当数据库中的项目数少于最大项目数（21）时，接受的答案无法正常工作。
这是我基于接受的答案构建的解决方案：
0)设置变量
private String lastMessageKey;
private int totalItemCount;
private int lastVisibleItem;
private boolean isLoadingMoreFollowers = false;
private boolean hasMoreItems = true;
private boolean hasShownNoMoreItemsToast = false;
private boolean initialLoad = true;
private final int MAX_LOAD_ITEMS = 11;
private final int VISIBLE_TRESHOLD = 1;
1) 设置监听器：
private void setMessageListener() {
if (childEventListener == null) {
mmessageArrayList.clear();
final ArrayList<Mmessage> tempMmessages = new ArrayList<>();
query = mDatabaseInstace.getReference().child(chat1).child(F.CHATS).child(F.MESSAGES).orderByKey().limitToLast(MAX_LOAD_ITEMS-1);
childEventListener = new ChildEventListener() {
@Override
public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
Log.d(TAG, "onChildAdded: -----------------> " + dataSnapshot);
Log.d(TAG, "onChildAdded: -----------------> " + s);
tempMmessages.add(dataSnapshot.getValue(Mmessage.class));
preloadMessagePics(tempMmessages);
if (initialLoad) {
lastMessageKey = tempMmessages.get(0).getMessagePushKey();
initialLoad = false;
}
mmessageArrayList.add(tempMmessages.size()-1, tempMmessages.get(0));
messageAdapter.notifyDataSetChanged();
tempMmessages.clear();
}
@Override
public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
Log.d(TAG, "onChildChanged: --------------------------------->");
}
@Override
public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {
Log.d(TAG, "onChildRemoved: ---------------------------------->");
}
@Override
public void onChildMoved(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
Log.d(TAG, "onChildMoved: ------------------------------------>");
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
Log.d(TAG, "onCancelled: ------------------------------------->");
}
};
query.addChildEventListener(childEventListener);
}
}
2) 设置加载更多监听器：
private void setLoadMoreListener(){
setup.RV_messages.addOnScrollListener(new RecyclerView.OnScrollListener() {
@Override
public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
super.onScrolled(recyclerView, dx, dy);
totalItemCount = linearLayoutManager.getItemCount();
lastVisibleItem = linearLayoutManager.findLastVisibleItemPosition();
if (!isLoadingMoreFollowers && totalItemCount <= (lastVisibleItem+VISIBLE_TRESHOLD) && (totalItemCount >= MAX_LOAD_ITEMS-1)){
if (hasMoreItems) {
getMoreMessages();
Toast.makeText(homeActivity, "load more items", Toast.LENGTH_SHORT).show();
}else {
if (!hasShownNoMoreItemsToast) {
Toast.makeText(homeActivity, "has no more items", Toast.LENGTH_SHORT).show();
hasShownNoMoreItemsToast = true;
}
}
}
}
});
}
3) 获取更多的项目：
private void getMoreMessages(){
final ArrayList<Mmessage> tempMmessages = new ArrayList<>();
isLoadingMoreFollowers = true;
loadMoreQuery = mDatabaseInstace.getReference().child(chat1).child(F.CHATS).child(F.MESSAGES).orderByKey().endAt(lastMessageKey).limitToLast(MAX_LOAD_ITEMS);
loadMoreChildEventListener = new ChildEventListener() {
@Override
public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
tempMmessages.add(dataSnapshot.getValue(Mmessage.class));
preloadMessagePics(tempMmessages);
if (tempMmessages.size() == MAX_LOAD_ITEMS){
lastMessageKey = tempMmessages.get(0).getMessagePushKey();
Collections.reverse(tempMmessages);
tempMmessages.remove(0);
mmessageArrayList.addAll(tempMmessages);
isLoadingMoreFollowers = false;
messageAdapter.notifyDataSetChanged();
tempMmessages.clear();
return;
}
if (lastMessageKey.equalsIgnoreCase(tempMmessages.get(tempMmessages.size()-1).getMessagePushKey())){
Collections.reverse(tempMmessages);
tempMmessages.remove(0);
mmessageArrayList.addAll(tempMmessages);
isLoadingMoreFollowers = false;
hasMoreItems = false;
messageAdapter.notifyDataSetChanged();
tempMmessages.clear();
}
}
@Override
public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
}
@Override
public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {
}
@Override
public void onChildMoved(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError) {
}
};
loadMoreQuery.addChildEventListener(loadMoreChildEventListener);
}
玩得开心！
这里是我认为会起作用的代码。