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diff --git a/source/chapter2/21_Protocols.md b/source/chapter2/21_Protocols.md
index 6b712689..4b5f854f 100755
--- a/source/chapter2/21_Protocols.md
+++ b/source/chapter2/21_Protocols.md
@@ -1,6 +1,5 @@
-> 翻译：geek5nan
-
-> 校对：dabing1022
+> 翻译：geek5nan  
+> 校对：dabing1022  
 
 # 协议
 -----------------
@@ -30,22 +29,27 @@
 
 `协议`的定义与类，结构体，枚举的定义非常相似，如下所示：
 
-	protocol SomeProtocol {
-		// 协议内容
-	}
-
+```swift
+protocol SomeProtocol {
+	// 协议内容
+}
+```
 
 在类，结构体，枚举的名称后加上`协议名称`，中间以冒号`:`分隔即可实现协议；实现多个协议时，各协议之间用逗号`,`分隔，如下所示：
 
-	struct SomeStructure: FirstProtocol, AnotherProtocol {
-		// 结构体内容
-	}
+```swift
+struct SomeStructure: FirstProtocol, AnotherProtocol {
+	// 结构体内容
+}
+```
 
 当某个类含有父类的同时并实现了协议，应当把父类放在所有的协议之前，如下所示：
 
-	class SomeClass: SomeSuperClass, FirstProtocol, AnotherProtocol {
-		// 类的内容
-	}
+```swift
+class SomeClass: SomeSuperClass, FirstProtocol, AnotherProtocol {
+	// 类的内容
+}
+```
 
 <a name="property_requirements"></a>
 ## 属性要求
@@ -54,46 +58,54 @@
 
 通常前置`var`关键字将属性声明为变量。在属性声明后写上`{ get set }`表示属性为可读写的。`{ get }`用来表示属性为可读的。即使你为可读的属性实现了`setter`方法，它也不会出错。
 
-	protocol SomeProtocol {
-		var musBeSettable : Int { get set }
-		var doesNotNeedToBeSettable: Int { get }
-	}
+```swift
+protocol SomeProtocol {
+	var musBeSettable : Int { get set }
+	var doesNotNeedToBeSettable: Int { get }
+}
+```
 
 用类来实现协议时，使用`class`关键字来表示该属性为类成员；用结构体或枚举实现协议时，则使用`static`关键字来表示：
 
-	protocol AnotherProtocol {
-		class var someTypeProperty: Int { get set }
-	}
+```swift
+protocol AnotherProtocol {
+	class var someTypeProperty: Int { get set }
+}
 
-	protocol FullyNamed {
-		var fullName: String { get }
-	}
+protocol FullyNamed {
+	var fullName: String { get }
+}
+```
 
 `FullyNamed`协议含有`fullName`属性。因此其`遵循者`必须含有一个名为`fullName`，类型为`String`的可读属性。
 
-	struct Person: FullyNamed{
-		var fullName: String
-	}
-	let john = Person(fullName: "John Appleseed")
-	//john.fullName 为 "John Appleseed"
+```swift
+struct Person: FullyNamed{
+	var fullName: String
+}
+let john = Person(fullName: "John Appleseed")
+//john.fullName 为 "John Appleseed"
+```
 
 `Person`结构体含有一个名为`fullName`的`存储型属性`，完整的`遵循`了协议。(*若协议未被完整遵循，编译时则会报错*)。
 
 如下所示，`Startship`类`遵循`了`FullyNamed`协议：
 
-	class Starship: FullyNamed {
-		var prefix: String?
-		var name: String
-		init(name: String, prefix: String? = nil ) {
-			self.anme = name
-			self.prefix = prefix
-		}
-		var fullName: String {
-		return (prefix ? prefix ! + " " : " ") + name
-		}
+```swift
+class Starship: FullyNamed {
+	var prefix: String?
+	var name: String
+	init(name: String, prefix: String? = nil ) {
+		self.anme = name
+		self.prefix = prefix
 	}
-	var ncc1701 = Starship(name: "Enterprise", prefix: "USS")
-	// ncc1701.fullName == "USS Enterprise"
+	var fullName: String {
+	return (prefix ? prefix ! + " " : " ") + name
+	}
+}
+var ncc1701 = Starship(name: "Enterprise", prefix: "USS")
+// ncc1701.fullName == "USS Enterprise"
+```
 
