Android Firebase addListenerForSingleValueEvent无法正常工作 [英] Android Firebase addListenerForSingleValueEvent is not working

问题描述

我知道这个问题已经提出很多，但我仍然可以解决.这是我的代码:

I know this has been asked a lot but I still can get it to work. Here's my code:

private int test;

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference userRef = rootRef.child("Users");

test = 0;
userRef.addListenerForSingleValueEvent(new ValueEventListener() {
   @Override
   public void onDataChange(DataSnapshot dataSnapshot) {
        if (dataSnapshot.hasChild(userID))
            test = 1;
        else
            test = 2;
   }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

Log.d(TAG, "Test = " + test);

这是Firebase上的内容:

Here's what on Firebase:

• appname-e545e
  • 用户
    • 用户ID
    • appname-e545e
      • Users
        • userID

        我不知道为什么，但是变量"test"总是返回值0，这意味着addListenerForSingleValueEvent不起作用.感谢您提供高级帮助

        I don't know why but the variable "test" always return value 0, which means addListenerForSingleValueEvent didn't work. Thanks for helping in advanced

        推荐答案

        数据是从Firebase异步加载的.到您的日志test时，onDataChange尚未运行.

        The data is loaded from Firebase asynchronously. By the time your log test, the onDataChange hasn't run yet.

        要查看实际操作，请在内部 onDataChange中添加更多日志记录:

        To see this in action, add some more logging inside onDataChange:

        私有int测试；

        test = 0;
        userRef.addListenerForSingleValueEvent(new ValueEventListener() {
           @Override
           public void onDataChange(DataSnapshot dataSnapshot) {
                if (dataSnapshot.hasChild(userID)) {
                    test = 1;
                }
                else {
                    test = 2;
                }
                Log.d(TAG, "Test2 = " + test);
           }
        
            @Override
            public void onCancelled(DatabaseError databaseError) {
                throw databaseError.toException(); // Don't ignore errors
            }
        });
        
        Log.d(TAG, "Test = " + test);
        

        现在您将首先看到:

        测试= 0

        Test = 0

        然后，一旦数据加载并调用onDataChange:

        And then once the data has loaded and onDataChange gets called:

        测试= 1

        Test = 1

        或

        测试= 2

        Test = 2

        因此，您应该始终从onDataChange()内放置(或调用)需要数据库中数据的代码.

        For this reason you should always put (or call) the code that needs the data from the database from within onDataChange().

        这篇关于Android Firebase addListenerForSingleValueEvent无法正常工作的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！