如何检测ValueEventListener是否在Firebase中获取数据 [英] How to detect if ValueEventListener has fetched data or not in Firebase

问题描述

我使用以下代码从Firebase DB中获取数据，但由于它在后台线程中发出网络请求，所以我想等到它完成请求并获取值。例如，

  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）{
 //在这里做某事
} 
 else {
 //在这里做某事
} 
  $ b $ p 
 $ b 
这段代码总是执行else部分，所以我想等到变量 isAvailable 从数据库中获取值，然后进一步执行。
解决方案
首先创建一个如下的界面： b 
 $ b 
  public interface IsAvailableCallback {
 void onAvailableCallback（boolean isAvailable）; 
 
 
 
 
 
 $ b 
假设你的上面的代码是在这个方法中，接口对象触发回调：
 
 
  public void isAvailable（IsAvailableCallback callback）{
 boolean isAvailable = false; 
 usernameReference.addListenerForSingleValueEvent（new ValueEventListener（）{
 @Override $ b $ public void onDataChange（DataSnapshot dataSnapshot）{
 isAvailable = true; 
 //这将触发true 
 callback.onAvailableCallback（isAvailable）; 
} 
 $ b $ @Override 
 public void onCancelled（DatabaseError databaseError）{
 progressBar.setVisibility（View.GONE）; 
 //这将触发false 
 callback.onAvailableCallback（isAvailable）; 
} 
}）; 
 
 $ / code> 
调用这个方法：
 $ b $ （
 $ p $  isAvailable（new IsAvailableCallback（）{
 @Override 
 public void onAvailableCallback（boolean isAvailable）{
 //你将在这里得到回调，在这里做你的条件
 
 
 code $ pre 
I'm using following code to fetch the data from Firebase DB but as it makes network request in background thread so I want to wait till it completes the request and get a value. For example,
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 
}
This snippet always execute the else part so I want to wait till the variable isAvailable get the value from database then further Execution will take place.
解决方案
First create an interface like this:
public interface IsAvailableCallback {
    void onAvailableCallback(boolean isAvailable);
}
Suppose your above code is in this method which takes interface object to trigger callback like :
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);
                }
            });                
}
Call this method like :
isAvailable(new IsAvailableCallback() {
    @Override
    public void onAvailableCallback(boolean isAvailable) {
        //you will get callback here, Do your if condition here
    }
}


                        
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