MyActivity从MyClass.java实现自定义监听器监听，但始终是NULL [英] MyActivity implements Custom Listener from MyClass.java but Listener is always NULL

问题描述

MyActivity实现在MyClass.java限定的CustomListener。我在活动中定义MyClass中定义的函数应触发监听器和做一些动作（完成（）MyActivity）。不知怎的，我得到空引用到侦听器。我在哪里去了？

请参阅下面的code！

MyActivity.java

 进口com.project.MyClass.CustomListener;公共类MyActivity扩展活动实现CustomListener {    上下文语境;
    CustomListener监听;    公共无效的onCreate（捆绑savedInstanceState）{        super.onCreate（savedInstanceState）;
        上下文=这一点;        MyClass的=新MyClass的（上下文）;
        myClass.setOnCustomListener（监听）;    }    公共无效doThisWhenTriggered（）
    {
    ...
    }}
 

MyClass.java

 公共类MyClass的{    私人客户服务联系CTX;
    ArrayList的＆LT; CustomListener＆GT;听众=新的ArrayList＆LT; CustomListener＆GT;（）;    公共MyClass的（上下文的背景下）
    {
        超（）;
        this.ctx =上下文
    }公共接口CustomListener
{
       公共无效doThisWhenTriggered（）;
}
公共无效setOnCustomListener（CustomListenerListener监听器）
{
    this.listeners.add（监听）;
}    公共无效generateTrigger（）
    {
        CustomListener听者= listeners.get（0）;        如果（听众！= NULL）
            listener.doThisWhenTriggered（）;
        其他
            Log.d（MYAPP，监听器是空）;
    }}
 

MyBroadcastReceiver.java

 公共无效的onReceive（上下文的背景下，意图意图）{
    ...
    callMyClass（上下文的背景下）
}公共无效callMyClass（上下文的背景下）
{
    MyClass的MyClass的=新MyClass的（上下文）;
    myClass.generateTrigger（上下文）;
}
 

修改

问题是因为在广播接收器和监听器列表访问MyClass的不同实例作为正确地指出的@Binyamin Sharet

的解决方案是声明侦听器列表为静态并且还公共无效generateTrigger（）为静态，以使接收机的callMyClass看起来像：

 公共无效callMyClass（上下文的背景下）
{
    MyClass.generateTrigger（上下文）;
}
 

解决方案

您从来没有实例监听器（声明为 CustomListener监听; ），因此它总是空，你只需要设置活动为监听器，因为它实现了接口：

  myClass.setOnCustomListener（本）;
 

由于在code看到的，你创建的接收器类的新实例，让你设置新实例的监听器列表不存在，因为列表不是静态的监听器。

MyActivity implements a CustomListener defined in MyClass.java. I have a function defined in MyClass that should trigger the listener and do some action(finish() MyActivity) defined in the activity. Somehow, I am getting null reference to the listener. Where am I going wrong?

Please refer to the code below!

MyActivity.java

import com.project.MyClass.CustomListener;

public class MyActivity extends Activity implements CustomListener {

    Context context;
    CustomListener listener;

    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        context = this;

        myClass = new MyClass(context);
        myClass.setOnCustomListener(listener);

    }

    public void doThisWhenTriggered()
    {
    ...
    }

}

MyClass.java

public class MyClass{

    private Contect ctx;
    ArrayList<CustomListener> listeners = new ArrayList<CustomListener>();

    public MyClass(Context context)
    {
        super();
        this.ctx = context
    }

public interface CustomListener
{
       public void doThisWhenTriggered();
}


public void setOnCustomListener(CustomListenerListener listener)
{
    this.listeners.add(listener);
}

    public void generateTrigger()
    {
        CustomListener listener = listeners.get(0);

        if(listener != null)
            listener.doThisWhenTriggered();
        else
            Log.d("MyAPP", "Listener is NULL");
    }

}

MyBroadcastReceiver.java

public void onReceive(Context context, Intent intent) {
    ...
    callMyClass(Context context)
}

public void callMyClass(Context context)
{
    MyClass myClass= new myClass(context);
    myClass.generateTrigger(context);
}

EDIT

The problem was BECAUSE of accessing different instances of MyClass in BroadcastReceiver and Listeners list as correctly pointed out by @Binyamin Sharet

The solution is to declare the listener list as static and also public void generateTrigger() as static so that the receiver's callMyClass looks like :

public void callMyClass(Context context)
{
    MyClass.generateTrigger(context);
}

解决方案

You never instantiate listener (declared as CustomListener listener;) and therefore its always null, you just need to set the activity as the listener, as it implements the interface:

myClass.setOnCustomListener(this);

As seen in your code, you create a new instance of the class in the receiver, so the listener you set does not exist in the listeners list of new instance, since the list is not static.

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