if 的条件式可以是一个实现了 BooleanType 协议的Class

结合 StringLiteralConvertible 协议 实现一个可以用字符串定义的条件式（对实现多类型条件式判定方法的统一）

class Condition: BooleanType,StringLiteralConvertible {
    
    //对BooleanType协议的实现
    var result: Bool = false
    
    var boolValue: Bool {
        get {
            return result
        }
    }
    
    init() {
    }
    
    //对StringLiteralConvertible协议的实现
    typealias ExtendedGraphemeClusterLiteralType = String
    typealias UnicodeScalarLiteralType = String
    
    required convenience init(stringLiteral value: StringLiteralType){
        self.init()
        if value == "" || value == "0" {
            result = false
        } else {
            result = true
        }
    }
    
    required convenience init(extendedGraphemeClusterLiteral value: ExtendedGraphemeClusterLiteralType){
          self.init()
    }
    
    required convenience init(unicodeScalarLiteral value: UnicodeScalarLiteralType){
       self.init()
    }
    
}

//可以用一个文字串来定义一个条件式
var condition: Condition = "0"

if condition {
    println("这是真的")
} else {
    println("这是假的")
}