 `Starship`类将`fullName`实现为可读的`计算型属性`。它的每一个实例都有一个名为`name`的必备属性和一个名为`prefix`的可选属性。 当`prefix`存在时，将`prefix`插入到`name`之前来为`Starship`构建`fullName`。
 
@@ -102,39 +114,42 @@
 
 `协议`能够要求其`遵循者`必备某些特定的`实例方法`和`类方法`。协议方法的声明与普通方法声明相似，但它不需要`方法`内容。
 
-> 注意：
->
+> 注意：  
 协议方法支持`变长参数(variadic parameter)`，不支持`默认参数(default parameter)`。
 
 前置`class`关键字表示协议中的成员为`类成员`；当协议用于被`枚举`或`结构体`遵循时，则使用`static`关键字。如下所示：
 
-	protocol SomeProtocol {
-		class func someTypeMethod()
-	}
+```swift
+protocol SomeProtocol {
+	class func someTypeMethod()
+}
 
-	protocol RandomNumberGenerator {
-		func random() -> Double
-	}
+protocol RandomNumberGenerator {
+	func random() -> Double
+}
+```
 
 `RandomNumberGenerator`协议要求其`遵循者`必须拥有一个名为`random`， 返回值类型为`Double`的实例方法。(我们假设随机数在[0，1]区间内)。
 
 `LinearCongruentialGenerator`类`遵循`了`RandomNumberGenerator`协议，并提供了一个叫做*线性同余生成器(linear congruential generator)*的伪随机数算法。
 
-	class LinearCongruentialGenerator: RandomNumberGenerator {
-		var lastRandom = 42.0
-		let m = 139968.0
-		let a = 3877.0
-		let c = 29573.0
-		func random() -> Double {
-			lastRandom = ((lastRandom * a + c) % m)
-			return lastRandom / m
-		}
+```swift
+class LinearCongruentialGenerator: RandomNumberGenerator {
+	var lastRandom = 42.0
+	let m = 139968.0
+	let a = 3877.0
+	let c = 29573.0
+	func random() -> Double {
+		lastRandom = ((lastRandom * a + c) % m)
+		return lastRandom / m
 	}
-	let generator = LinearCongruentialGenerator()
-	println("Here's a random number: \(generator.random())")
-	// 输出 : "Here's a random number: 0.37464991998171"
-	println("And another one: \(generator.random())")
-	// 输出 : "And another one: 0.729023776863283"
+}
+let generator = LinearCongruentialGenerator()
+println("Here's a random number: \(generator.random())")
+// 输出 : "Here's a random number: 0.37464991998171"
+println("And another one: \(generator.random())")
+// 输出 : "And another one: 0.729023776863283"
+```
 
 <a name="mutating_method_requirements"></a>
 ## 突变方法要求
@@ -143,34 +158,37 @@
 
 (*译者注：类中的成员为`引用类型(Reference Type)`，可以方便的修改实例及其属性的值而无需改变类型；而`结构体`和`枚举`中的成员均为`值类型(Value Type)`，修改变量的值就相当于修改变量的类型，而`Swift`默认不允许修改类型，因此需要前置`mutating`关键字用来表示该`函数`中能够修改类型*)
 
-> 注意：
->
+> 注意：  
 用`class`实现协议中的`mutating`方法时，不用写`mutating`关键字；用`结构体`，`枚举`实现协议中的`mutating`方法时，必须写`mutating`关键字。
 
 如下所示，`Togglable`协议含有`toggle`函数。根据函数名称推测，`toggle`可能用于**切换或恢复**某个属性的状态。`mutating`关键字表示它为`突变方法`：
 
-	protocol Togglable {
-		mutating func toggle()
-	}
+```swift
+protocol Togglable {
+	mutating func toggle()
+}
+```
 
 当使用`枚举`或`结构体`来实现`Togglabl`协议时，必须在`toggle`方法前加上`mutating`关键字。
 
 如下所示，`OnOffSwitch`枚举`遵循`了`Togglable`协议，`On`，`Off`两个成员用于表示当前状态
 
-	enum OnOffSwitch: Togglable {
-		case Off, On
-		mutating func toggle() {
-			switch self {
-			case Off:
-				self = On
-			case On:
-				self = Off
-			}
+```swift
+enum OnOffSwitch: Togglable {
+	case Off, On
+	mutating func toggle() {
+		switch self {
+		case Off:
+			self = On
+		case On:
+			self = Off
 		}
 	}
-	var lightSwitch = OnOffSwitch.Off
-	lightSwitch.toggle()
-	//lightSwitch 现在的值为 .On
+}
+var lightSwitch = OnOffSwitch.Off
+lightSwitch.toggle()
+//lightSwitch 现在的值为 .On
+```
 
