一天一个设计模式(3):观察者模式

观察者模式: 又名通知者模式,对象A对已订阅的若干个对象(B, C, D)进行通知,(B, C, D)可以选择退订或再订阅;

例子(javascript): 老师D对已签到的学生进行通知报数

var D = {
 students: [],
 sign:function(student){
  var i;
  for(i in D.students){
   if(D.students[i] == student) return;
  }
  D.students[D.students.length] = student;
 },
 unsign: function(student){
  var i;
  for(i in D.students){
   if(D.students[i] == student){
    D.students[i] = null;
    return;
   }
  }
  D.students[D.students.length] = student;
 },

 notify: function(){
  var i, num = 1;
  for(i in D.students){
   D.students[i] && D.students[i].speak(num++);
  }
 }

};
var student = function(){};
student.prototype.speak = function(n){
  alert(n);
 };
var A = new student;
var B = new student;
var C = new student;

D.sign(A);
D.sign(B);
D.sign(C);

D.notify();