;
private boolean mSnapLastItemEnabled;
SnapListener mSnapListener;
boolean mSnapping;
private RecyclerView.OnScrollListener mScrollListener = new RecyclerView.OnScrollListener() {
@Override
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
super.onScrollStateChanged(recyclerView, newState);
if (newState == RecyclerView.SCROLL_STATE_SETTLING) {
mSnapping = false;
}
if (newState == RecyclerView.SCROLL_STATE_IDLE && mSnapping && mSnapListener != null) {
int position = getSnappedPosition(recyclerView);
if (position != RecyclerView.NO_POSITION) {
mSnapListener.onSnap(position);
}
mSnapping = false;
}
}
};
public GravitySnapHelper(int gravity) {
this(gravity, false, null);
}
public GravitySnapHelper(int gravity, boolean enableSnapLastItem) {
this(gravity, enableSnapLastItem, null);
}
public GravitySnapHelper(int gravity, boolean enableSnapLastItem, SnapListener snapListener) {
if (gravity != Gravity.START && gravity != Gravity.END
&& gravity != Gravity.BOTTOM && gravity != Gravity.TOP) {
throw new IllegalArgumentException("Invalid gravity value. Use START " +
"| END | BOTTOM | TOP constants");
}
mSnapListener = snapListener;
mGravity = gravity;
mSnapLastItemEnabled = enableSnapLastItem;
}
@Override
public void attachToRecyclerView(@Nullable RecyclerView recyclerView)
throws IllegalStateException {
if (recyclerView != null) {
if (mGravity == Gravity.START || mGravity == Gravity.END) {
mIsRtlHorizontal
= false;
}
if (mSnapListener != null) {
recyclerView.addOnScrollListener(mScrollListener);
}
}
super.attachToRecyclerView(recyclerView);
}
@Override
public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager,
@NonNull View targetView) {
int[] out = new int[2];
if (layoutManager.canScrollHorizontally()) {
if (mGravity == Gravity.START) {
out[0] = distanceToStart(targetView, getHorizontalHelper(layoutManager), false);
} else { // END
out[0] = distanceToEnd(targetView, getHorizontalHelper(layoutManager), false);
}
} else {
out[0] = 0;
}
if (layoutManager.canScrollVertically()) {
if (mGravity == Gravity.TOP) {
out[1] = distanceToStart(targetView, getVerticalHelper(layoutManager), false);
} else { // BOTTOM
out[1] = distanceToEnd(targetView, getVerticalHelper(layoutManager), false);
}
} else {
out[1] = 0;
}
return out;
}
@Override
public View findSnapView(RecyclerView.LayoutManager layoutManager) {
View snapView = null;
if (layoutManager instanceof LinearLayoutManager) {
switch (mGravity) {
case Gravity.START:
snapView = findStartView(layoutManager, getHorizontalHelper(layoutManager));
break;
case Gravity.END:
snapView = findEndView(layoutManager, getHorizontalHelper(layoutManager));
break;
case Gravity.TOP:
snapView = findStartView(layoutManager, getVerticalHelper(layoutManager));
break;
case Gravity.BOTTOM:
snapView = findEndView(layoutManager, getVerticalHelper(layoutManager));
break;
}
}
mSnapping = snapView != null;
return snapView;
}
/**
* Enable snapping of the last item that's snappable.
* The default value is false, because you can't see the last item completely
* if this is enabled.
*
* @param snap true if you want to enable snapping of the last snappable item
*/
public void enableLastItemSnap(boolean snap) {
mSnapLastItemEnabled = snap;
}
private int distanceToStart(View targetView, OrientationHelper helper, boolean fromEnd) {
if (mIsRtlHorizontal && !fromEnd) {
return distanceToEnd(targetView, helper, true);
}
return helper.getDecoratedStart(targetView) - helper.getStartAfterPadding();
}
private int distanceToEnd(View targetView, OrientationHelper helper, boolean fromStart) {
if (mIsRtlHorizontal && !fromStart) {
return distanceToStart(targetView, helper, true);
}
return helper.getDecoratedEnd(targetView) - helper.getEndAfterPadding();
}
/**
* Returns the first view that we should snap to.