 <a name="protocols_as_types"></a>
 ## 协议类型
@@ -183,25 +201,26 @@
 * 作为常量，变量，属性的类型
 * 作为数组，字典或其他容器中的元素类型
 
-> 注意：
->
+> 注意：  
 协议类型应与其他类型(Int，Double，String)的写法相同，使用驼峰式
 
-	class Dice {
-		let sides: Int
-		let generator: RandomNumberGenerator
-		init(sides: Int, generator: RandomNumberGenerator) {
-			self.sides = sides
-			self.generator = generator
-		}
-		func roll() -> Int {
-			return Int(generator.random() * Double(sides)) +1
-		}
+```swift
+class Dice {
+	let sides: Int
+	let generator: RandomNumberGenerator
+	init(sides: Int, generator: RandomNumberGenerator) {
+		self.sides = sides
+		self.generator = generator
 	}
+	func roll() -> Int {
+		return Int(generator.random() * Double(sides)) +1
+	}
+}
+```
 
 这里定义了一个名为 `Dice`的类，用来代表桌游中的N个面的骰子。
 
- `Dice`含有`sides`和`generator`两个属性，前者用来表示骰子有几个面，后者为骰子提供一个随机数生成器。由于后者为`RandomNumberGenerator`的协议类型。所以它能够被赋值为任意`遵循`该协议的类型。
+`Dice`含有`sides`和`generator`两个属性，前者用来表示骰子有几个面，后者为骰子提供一个随机数生成器。由于后者为`RandomNumberGenerator`的协议类型。所以它能够被赋值为任意`遵循`该协议的类型。
 
 此外，使用`构造器(init)`来代替之前版本中的`setup`操作。构造器中含有一个名为`generator`，类型为`RandomNumberGenerator`的形参，使得它可以接收任意遵循`RandomNumberGenerator`协议的类型。
 
@@ -209,16 +228,18 @@
 
 如下所示，`LinearCongruentialGenerator`的实例作为随机数生成器传入`Dice`的`构造器`
 
-	var d6 = Dice(sides: 6,generator: LinearCongruentialGenerator())
-	for _ in 1...5 {
-		println("Random dice roll is \(d6.roll())")
-	}
-	//输出结果
-	//Random dice roll is 3
-	//Random dice roll is 5
-	//Random dice roll is 4
-	//Random dice roll is 5
-	//Random dice roll is 4
+```swift
+var d6 = Dice(sides: 6,generator: LinearCongruentialGenerator())
+for _ in 1...5 {
+	println("Random dice roll is \(d6.roll())")
+}
+//输出结果
+//Random dice roll is 3
+//Random dice roll is 5
+//Random dice roll is 4
+//Random dice roll is 5
+//Random dice roll is 4
+```
 
 <a name="delegation"></a>
 ## 委托(代理)模式
@@ -231,55 +252,59 @@
 
 下文是两个基于骰子游戏的协议：
 
-	protocol DiceGame {
-		var dice: Dice { get }
-		func play()
-	}
-	protocol DiceGameDelegate {
-		func gameDidStart(game: DiceGame)
-		func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll:Int)
-		func gameDidEnd(game: DiceGame)
-	}
+```swift
+protocol DiceGame {
+	var dice: Dice { get }
+	func play()
+}
+
+protocol DiceGameDelegate {
+	func gameDidStart(game: DiceGame)
+	func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll:Int)
+	func gameDidEnd(game: DiceGame)
+}
+```
 
 `DiceGame`协议可以在任意含有骰子的游戏中实现，`DiceGameDelegate`协议可以用来追踪`DiceGame`的游戏过程。
 
 如下所示，`SnakesAndLadders`是`Snakes and Ladders`(译者注：[控制流](05_Control_Flow.html)章节有该游戏的详细介绍)游戏的新版本。新版本使用`Dice`作为骰子，并且实现了`DiceGame`和`DiceGameDelegate`协议
 
