如何检测ValueEventListener是否在Firebase中提取数据

Question

boolean isAvailable=false;
    usernameReference.addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        isAvailable = true;
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {
                        progressBar.setVisibility(View.GONE);
                    }
                });
if(isAvailable){
       //do something here
}
else{
      //do something here 
}

这个代码段总是执行else部分，所以我想等到变量可用从数据库中获取值后再执行。

Answer 1

首先创建一个接口，如下所示：

public interface IsAvailableCallback {
    void onAvailableCallback(boolean isAvailable);
}

假设上面的代码在这个方法中，它采用接口对象来触发回调，如下所示：

public void isAvailable(IsAvailableCallback callback) {
    boolean isAvailable=false;
    usernameReference.addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    isAvailable = true;
                    //this will trigger true
                    callback.onAvailableCallback(isAvailable);
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    progressBar.setVisibility(View.GONE);
                    //this will trigger false
                    callback.onAvailableCallback(isAvailable);
                }
            });                
}

像这样调用此方法：

isAvailable(new IsAvailableCallback() {
    @Override
    public void onAvailableCallback(boolean isAvailable) {
        //you will get callback here, Do your if condition here
    }
}