Firebase:Firestore:在Realtime Database中找到的ChildEventListener的等价物

Question

我有这个用于Firestore.

FirebaseFirestore   db  = FirebaseFirestore.getInstance();
        CollectionReference ref = db.collection("app/appdata/notifications");
        ref.addSnapshotListener((snapshot, e) -> {
            if (e != null) {
                Log.w(TAG, "Listen failed.", e);
                return;
            }

            for (DocumentSnapshot x : snapshot.getDocuments()) {
                System.out.println(x.getData());
            }
        });

但我不想使用那个循环,而是我只需要获得新的孩子.我想在Realtime Db中看到类似下面的内容.

ref.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String prevChildKey) {
        Post newPost = dataSnapshot.getValue(Post.class);
        System.out.println("Author: " + newPost.author);
        System.out.println("Title: " + newPost.title);
        System.out.println("Previous Post ID: " + prevChildKey);
    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String prevChildKey) {}

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {}

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String prevChildKey) {}

    @Override
    public void onCancelled(DatabaseError databaseError) {}
});

Answer 1

您需要.getDocumentChanges()在QuerySnapshot对象上使用以获取自上次快照以来的更改列表.这相当于实时数据库中的子更改事件.例如:

FirebaseFirestore   db  = FirebaseFirestore.getInstance();
CollectionReference ref = db.collection("app/appdata/notifications");
ref.addSnapshotListener((snapshot, e) -> {
    if (e != null) {
        Log.w(TAG, "Listen failed.", e);
        return;
    }

    for (DocumentChange dc : snapshots.getDocumentChanges()) {
        switch (dc.getType()) {
            case ADDED:
                // handle added documents...
                break;
            case MODIFIED:
                // handle modified documents...
                break;
            case REMOVED:
                // handle removed documents...
                break;
            }
        }
    }
});

有关详细信息,请参阅https://firebase.google.com/docs/firestore/query-data/listen#view_changes_between_snapshots.