RecyclerView 获取 item 的位置

/**
* Created by JokAr on 2016/2/17.
*/
public class RecyclerViewPositionHelper {
final RecyclerView recyclerView;
final RecyclerView.LayoutManager layoutManager;

public RecyclerViewPositionHelper(RecyclerView recyclerView) {
this.recyclerView = recyclerView;
this.layoutManager = recyclerView.getLayoutManager();
}

public static RecyclerViewPositionHelper createHelper(RecyclerView recyclerView) {
if (recyclerView == null) {
throw new NullPointerException("Recycler View is null");
}
return new RecyclerViewPositionHelper(recyclerView);
}

/**
* Returns the adapter item count.
*
* @return The total number on items in a layout manager
*/
public int getItemCount() {
return layoutManager == null ? 0 : layoutManager.getItemCount();
}

/**
* Returns the adapter position of the first visible view. This position does not include
* adapter changes that were dispatched after the last layout pass.
*
* @return The adapter position of the first visible item or {@link RecyclerView#NO_POSITION} if
* there aren't any visible items.
*/
public int findFirstVisibleItemPosition() {
final View child = findOneVisibleChild(0, layoutManager.getChildCount(), false, true);
return child == null ? RecyclerView.NO_POSITION : recyclerView.getChildAdapterPosition(child);
}

/**
* Returns the adapter position of the first fully visible view. This position does not include
* adapter changes that were dispatched after the last layout pass.
*
* @return The adapter position of the first fully visible item or
* {@link RecyclerView#NO_POSITION} if there aren't any visible items.
*/
public int findFirstCompletelyVisibleItemPosition() {
final View child = findOneVisibleChild(0, layoutManager.getChildCount(), true, false);
return child == null ? RecyclerView.NO_POSITION : recyclerView.getChildAdapterPosition(child);
}

/**
* Returns the adapter position of the last visible view. This position does not include
* adapter changes that were dispatched after the last layout pass.
*
* @return The adapter position of the last visible view or {@link RecyclerView#NO_POSITION} if
* there aren't any visible items
*/
public int findLastVisibleItemPosition() {
final View child = findOneVisibleChild(layoutManager.getChildCount() - 1, -1, false, true);
return child == null ? RecyclerView.NO_POSITION : recyclerView.getChildAdapterPosition(child);
}

/**
* Returns the adapter position of the last fully visible view. This position does not include
* adapter changes that were dispatched after the last layout pass.
*
* @return The adapter position of the last fully visible view or
* {@link RecyclerView#NO_POSITION} if there aren't any visible items.
*/
public int findLastCompletelyVisibleItemPosition() {
final View child = findOneVisibleChild(layoutManager.getChildCount() - 1, -1, true, false);
return child == null ? RecyclerView.NO_POSITION : recyclerView.getChildAdapterPosition(child);
}

View findOneVisibleChild(int fromIndex, int toIndex, boolean completelyVisible,
boolean acceptPartiallyVisible) {
OrientationHelper helper;
if (layoutManager.canScrollVertically()) {
helper = OrientationHelper.createVerticalHelper(layoutManager);
} else {
helper = OrientationHelper.createHorizontalHelper(layoutManager);
}

final int start = helper.getStartAfterPadding();
final int end = helper.getEndAfterPadding();
final int next = toIndex > fromIndex ? 1 : -1;
View partiallyVisible = null;
for (int i = fromIndex; i != toIndex; i += next) {
final View child = layoutManager.getChildAt(i);
final int childStart = helper.getDecoratedStart(child);
final int childEnd = helper.getDecoratedEnd(child);
if (childStart < end && childEnd > start) {
if (completelyVisible) {
if (childStart >= start && childEnd <= end) {
return child;
} else if (acceptPartiallyVisible && partiallyVisible == null) {
partiallyVisible = child;
}
} else {
return child;
}
}
}
return partiallyVisible;
}
}