Firebase检索数据Null外部方法 [英] Firebase retrieve data Null outside method

问题描述

我将以下方法放入onCreate()中，当应用启动时，它将触发此方法.但是，当我尝试将mName移到ValueEventListener()之外时，它返回null并且应用程序崩溃了.

I put the below method inside an onCreate(), when the app launch it will trigger this method. But when I try to to get mName outside the ValueEventListener(), it return null and the app crashed.

任何人都知道为什么会发生此问题吗?

Anyone know why this issue happen?

 ValueEventListener postListener = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            UserDetails info = dataSnapshot.getValue(UserDetails.class);
            Log.d(TAG, String.valueOf(dataSnapshot));

            mName = info.getName();
            **Log.d(TAG, mName); // HERE GET THE NAME**

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.w(TAG, "getUser:onCancelled", databaseError.toException());
        }
    };
    mDatabase.child("Admin")
            .child("Info")
            .child(uid)
            .addValueEventListener(postListener);

    **Log.d(TAG, mName); // HERE GET NULL**

推荐答案

正如其他人所说:Firebase数据库中的所有数据都是异步读取的.通过在代码中添加更多日志语句，最容易看到这种情况:

As the others have said: all data from the Firebase Database is read asynchronously. It's easiest to see this by adding some more log statements to your code:

Log.d(TAG, "Before attaching listener");
mDatabase.child("Admin")
        .child("Info")
        .child(uid)
        .addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        Log.d(TAG, "In listener's onDataChange");
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.w(TAG, "getUser:onCancelled", databaseError.toException());
    }
});
Log.d(TAG, "After attaching listener");

与您期望的不同，它的输出是:

Unlike what you may expect, the output of this is:

在附加侦听器之前

Before attaching listener

附加监听器后

内部侦听器的onDataChange

Inside listener's onDataChange

处理异步行为的方法是重新构造问题.

The way to deal with the asynchronous behavior is to reframe your problem.

现在，您的代码已被写入:首先我们加载数据，然后我们对其进行记录.

Right now your code is written: first we load the data, then we log it.

如果将问题重新组织为:每当数据加载时，我们都会记录"，您将获得如下代码:

If you reframe the problem to: "whenever the data is loaded, we log it", you get code like this:

mDatabase.child("Admin")
        .child("Info")
        .child(uid)
        .addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        UserDetails info = dataSnapshot.getValue(UserDetails.class);
        Log.d(TAG, String.valueOf(dataSnapshot));

        mName = info.getName();
        Log.d(TAG, mName);

    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.w(TAG, "getUser:onCancelled", databaseError.toException());
    }
});

因此，所有需要数据的代码都必须位于onDataChange()内部.或者，您可以创建自己的自定义回调方法，然后从onDataChange()调用该方法.但是逻辑总是相同的:需要数据的代码是在数据可用时从onDataChange() 触发的.

So all code that needs the data, needs to be inside the onDataChange(). Alternatively, you can make your own custom callback method and invoke that from onDataChange(). But the logic is always the same: the code that needs toe data, is triggered from onDataChange() when the data is available.

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