详解JavaScript的策略模式编程

// Greeter is a class of object that can greet people.
// It can learn different ways of greeting people through
// 'Strategies.'
//
// This is the Greeter constructor.
var Greeter = function(strategy) {
this.strategy = strategy;
};
 
// Greeter provides a greet function that is going to
// greet people using the Strategy passed to the constructor.
Greeter.prototype.greet = function() {
return this.strategy();
};
 
// Since a function encapsulates an algorithm, it makes a perfect
// candidate for a Strategy.
//
// Here are a couple of Strategies to use with our Greeter.
var politeGreetingStrategy = function() {
console.log("Hello.");
};
 
var friendlyGreetingStrategy = function() {
console.log("Hey!");
};
 
var boredGreetingStrategy = function() {
console.log("sup.");
};
 
// Let's use these strategies!
var politeGreeter = new Greeter(politeGreetingStrategy);
var friendlyGreeter = new Greeter(friendlyGreetingStrategy);
var boredGreeter = new Greeter(boredGreetingStrategy);
 
console.log(politeGreeter.greet()); //=> Hello.
console.log(friendlyGreeter.greet()); //=> Hey!
console.log(boredGreeter.greet()); //=> sup.

// We can also leverage the power of Prototypes in Javascript to create
// classes that act as strategies.
//
// Here, we create an abstract class that will serve as the interface
// for all our strategies. It isn't needed, but it's good for documenting
// purposes.
var Strategy = function() {};
 
Strategy.prototype.execute = function() {
 throw new Error('Strategy#execute needs to be overridden.')
};
 
// Like above, we want to create Greeting strategies. Let's subclass
// our Strategy class to define them. Notice that the parent class
// requires its children to override the execute method.
var GreetingStrategy = function() {};
GreetingStrategy.prototype = Object.create(Strategy.prototype);
 
// Here is the `execute` method, which is part of the public interface of
// our Strategy-based objects. Notice how I implemented this method in term of
// of other methods. This pattern is called a Template Method, and you'll see
// the benefits later on.
GreetingStrategy.prototype.execute = function() {
 return this.sayHi() + this.sayBye();
};
 
GreetingStrategy.prototype.sayHi = function() {
 return "Hello, ";
};
 
GreetingStrategy.prototype.sayBye = function() {
 return "Goodbye.";
};
 
// We can already try out our Strategy. It requires a little tweak in the
// Greeter class before, though.
Greeter.prototype.greet = function() {
 return this.strategy.execute();
};
 
var greeter = new Greeter(new GreetingStrategy());
greeter.greet() //=> 'Hello, Goodbye.'

// Since the GreetingStrategy#execute method uses methods to define its algorithm,
// the Template Method pattern, we can subclass it and simply override one of those
// methods to alter the behavior without changing the algorithm.
 
var PoliteGreetingStrategy = function() {};
PoliteGreetingStrategy.prototype = Object.create(GreetingStrategy.prototype);
PoliteGreetingStrategy.prototype.sayHi = function() {
 return "Welcome sir, ";
};
 
var FriendlyGreetingStrategy = function() {};
FriendlyGreetingStrategy.prototype = Object.create(GreetingStrategy.prototype);
FriendlyGreetingStrategy.prototype.sayHi = function() {
 return "Hey, ";
};
 
var BoredGreetingStrategy = function() {};
BoredGreetingStrategy.prototype = Object.create(GreetingStrategy.prototype);
BoredGreetingStrategy.prototype.sayHi = function() {
 return "sup, ";
};
 
var politeGreeter  = new Greeter(new PoliteGreetingStrategy());
var friendlyGreeter = new Greeter(new FriendlyGreetingStrategy());
var boredGreeter  = new Greeter(new BoredGreetingStrategy());
 
politeGreeter.greet();  //=> 'Welcome sir, Goodbye.'
friendlyGreeter.greet(); //=> 'Hey, Goodbye.'
boredGreeter.greet();  //=> 'sup, Goodbye.'

var greeters = [
 new Greeter(new BoredGreetingStrategy()),
 new Greeter(new PoliteGreetingStrategy()),
 new Greeter(new FriendlyGreetingStrategy()),
];
 
greeters.forEach(function(greeter) {
  
 // Since each greeter knows its strategy, there's no need
 // to do any type checking. We just greet, and the object
 // knows how to handle it.
 greeter.greet();
});

// Taken from http://passportjs.org
 
var passport = require('passport')
   
  // Each authentication mechanism is provided as an npm package.
  // These packages expose a Strategy object.
 , LocalStrategy = require('passport-local').Strategy
 , FacebookStrategy = require('passport-facebook').Strategy;
 
// Passport can be instanciated using any Strategy.
passport.use(new LocalStrategy(
 function(username, password, done) {
  User.findOne({ username: username }, function (err, user) {
   if (err) { return done(err); }
   if (!user) {
    return done(null, false, { message: 'Incorrect username.' });
   }
   if (!user.validPassword(password)) {
    return done(null, false, { message: 'Incorrect password.' });
   }
   return done(null, user);
  });
 }
));
 
// In this case, we instanciate a Facebook Strategy
passport.use(new FacebookStrategy({
  clientID: FACEBOOK_APP_ID,
  clientSecret: FACEBOOK_APP_SECRET,
  callbackURL: "http://www.example.com/auth/facebook/callback"
 },
 function(accessToken, refreshToken, profile, done) {
  User.findOrCreate(..., function(err, user) {
   if (err) { return done(err); }
   done(null, user);
  });
 }
));