学习笔记--十一.设计模式设计模式单例模式1 创建型的设计模式，主要考虑的是创建对象的方式。当我们需要创建一种类型或一

设计模式

单例模式1

创建型的设计模式，主要考虑的是创建对象的方式。当我们需要创建一种类型或一个类的唯一对象时，就可以使用该模式。

最基础的单例模式实现是使用对象文本标识法，例如：

var single = {};

单例模式2

可以用类似类的语法来实现单例模式。使对构造器多次调用new，但实际上创建的对象实例始终只有一个，且后续的构造器调用过程中所返回的始终是这个对象。

a. 全局变量

function Logger() {
  if (typeof global_log === 'undefined') {
    global_log = this; //全局变量没有global_log，则赋值
  }
  return global_log;
}

缺点是：可能会覆写全局变量导致实例丢失，也有可能覆写其他变量

b. 构造器属性

将唯一的实例保存到构造器本身的属性。

function Logger2() {
  if (!Logger2.single_instance) {
    Logger2.single_instance = this;
  }
  return Logger2.single_instance;
}

缺点是：构造器的属性是公有的，仍然有可能会被覆写，唯一实例可能会被修改或丢失。

c . 私有属性

function Logger3() {
  var single_instance;
  return (function () {
    if (!single_instance) {
      single_instance = this;
    }
    return single_instance;
  })();
}

工厂模式
装饰器模式

var tree = {};
tree.decroate = function () {
  alert("Make sure the tree won't fall");
};

tree.getDecorator = function (deco) {
  tree[deco].prototype = this;
  return new tree[deco]();
};

tree.RedBalls = function () {
  this.decroate = function () {
    tree.RedBalls.prototype.decroate();
    alert('Put on some red balls');
  };
};

tree.BlueBalls = function () {
  this.decroate = function () {
    this.BlueBalls.prototype.decroate();
    alert('Add blue balls');
  };
};

tree.Angel = function () {
  this.decroate = function () {
    this.Angel.prototype.decroate();
    alert('An angel on the top');
  };
};

tree = tree.getDecorator('BlueBalls');
tree = tree.getDecorator('Angel');
tree = tree.getDecorator('RedBalls');

tree.decroate();

观察者模式

var observer = {
  addSubscriber: function (callback) {
    if (typeof callback === 'function') {
      this.subscribers[this.subscribers.length] = callback;
    }
  },
  removeSubscriber: function (callback) {
    for (var i = 0; i < this.subscribers.length; i++) {
      if (this.subscribers[i] === callback) {
        delete this.subscribers[i];
      }
    }
  },
  publish: function (what) {
    for (var i = 0; i < this.subscribers.length; i++) {
      if (typeof this.subscribers[i] === 'function') {
        this.subscribers[i](what);
      }
    }
  },
  make: function (o) {
    for (i in this) {
      if (this.hasOwnProperty(i)) {
        o[i] = this[i];
        o.subscribers = [];
      }
    }
  },
};

var blogger = {
  writeBlogPost: function () {
    var content = 'Today is ' + new Date();
    this.publish(content);
  },
};
var la_times = {
  newIssue: function () {
    var paper = 'Martains have landed on Earth!';
    this.publish(paper);
  },
};

observer.make(blogger);
observer.make(la_times);

var jack = {
  read: function (what) {
    console.log('I am just read that ' + what);
  },
};

var jill = {
  gossip: function (what) {
    console.log("You didn't hear it from me, bu " + what);
  },
};

blogger.addSubscriber(jack.read);
blogger.addSubscriber(jill.gossip);


blogger.writeBlogPost();
/**
I am just read that Today is Tue Oct 31 2023 13:21:11 GMT+0800 (China Standard Time)
test07.js:49
You didn't hear it from me, bu Today is Tue Oct 31 2023 13:21:11 GMT+0800 (China Standard Time)
test07.js:55
*/

blogger.removeSubscriber(jill.gossip);
blogger.writeBlogPost();
/*
I am just read that Today is Tue Oct 31 2023 13:21:12 GMT+0800 (China Standard Time)
test07.js:49
*/

la_times.addSubscriber(jill.gossip);
la_times.newIssue();
/*You didn't hear it from me, bu Martains have landed on Earth!*/