监听者

     监听者对大家来说可能并不陌生，所谓监听者就是用来监听自已感兴趣的事件的，当收到自已感兴趣的事件时会做出相应的反映。

    它的应用是非常广的，比如win32的消息机制，它跟监听者模式就异曲同工，操作系统将产生的各种事件包装成消息(MSG),该消息结构包装了发生的事件的描述信息，比如消息值，屏幕坐标等，然后将它投递到相应的进程中，当该进程收到该消息时会查看该MSG结构中的信息，也就知道了发生了什么事件，并对自已感兴趣的事件进行响应。

    下面我们就来用C++实现一个简单的监听者

using namespace std;

//Listener基类(抽象类)
class AbstractListener
{
public:
    AbstractListener(){};
    virtual ~AbstractListener(){};
    virtual void ActionPerformed(int state) = 0;
};
//Listener实现类(它类似于操作系统中的进程)
class StateListener:public AbstractListener
{
public:
    StateListener(){};
    virtual ~StateListener(){};
    virtual void ActionPerformed(int state);
};
void StateListener::ActionPerformed(int state)
{
         //这里就是对自已感兴趣的事件的相应处理
    if(state==1){
        cout<<"event 1 is fired."<<endl;
    }else if(state==2){
        cout<<"event 2 is fired."<<endl;
    }else{
        cout<<"other event is fired."<<endl;
    }
}
//Listener Center(类似于操作系统(OS))
class ListenerCenter
{
private:
    StateListener listenerList[50];//存储listener的数组
    int count;//当前中心中已有的listener的个数
public:
    ListenerCenter();
    virtual ~ListenerCenter(){};
    int GetCount();
    void AddListener(StateListener);
    void FireStateListener(int state);
};
ListenerCenter::ListenerCenter()
{
    this->count=0;
}
int ListenerCenter::GetCount()
{
    return count;
}
void ListenerCenter::AddListener(StateListener statelistener)
{
    listenerList[this->count]=statelistener;
    this->count++;
}
void ListenerCenter::FireStateListener(int state)
{
    for(int i=0;i<this->count;i++){
        listenerList[i].ActionPerformed(state);
    }
}

int main(int argc, char* argv[])
{
    ListenerCenter listenerCenter;

    StateListener statelis1;
    StateListener statelis2;

    listenerCenter.AddListener(statelis1);
    listenerCenter.AddListener(statelis2);

    cout<<"=========================="<<endl;
    listenerCenter.FireStateListener(1);
    cout<<"=========================="<<endl;
    listenerCenter.FireStateListener(2);
    cout<<"=========================="<<endl;
    listenerCenter.FireStateListener(3);
    cout<<"=========================="<<endl;

    return 0;
}

#include <iostream> using namespace std; //Listener基类(抽象类) class AbstractListener { public: AbstractListener(){}; virtual ~AbstractListener(){}; virtual void ActionPerformed(int state) = 0; }; //Listener实现类(它类似于操作系统中的进程) class StateListener:public AbstractListener { public: StateListener(){}; virtual ~StateListener(){}; virtual void ActionPerformed(int state); }; void StateListener::ActionPerformed(int state) { //这里就是对自已感兴趣的事件的相应处理 if(state==1){ cout<<"event 1 is fired."<<endl; }else if(state==2){ cout<<"event 2 is fired."<<endl; }else{ cout<<"other event is fired."<<endl; } } //Listener Center(类似于操作系统(OS)) class ListenerCenter { private: StateListener listenerList[50];//存储listener的数组 int count;//当前中心中已有的listener的个数 public: ListenerCenter(); virtual ~ListenerCenter(){}; int GetCount(); void AddListener(StateListener); void FireStateListener(int state); }; ListenerCenter::ListenerCenter() { this->count=0; } int ListenerCenter::GetCount() { return count; } void ListenerCenter::AddListener(StateListener statelistener) { listenerList[this->count]=statelistener; this->count++; } void ListenerCenter::FireStateListener(int state) { for(int i=0;i<this->count;i++){ listenerList[i].ActionPerformed(state); } } int main(int argc, char* argv[]) { ListenerCenter listenerCenter; StateListener statelis1; StateListener statelis2; listenerCenter.AddListener(statelis1); listenerCenter.AddListener(statelis2); cout<<"=========================="<<endl; listenerCenter.FireStateListener(1); cout<<"=========================="<<endl; listenerCenter.FireStateListener(2); cout<<"=========================="<<endl; listenerCenter.FireStateListener(3); cout<<"=========================="<<endl; return 0; }

输出结果:

 ==========================
event 1 is fired.
event 1 is fired.
==========================
event 2 is fired.
event 2 is fired.
==========================
other event is fired.
other event is fired.
==========================

 

该模式的应用非常广，比较经典的应用是AWT的事件处理机制，如果应用得当，会使程序的可读性变得很好。