-	class SnakesAndLadders: DiceGame {
-		let finalSquare = 25
-		let dic = Dice(sides: 6, generator: LinearCongruentialGenerator())
-		var square = 0
-		var board: Int[]
-		init() {
-			board = Int[](count: finalSquare + 1, repeatedValue: 0)
-			board[03] = +08; board[06] = +11; borad[09] = +09; board[10] = +02
-			borad[14] = -10; board[19] = -11; borad[22] = -02; board[24] = -08
-		}
- 		var delegate: DiceGameDelegate?
- 		func play() {
- 			square = 0
- 			delegate?.gameDidStart(self)
- 			gameLoop: while square != finalSquare {
- 				let diceRoll = dice.roll()
- 				delegate?.game(self,didStartNewTurnWithDiceRoll: diceRoll)
- 				switch square + diceRoll {
- 				case finalSquare:
-	 				break gameLoop
- 				case let newSquare where newSquare > finalSquare:
- 					continue gameLoop
- 				default:
- 				square += diceRoll
- 				square += board[square]
- 				}
- 			}
- 			delegate?.gameDIdEnd(self)
- 		}
+```swift
+class SnakesAndLadders: DiceGame {
+	let finalSquare = 25
+	let dic = Dice(sides: 6, generator: LinearCongruentialGenerator())
+	var square = 0
+	var board: Int[]
+	init() {
+		board = Int[](count: finalSquare + 1, repeatedValue: 0)
+		board[03] = +08; board[06] = +11; borad[09] = +09; board[10] = +02
+		borad[14] = -10; board[19] = -11; borad[22] = -02; board[24] = -08
 	}
+ 	var delegate: DiceGameDelegate?
+ 	func play() {
+ 		square = 0
+ 		delegate?.gameDidStart(self)
+ 		gameLoop: while square != finalSquare {
+ 			let diceRoll = dice.roll()
+ 			delegate?.game(self,didStartNewTurnWithDiceRoll: diceRoll)
+ 			switch square + diceRoll {
+ 			case finalSquare:
+ 				break gameLoop
+ 			case let newSquare where newSquare > finalSquare:
+ 				continue gameLoop
+ 			default:
+ 			square += diceRoll
+ 			square += board[square]
+ 			}
+ 		}
+ 		delegate?.gameDIdEnd(self)
+ 	}
+}
+```
 
 游戏的`初始化设置(setup)`被`SnakesAndLadders`类的`构造器(initializer)`实现。所有的游戏逻辑被转移到了`play`方法中。
 
-> 注意：
->
+> 注意：  
 因为`delegate`并不是该游戏的必备条件，`delegate`被定义为遵循`DiceGameDelegate`协议的可选属性
 
 `DicegameDelegate`协议提供了三个方法用来追踪游戏过程。被放置于游戏的逻辑中，即`play()`方法内。分别在游戏开始时，新一轮开始时，游戏结束时被调用。
@@ -288,23 +313,25 @@
 
 如下所示，`DiceGameTracker`遵循了`DiceGameDelegate`协议
 
-	class DiceGameTracker: DiceGameDelegate {
-    	var numberOfTurns = 0
-	    func gameDidStart(game: DiceGame) {
-        	numberOfTurns = 0
-    	    if game is SnakesAndLadders {
-	            println("Started a new game of Snakes and Ladders")
-	        }
-    	    println("The game is using a \(game.dice.sides)-sided dice")
-	    }
-    	func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int) {
-	        ++numberOfTurns
-        	println("Rolled a \(diceRoll)")
-    	}
-	    func gameDidEnd(game: DiceGame) {
-        	println("The game lasted for \(numberOfTurns) turns")
-    	}
-	}
+```swift
+class DiceGameTracker: DiceGameDelegate {
+    var numberOfTurns = 0
+    func gameDidStart(game: DiceGame) {
+        numberOfTurns = 0
+        if game is SnakesAndLadders {
+            println("Started a new game of Snakes and Ladders")
+        }
+        println("The game is using a \(game.dice.sides)-sided dice")
+    }
+    func game(game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int) {
+        ++numberOfTurns
+        println("Rolled a \(diceRoll)")
+    }
+    func gameDidEnd(game: DiceGame) {
+        println("The game lasted for \(numberOfTurns) turns")
+    }
+}
+```
 
