我使用下面的代碼將一個PagerSnapHelper附加到一個帶有水平LinearLayoutManager的RecyclerView。如何啓用PagerSnapHelper在RecyclerView中雙向捕捉？
SnapHelper snapHelper = new PagerSnapHelper();
snapHelper.attachToRecyclerView(myRecyclerView);
但是，當我執行滾動時，貼緊僅發生在一個方向上。即，其捕捉到右側滑動中的下一個項目，但不捕捉到左側滑動中的前一個項目。
這是默認行爲嗎？我可以重寫這個以啓用捕捉到前一個項目以及左側滑動？
這裏的任何指針都會有很大的幫助。謝謝！
SnapHelper有時也有問題，在某些情況下，如項目計數等
您可以使用它從LinearSnapHelper擴展這個GravitySnapHelper類。
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);
}
}
用法很簡單：
SnapHelper snapHelper = new GravitySnapHelper(Gravity.START);
snapHelper.attachToRecyclerView(recyclerView);