观察者模式

优点

• 需要对应用的各个组件有一个较深的思考
• 帮助我们减少依赖
• 帮助我们减少耦合

缺点

• 检查程序完整性可能会比较困难
• 从订阅者到发布者的切换可能开销比较大

// build the Subject base class
var Subject = ( function( window, undefined ) {

  function Subject() {
    this._list = [];
  }

  // this method will handle adding observers to the internal list
  Subject.prototype.observe = function observeObject( obj ) {
    console.log( 'added new observer' );
    this._list.push( obj );
  };

  Subject.prototype.unobserve = function unobserveObject( obj ) {
    for( var i = 0, len = this._list.length; i < len; i++ ) {
      if( this._list[ i ] === obj ) {
        this._list.splice( i, 1 );
        console.log( 'removed existing observer' );
        return true;
      }
    }
    return false;
  };

  Subject.prototype.notify = function notifyObservers() {
    var args = Array.prototype.slice.call( arguments, 0 );
    for( var i = 0, len = this._list.length; i < len; i++ ) {
      this._list[ i ].update.apply( null, args );
    }
  };

  return Subject;

} )( window );

// setup an object that fetchs stocks
function StockGrabber() {

  var subject = new Subject();

  this.addObserver = function addObserver( newObserver ) {
    subject.observe( newObserver );
  };

  this.removeObserver = function removeObserver( deleteObserver ) {
    subject.unobserve( deleteObserver );
  };

  this.fetchStocks = function fetchStocks() {
    // fake fetching the stocks
    var stocks = {
      aapl : 167.00,
      goog : 243.67,
      msft : 99.34
    };

    // notify our observers of the stock change
    subject.notify( stocks );
  };

}

// define a couple of different observers
var StockUpdaterComponent = {
  update : function() {
    console.log( '"update" called on StockUpdater with: ', arguments );
  }
};
var StockChartsComponent = {
  update : function() {
    console.log( '"update" called on StockCharts with: ', arguments );
  }
};

// example usage
var stockApp = new StockGrabber();
stockApp.addObserver( StockUpdaterComponent );
stockApp.fetchStocks(); // console logs: "update" called on StockUpdater with...
stockApp.addObserver( StockChartsComponent );
stockApp.fetchStocks(); // console logs: "update" called on StockUpdater with... "update" called on StockCarts with...
stockApp.removeObserver( StockUpdaterComponent );
stockApp.fetchStocks(); // console logs: "update" called on StockCharts with...
stockApp.removeObserver( StockChartsComponent );
stockApp.fetchStocks(); // does nothing; no observers