 `DiceGameTracker`实现了`DiceGameDelegate`协议的方法要求，用来记录游戏已经进行的轮数。 当游戏开始时，`numberOfTurns`属性被赋值为0；在每新一轮中递加；游戏结束后，输出打印游戏的总轮数。
 
@@ -312,79 +339,91 @@
 
 `DiceGameTracker`的运行情况，如下所示：
 
-	“let tracker = DiceGameTracker()
-	let game = SnakesAndLadders()
-	game.delegate = tracker
-	game.play()
-	// Started a new game of Snakes and Ladders
-	// The game is using a 6-sided dice
-	// Rolled a 3
-	// Rolled a 5
-	// Rolled a 4
-	// Rolled a 5
-	// The game lasted for 4 turns”
+```swift
+“let tracker = DiceGameTracker()
+let game = SnakesAndLadders()
+game.delegate = tracker
+game.play()
+// Started a new game of Snakes and Ladders
+// The game is using a 6-sided dice
+// Rolled a 3
+// Rolled a 5
+// Rolled a 4
+// Rolled a 5
+// The game lasted for 4 turns”
+```
 
 <a name="adding_protocol_conformance_with_an_extension"></a>
 ## 在扩展中添加协议成员
 
 即便无法修改源代码，依然可以通过`扩展(Extension)`来扩充已存在类型(*译者注： 类，结构体，枚举等*)。`扩展`可以为已存在的类型添加`属性`，`方法`，`下标`，`协议`等成员。详情请在[扩展](20_Extensions.html)章节中查看。
 
-> 注意：
->
+> 注意：  
 通过`扩展`为已存在的类型`遵循`协议时，该类型的所有实例也会随之添加协议中的方法
 
 `TextRepresentable`协议含有一个`asText`，如下所示：
 
-	protocol TextRepresentable {
-		func asText() -> String
-	}
+```swift
+protocol TextRepresentable {
+	func asText() -> String
+}
+```
 
 通过`扩展`为上一节中提到的`Dice`类遵循`TextRepresentable`协议
 
-	extension Dice: TextRepresentable {
-		func asText() -> String {
-			return "A \(sides)-sided dice"
-		}
+```swift
+extension Dice: TextRepresentable {
+	cun asText() -> String {
+		return "A \(sides)-sided dice"
 	}
+}
+```
 
 从现在起，`Dice`类型的实例可被当作`TextRepresentable`类型：
 
-	let d12 = Dice(sides: 12,generator: LinearCongruentialGenerator())
-	println(d12.asText())
-	// 输出 "A 12-sided dice"
+```swift
+let d12 = Dice(sides: 12,generator: LinearCongruentialGenerator())
+println(d12.asText())
+// 输出 "A 12-sided dice"
+```
 
 `SnakesAndLadders`类也可以通过`扩展`的方式来遵循协议：
 
-	extension SnakeAndLadders: TextRepresentable {
-		func asText() -> String {
-			return "A game of Snakes and Ladders with \(finalSquare) squares"
-		}
+```swift
+extension SnakeAndLadders: TextRepresentable {
+	func asText() -> String {
+		return "A game of Snakes and Ladders with \(finalSquare) squares"
 	}
-	println(game.asText())
-	// 输出 "A game of Snakes and Ladders with 25 squares"
+}
+println(game.asText())
+// 输出 "A game of Snakes and Ladders with 25 squares"
+```
 
 <a name="declaring_protocol_adoption_with_an_extension"></a>
 ## 通过延展补充协议声明
 
 当一个类型已经实现了协议中的所有要求，却没有声明时，可以通过`扩展`来补充协议声明：
 
-	struct Hamster {
-		var name: String
-		func asText() -> String {
-			return "A hamster named \(name)"
-		}
+```swift
+struct Hamster {
+	var name: String
+	func asText() -> String {
+		return "A hamster named \(name)"
 	}
-	extension Hamster: TextRepresentabl {}
+}
+extension Hamster: TextRepresentabl {}
+```
 