*
* @param layoutManager the recyclerview's layout manager
* @param helper orientation helper to calculate view sizes
* @return the first view in the LayoutManager to snap to
*/
private View findStartView(RecyclerView.LayoutManager layoutManager,
OrientationHelper helper) {
if (layoutManager instanceof LinearLayoutManager) {
int firstChild = ((LinearLayoutManager) layoutManager).findFirstVisibleItemPosition();
if (firstChild == RecyclerView.NO_POSITION) {
return null;
}
View child = layoutManager.findViewByPosition(firstChild);
float visibleWidth;
// We should return the child if it's visible width
// is greater than 0.5 of it's total width.
// In a RTL configuration, we need to check the start point and in LTR the end point
if (mIsRtlHorizontal) {
visibleWidth = (float) (helper.getTotalSpace() - helper.getDecoratedStart(child))
/ helper.getDecoratedMeasurement(child);
} else {
visibleWidth = (float) helper.getDecoratedEnd(child)
/ helper.getDecoratedMeasurement(child);
}
// If we're at the end of the list, we shouldn't snap
// to avoid having the last item not completely visible.
boolean endOfList = ((LinearLayoutManager) layoutManager)
.findLastCompletelyVisibleItemPosition()
== layoutManager.getItemCount() - 1;
if (visibleWidth > 0.5f && !endOfList) {
return child;
} else if (mSnapLastItemEnabled && endOfList) {
return child;
} else if (endOfList) {
return null;
} else {
// If the child wasn't returned, we need to return
// the next view close to the start.
return layoutManager.findViewByPosition(firstChild + 1);
}
}
return null;
}
private View findEndView(RecyclerView.LayoutManager layoutManager,
OrientationHelper helper) {
if (layoutManager instanceof LinearLayoutManager) {
int lastChild = ((LinearLayoutManager) layoutManager).findLastVisibleItemPosition();
if (lastChild == RecyclerView.NO_POSITION) {
return null;
}
View child = layoutManager.findViewByPosition(lastChild);
float visibleWidth;
if (mIsRtlHorizontal) {
visibleWidth = (float) helper.getDecoratedEnd(child)
/ helper.getDecoratedMeasurement(child);
} else {
visibleWidth = (float) (helper.getTotalSpace() - helper.getDecoratedStart(child))
/ helper.getDecoratedMeasurement(child);
}
// If we're at the start of the list, we shouldn't snap
// to avoid having the first item not completely visible.
boolean startOfList = ((LinearLayoutManager) layoutManager)
.findFirstCompletelyVisibleItemPosition() == 0;
if (visibleWidth > 0.5f && !startOfList) {
return child;
} else if (mSnapLastItemEnabled && startOfList) {
return child;
} else if (startOfList) {
return null;
} else {
// If the child wasn't returned, we need to return the previous view
return layoutManager.findViewByPosition(lastChild - 1);
}
}
return null;
}
int getSnappedPosition(RecyclerView recyclerView) {
RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
if (layoutManager instanceof LinearLayoutManager) {
if (mGravity == Gravity.START || mGravity == Gravity.TOP) {
return ((LinearLayoutManager) layoutManager).findFirstCompletelyVisibleItemPosition();
} else if (mGravity == Gravity.END || mGravity == Gravity.BOTTOM) {
return ((LinearLayoutManager) layoutManager).findLastCompletelyVisibleItemPosition();
}
}
return RecyclerView.NO_POSITION;
}
private OrientationHelper getVerticalHelper(RecyclerView.LayoutManager layoutManager) {
if (mVerticalHelper == null) {
mVerticalHelper = OrientationHelper.createVerticalHelper(layoutManager);
}
return mVerticalHelper;
}
private OrientationHelper getHorizontalHelper(RecyclerView.LayoutManager layoutManager) {
if (mHorizontalHelper == null) {
mHorizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager);
}
return mHorizontalHelper;
}
public interface SnapListener {
void onSnap(int position);
}
}