对象未使用valueEventListerner从Datasnapshot获取，但工作正常，但是AddChildEventListern [英] Object not getting From Datasnapshot using valueEventListerner but Working find but AddChildEventListern

问题描述

当我尝试从addChildListerner(DataSnapShot)获取对象时，它工作正常并分配给对象的DataSnapshot

When i try to get a Object from addChildListerner(DataSnapShot) it works fine and assign to DataSnapshot to object

工作正常:

  myRef = database.getReference("Chat").child(Combine);
    myRef.orderByKey().limitToLast(1).addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
            ChatData user = dataSnapshot.getValue(ChatData.class);
            userChild.add(user);

但是当我尝试使用ValueListerner(snapshot)应用程序崩溃获取相同的对象时 我已经使用了snapshot.getChildern () snapshot.getValue的所有内容，然后应用崩溃了.

But when i try to get same Object using ValueListerner(snapshot) app crash i have use everything snapshot.getChildern () snapshot.getValue then app crash.

错误

myRef = database.getReference("Chat").child(Combine);
myRef.orderByKey().limitToLast(1).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        ChatData user = dataSnapshot.getValue(ChatData.class);
                        userChild.add(user);

                }

我想找回 ChatData用户= dataSnapshot.getValue(ChatData.class); userChild.add(user);

i want to retrieve ChatData user = dataSnapshot.getValue(ChatData.class); userChild.add(user);

Debug time : 

         DataSnapshot { key = 
                     1123469ACDEFFFFGJKLNOOOPQSTUUVWZabccdehhkkloopqruuuuwxyy, 
                     value
                           = {-LrjsM3ZO0pzQbvCcRuQ
                           ={time=Tue Oct 22 00:53:10 GMT+05:00 2019
                           , msg=hi
                            , user_ID=LuFro93OCcPEpoFTKuQhUkeuw462}} 
                              }

如何获取此值

推荐答案

对Firebase数据库执行查询时，可能会有多个结果.因此，快照包含这些结果的列表.即使只有一个结果，快照也会包含一个结果的列表.

When you execute a query against the Firebase Database, there will potentially be multiple results. So the snapshot contains a list of those results. Even if there is only a single result, the snapshot will contain a list of one result.

在您的第一个示例中，Firebase客户端处理列表，并为每个结果调用您的onChildAdded.但是使用ValueEventListener，您将获得包含所有结果的单个快照.因此，您的onDataChange将需要处理此列表，方法是遍历DataSnapshot.getChildren().

In your first example, the Firebase client handles the list, and calls your onChildAdded for each result. But with a ValueEventListener, you get a single snapshot with all results. So your onDataChange will need to handle this list, which you do by iterating over the DataSnapshot.getChildren().

类似这样的东西:

myRef.orderByKey().limitToLast(1).addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        for (DataSnapshot userSnapshot: dataSnapshot.getChildren()) {
            ChatData user = userSnapshot.getValue(ChatData.class);
            userChild.add(user);
        }
    }

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