 从现在起，`Hamster`的实例可以作为`TextRepresentable`类型使用
 
-	let simonTheHamster = Hamster(name: "Simon")
-	let somethingTextRepresentable: TextRepresentabl = simonTheHamester
-	println(somethingTextRepresentable.asText())
-	// 输出 "A hamster named Simon"
+```swift
+let simonTheHamster = Hamster(name: "Simon")
+let somethingTextRepresentable: TextRepresentabl = simonTheHamester
+println(somethingTextRepresentable.asText())
+// 输出 "A hamster named Simon"
+```
 
-> 注意：
->
+> 注意：  
 即时满足了协议的所有要求，类型也不会自动转变，因此你必须为它做出明显的协议声明
 
 <a name="collections_of_protocol_types"></a>
@@ -392,16 +431,20 @@
 
 协议类型可以被集合使用，表示集合中的元素均为协议类型：
 
-	let things: TextRepresentable[] = [game,d12,simoTheHamster]
+```swift
+let things: TextRepresentable[] = [game,d12,simoTheHamster]
+```
 
 如下所示，`things`数组可以被直接遍历，并调用其中元素的`asText()`函数：
 
-	for thing in things {
-		println(thing.asText())
-	}
-	// A game of Snakes and Ladders with 25 squares
-	// A 12-sided dice
-	// A hamster named Simon
+```swift
+for thing in things {
+	println(thing.asText())
+}
+// A game of Snakes and Ladders with 25 squares
+// A 12-sided dice
+// A hamster named Simon
+```
 
 `thing`被当做是`TextRepresentable`类型而不是`Dice`，`DiceGame`，`Hamster`等类型。因此能且仅能调用`asText`方法
 
@@ -410,36 +453,42 @@
 
 协议能够*继承*一到多个其他协议。语法与类的继承相似，多个协议间用逗号`,`分隔
 
-	protocol InheritingProtocol: SomeProtocol, AnotherProtocol {
-		// 协议定义
-	}
+```swift
+protocol InheritingProtocol: SomeProtocol, AnotherProtocol {
+	// 协议定义
+}
+```
 
 如下所示，`PrettyTextRepresentable`协议继承了`TextRepresentable`协议
 
-	protocol PrettyTextRepresentable: TextRepresentable {
-		func asPrettyText() -> String
-	}
+```swift
+protocol PrettyTextRepresentable: TextRepresentable {
+	func asPrettyText() -> String
+}
+```
 
 `遵循``PrettyTextRepresentable`协议的同时，也需要`遵循`TextRepresentable`协议。
 
 如下所示，用`扩展`为`SnakesAndLadders`遵循`PrettyTextRepresentable`协议：
 
-	extension SnakesAndLadders: PrettyTextRepresentable {
-    	func asPrettyText() -> String {
-        	var output = asText() + ":\n"
-        	for index in 1...finalSquare {
-            	switch board[index] {
-           	 	case let ladder where ladder > 0:
-        	        output += "▲ "
-    	        case let snake where snake < 0:
-	                output += "▼ "
-            	default:
-                	output += "○ "
-            	}
-        	}
-        	return output
-    	}
-	}
+```swift
+extension SnakesAndLadders: PrettyTextRepresentable {
+    func asPrettyText() -> String {
+        var output = asText() + ":\n"
+        for index in 1...finalSquare {
+            switch board[index] {
+            	case let ladder where ladder > 0:
+                output += "▲ "
+            case let snake where snake < 0:
+                output += "▼ "
+            default:
+                output += "○ "
+            }
+        }
+        return output
+    }
+}
+```
 
 在`for in`中迭代出了`board`数组中的每一个元素：
 
@@ -449,9 +498,11 @@
 
 任意`SankesAndLadders`的实例都可以使用`asPrettyText()`方法。
 
-	println(game.asPrettyText())
-	// A game of Snakes and Ladders with 25 squares:
-	// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○
+```swift
+println(game.asPrettyText())
+// A game of Snakes and Ladders with 25 squares:
+// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○
+```
 
