本文介绍了如何使PagerSnapHelper在RecyclerView中双向捕捉?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我已使用以下代码通过水平LinearLayoutManager将PagerSnapHelper附加到RecyclerView.
I've attached a PagerSnapHelper to a RecyclerView with a Horizontal LinearLayoutManager using below code.
SnapHelper snapHelper = new PagerSnapHelper();
snapHelper.attachToRecyclerView(myRecyclerView);
但是，当我执行滚动时，捕捉仅发生在一个方向.也就是说，它会在向右滑动时捕捉到下一个项目，但不会在向左滑动时捕捉到上一个项目.
However, when I perform scroll, snapping happens only in one direction. i.e. it snaps to next item on a right swipe but doesn't snap to previous item on a left swipe.
这是默认行为吗?我是否可以覆盖此设置，以便在向左滑动时也可以捕捉到上一个项目?
Is this the default behavior? Could I override this to enable snapping to previous item as well on a left swipe?
这里的任何指针都会有很大的帮助.谢谢！
Any pointers here would be of great help. Thanks!
推荐答案
SnapHelper在某些情况下有时会出现问题，例如项目数等.
SnapHelper sometimes has issues in some cases, like item count etc.
您可以使用此 GravitySnapHelper 类，该类是从LinearSnapHelper扩展的.
You can use this GravitySnapHelper class which is extended from LinearSnapHelper.
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);
}
}
用法很简单:
SnapHelper snapHelper = new GravitySnapHelper(Gravity.START);
snapHelper.attachToRecyclerView(recyclerView);
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