Swift: The Basics

1. Swift是类型安全的语言；
2. Swift introduces optional types, which handle the absence of a value. Optional say either "there is a value, and it equals x" or "there isn't a value at all".
3. 类型注解

   var welcomeMessage: String
   var red, green, double: Double

4. 类型别名（Type Alias）

   typealias AudioSample = UInt16
   
   var maxAmplitudeFound = AudioSample.min

5. 元组（Tuples）：元组内的值可以是不同类型的任何值。

   let http404Error = (, "Not Found")
   //http404Error的类型是(Int, String)

   分解（decompose）元组：可以使用下划线来忽略一些值

   let (statusCode, _) = http404Error
   print("The status code is \(status)")

   也可以使用下标来取出单个值:

    print("The status code is \(http404Error.0)")
    // Prints "The status code is 404"
    print("The status message is \(http404Error.1)")
    // Prints "The status message is Not Found"

   可以在定义元组的时候，给每个元素命名：

   let http200Status = (statusCode: , description: "OK")

   这个时候，可以这样获取每个元素的值：

   print("The status code is \(http200Status.statusCode)")

6. Optional type

   let possibleNumber = ""
   let convertedNumber = Int(possibleNumber)
   //convertedNumber的类型是 Int?， 不是Int

   Swift的nil跟oc中的nil不同。In Objective-C, nil is a pointer to a nonexistent object. In Swift, nil is not a pointer -- it's the absence of a value of certain type. Optionals of any type can be set to nil, not just object types.
   forced unwrapping
   optional binding
   implicity unwrapped optionals