 <a name="protocol_composition"></a>
 ## 协议合成
@@ -460,29 +511,30 @@
 
 举个例子：
 
-	protocol Named {
-    	var name: String { get }
-	}
-	protocol Aged {
-	    var age: Int { get }
-	}
-	struct Person: Named, Aged {
-    	var name: String
-    	var age: Int
-	}
-	func wishHappyBirthday(celebrator: protocol<Named, Aged>) {
-    	println("Happy birthday \(celebrator.name) - you're \(celebrator.age)!")
-	}
-	let birthdayPerson = Person(name: "Malcolm", age: 21)
-	wishHappyBirthday(birthdayPerson)
-	// 输出 "Happy birthday Malcolm - you're 21!
+```swift
+protocol Named {
+    var name: String { get }
+}
+protocol Aged {
+    var age: Int { get }
+}
+struct Person: Named, Aged {
+    var name: String
+    var age: Int
+}
+func wishHappyBirthday(celebrator: protocol<Named, Aged>) {
+    println("Happy birthday \(celebrator.name) - you're \(celebrator.age)!")
+}
+let birthdayPerson = Person(name: "Malcolm", age: 21)
+wishHappyBirthday(birthdayPerson)
+// 输出 "Happy birthday Malcolm - you're 21!
+```
 
 `Named`协议包含`String`类型的`name`属性；`Aged`协议包含`Int`类型的`age`属性。`Person`结构体`遵循`了这两个协议。
 
 `wishHappyBirthday`函数的形参`celebrator`的类型为`protocol<Named,Aged>`。可以传入任意`遵循`这两个协议的类型的实例
 
-> 注意：
->
+> 注意：  
 `协议合成`并不会生成一个新协议类型，而是将多个协议合成为一个临时的协议，超出范围后立即失效。
 
 <a name="checking_for_protocol_conformance"></a>
@@ -494,57 +546,64 @@
 * `as?`返回一个可选值，当实例`遵循`协议时，返回该协议类型；否则返回`nil`
 * `as`用以强制向下转换型。
 
-```
+```swift
 @objc protocol HasArea {
-	var area: Double { get }
+    var area: Double { get }
 }
 ```
 
 
-> 注意：
->
+> 注意：  
 `@objc`用来表示协议是可选的，也可以用来表示暴露给`Objective-C`的代码，此外，`@objc`型协议只对`类`有效，因此只能在`类`中检查协议的一致性。详情查看*[Using Siwft with Cocoa and Objectivei-c](https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/BuildingCocoaApps/index.html#//apple_ref/doc/uid/TP40014216)*。
 
-	class Circle: HasArea {
-    	let pi = 3.1415927
-    	var radius: Double
-    	var area:≈radius }
-    	init(radius: Double) { self.radius = radius }
-	}
-	class Country: HasArea {
-    	var area: Double
-    	init(area: Double) { self.area = area }
-	}
+```swift
+class Circle: HasArea {
+    let pi = 3.1415927
+    var radius: Double
+    var area:≈radius }
+    init(radius: Double) { self.radius = radius }
+}
+class Country: HasArea {
+    var area: Double
+    init(area: Double) { self.area = area }
+}
+```
 
 `Circle`和`Country`都遵循了`HasArea`协议，前者把`area`写为计算型属性（computed property），后者则把`area`写为存储型属性（stored property）。
 
 如下所示，`Animal`类没有实现任何协议
 
-	class Animal {
-		var legs: Int
-		init(legs: Int) { self.legs = legs }
-	}
+```swift
+class Animal {
+	var legs: Int
+	init(legs: Int) { self.legs = legs }
+}
+```
 
 `Circle,Country,Animal`并没有一个相同的基类，所以采用`AnyObject`类型的数组来装载在它们的实例，如下所示：
 
-	let objects: AnyObject[] = [
-		Circle(radius: 2.0),
-		Country(area: 243_610),
-		Animal(legs: 4)
-	]
+```swift
+let objects: AnyObject[] = [
+	Circle(radius: 2.0),
+	Country(area: 243_610),
+	Animal(legs: 4)
+]
+```
 
 如下所示，在迭代时检查`object`数组的元素是否`遵循`了`HasArea`协议：
 
-	for object in objects {
-	    if let objectWithArea = object as? HasArea {
-	        println("Area is \(objectWithArea.area)")
-	    } else {
-	        println("Something that doesn't have an area")
-	    }
-	}
-	// Area is 12.5663708
-	// Area is 243610.0
-	// Something that doesn't have an area
+```swift
+for object in objects {
+    if let objectWithArea = object as? HasArea {
+        println("Area is \(objectWithArea.area)")
+    } else {
+        println("Something that doesn't have an area")
+    }
+}
+// Area is 12.5663708
+// Area is 243610.0
+// Something that doesn't have an area
+```
 
 当数组中的元素遵循`HasArea`协议时，通过`as?`操作符将其`可选绑定(optional binding)`到`objectWithArea`常量上。
 
@@ -559,36 +618,38 @@
 
 像`someOptionalMethod?(someArgument)`一样，你可以在可选方法名称后加上`?`来检查该方法是否被实现。`可选方法`和`可选属性`都会返回一个`可选值(optional value)`，当其不可访问时，`?`之后语句不会执行，并返回`nil`。
 
-> 注意：
->
+> 注意：  
 可选协议只能在含有`@objc`前缀的协议中生效。且`@objc`的协议只能被`类`遵循。
 
 `Counter`类使用`CounterDataSource`类型的外部数据源来提供`增量值(increment amount)`，如下所示：
 
-	@objc protocol CounterDataSource {
-    	@optional func incrementForCount(count: Int) -> Int
-    	@optional var fixedIncrement: Int { get }
-	}
+```swift
+@objc protocol CounterDataSource {
+    @optional func incrementForCount(count: Int) -> Int
+    @optional var fixedIncrement: Int { get }
+}
+```
 
 `CounterDataSource`含有`incrementForCount`的`可选方法`和`fiexdIncrement`的`可选属性`。
 
-> 注意：
->
+> 注意：  
 `CounterDataSource`中的属性和方法都是可选的，因此可以在类中声明但不实现这些成员，尽管技术上允许这样做，不过最好不要这样写。
 
 `Counter`类含有`CounterDataSource?`类型的可选属性`dataSource`，如下所示：
 
-	@objc class Counter {
-	    var count = 0
-	    var dataSource: CounterDataSource?
-	    func increment() {
-	        if let amount = dataSource?.incrementForCount?(count) {
-	            count += amount
-	        } else if let amount = dataSource?.fixedIncrement? {
-	            count += amount
-	        }
-	    }
-	}
+```swift
+@objc class Counter {
+    var count = 0
+    var dataSource: CounterDataSource?
+    func increment() {
+        if let amount = dataSource?.incrementForCount?(count) {
+            count += amount
+        } else if let amount = dataSource?.fixedIncrement? {
+            count += amount
+        }
+    }
+}
+```
 
 `count`属性用于存储当前的值，`increment`方法用来为`count`赋值。
 
@@ -603,47 +664,55 @@
 
 `ThreeSource`实现了`CounterDataSource`协议，如下所示：
 
-	class ThreeSource: CounterDataSource {
-		let fixedIncrement = 3
-	}
+```swift
+class ThreeSource: CounterDataSource {
+	let fixedIncrement = 3
+}
+```
 
 使用`ThreeSource`作为数据源开实例化一个`Counter`：
 
-	var counter = Counter()
-	counter.dataSource = ThreeSource()
-	for _ in 1...4 {
-	    counter.increment()
-	    println(counter.count)
-	}
-	// 3
-	// 6
-	// 9
-	// 12
+```swift
+var counter = Counter()
+counter.dataSource = ThreeSource()
+for _ in 1...4 {
+    counter.increment()
+    println(counter.count)
+}
+// 3
+// 6
+// 9
+// 12
+```
 
 `TowardsZeroSource`实现了`CounterDataSource`协议中的`incrementForCount`方法，如下所示：
 
-	class TowardsZeroSource: CounterDataSource {
-    func incrementForCount(count: Int) -> Int {
-	        if count == 0 {
-	            return 0
-	        } else if count < 0 {
-	            return 1
-	        } else {
-	            return -1
-	        }
-    	}
-	}
+```swift
+class TowardsZeroSource: CounterDataSource {
+func incrementForCount(count: Int) -> Int {
+        if count == 0 {
+            return 0
+        } else if count < 0 {
+            return 1
+        } else {
+            return -1
+        }
+    }
+}
+```
 
 下边是执行的代码：
 
-	counter.count = -4
-	counter.dataSource = TowardsZeroSource()
-	for _ in 1...5 {
-	    counter.increment()
-	    println(counter.count)
-	}
-	// -3
-	// -2
-	// -1
-	// 0
-	// 0
+```swift
+counter.count = -4
+counter.dataSource = TowardsZeroSource()
+for _ in 1...5 {
+    counter.increment()
+    println(counter.count)
+}
+// -3
+// -2
+// -1
+// 0
+// 0
+```