本文是學(xué)習(xí)《The Swift Programming Language》整理的相關(guān)隨筆顺饮，基本的語法不作介紹，主要介紹Swift中的一些特性或者與OC差異點(diǎn)凌那。
系列文章：
比較類與結(jié)構(gòu)體(Comparing Classes and Structures)
Classes and structures in Swift have many things in common. Both can:
- Define properties to store values
- Define methods to provide functionality
- Define subscripts to provide access to their values using subscript syntax
- Define initializers to set up their initial state
- Be extended to expand their functionality beyond a default implementation
- Conform to protocols to provide standard functionality of a certain kind
-
For more information, see Properties, Methods, Subscripts, Initialization, Extensions, and Protocols.
Classes have additional capabilities that structures do not:
- Inheritance enables one class to inherit the characteristics of another.
- Type casting enables you to check and interpret the type of a class instance at runtime.
- Deinitializers enable an instance of a class to free up any resources it has assigned.
- Reference counting allows more than one reference to a class instances
Swift 中類和結(jié)構(gòu)體有很多共同點(diǎn)兼雄。共同處在于：
- 定義屬性用于存儲(chǔ)值
- 定義方法用于提供功能
- 定義附屬腳本用于訪問值
- 定義構(gòu)造器用于生成初始化值
- 通過擴(kuò)展以增加默認(rèn)實(shí)現(xiàn)的功能
- 符合協(xié)議以對(duì)某類提供標(biāo)準(zhǔn)功能
與結(jié)構(gòu)體相比，類還有如下的附加功能：
- 繼承允許一個(gè)類繼承另一個(gè)類的特征
- 類型轉(zhuǎn)換允許在運(yùn)行時(shí)檢查和解釋一個(gè)類實(shí)例的類型
- 解構(gòu)器允許一個(gè)類實(shí)例釋放任何其所被分配的資源
- 引用計(jì)數(shù)允許對(duì)一個(gè)類的多次引用
定義語法(Definition Syntax)
Classes and structures have a similar definition syntax.
You introduce classes with the class keyword and
structures with the struct keyword.
例子：
struct Resolution {
var width = 0
var height = 0
}
class VideoMode {
var resolution = Resolution()
var interlaced = false
var frameRate = 0.0
var name: String?
}
var someResolution = Resolution();
someResolution.width = 10;
someResolution.height = 10;
var someVideoMode = VideoMode();
someVideoMode.resolution = someResolution;
someVideoMode.frameRate = 1.0;
someVideoMode.name = "mp4";
print("someVideoMode.resolution: (\(someVideoMode.resolution.width) \(someVideoMode.resolution.height)),someVideoMode.name \(someVideoMode.name)");
執(zhí)行結(jié)果：
someVideoMode.resolution: (10 10),someVideoMode.name Optional("mp4")
結(jié)構(gòu)體與枚舉都是值類型數(shù)據(jù)(Structures and Enumerations Are Value Types)
A value type is a type whose value is copied when it is
assigned to a variable or constant, or when it is passed
to a function.
結(jié)構(gòu)體例子：
struct Resolution {
var width = 0
var height = 0
}
let hd = Resolution(width:1920,height:1080)
var cinema = hd
cinema.width = 2048
print("cinema is now \(cinema.width) pixels wide")
print("hd is still \(hd.width) pixels wide")
執(zhí)行結(jié)果：
cinema is now 2048 pixels wide
hd is still 1920 pixels wide
枚舉例子：
enum CompassPoint {
case north, south, east, west
}
var currentDirection = CompassPoint.west
let rememberedDirection = currentDirection
currentDirection = .east
if rememberedDirection == .west {
print("The remembered direction is still .west")
}
執(zhí)行結(jié)果:
The remembered direction is still .west
類是引用類型數(shù)據(jù)(Classes Are Reference Types)
Unlike value types, reference types are not copied when
they are assigned to a variable or constant, or when they
are passed to a function. Rather than a copy, a reference
to the same existing instance is used instead.
例子：
struct Resolution {
var width = 0
var height = 0
}
class VideoMode {
var resolution = Resolution()
var interlaced = false
var frameRate = 0.0
var name: String?
}
var someVideoMode = VideoMode();
someVideoMode.name = "mp4";
var otherVideoMode = someVideoMode;
otherVideoMode.name = "AVI"
print("someVideoMode.name : (\(someVideoMode.name)");
執(zhí)行結(jié)果：
someVideoMode.name : (Optional("AVI")
恒等式操作符(Identity Operators)
It can sometimes be useful to find out if two constants or
variables refer to exactly the same instance of a class.
To enable this, Swift provides two identity operators:
Identical to (===)
Not identical to (!==)
接著上述的例子：
struct Resolution {
var width = 0
var height = 0
}
class VideoMode {
var resolution = Resolution()
var interlaced = false
var frameRate = 0.0
var name: String?
}
var someVideoMode = VideoMode();
someVideoMode.name = "mp4";
var otherVideoMode = someVideoMode;
otherVideoMode.name = "AVI"
var thirdVideoMode = VideoMode();
if someVideoMode === otherVideoMode{
print("someVideoMode === otherVideoMode")
}
if someVideoMode !== thirdVideoMode{
print("someVideoMode !== thirdVideoMode")
}
執(zhí)行結(jié)果：
someVideoMode === otherVideoMode
someVideoMode !== thirdVideoMode
類和結(jié)構(gòu)體的選擇(Choosing Between Classes and Structures)
You can use both classes and structures to define custom data types to use as the building blocks of your program’s
code.
However, structure instances are always passed by value,
and class instances are always passed by reference. This
means that they are suited to different kinds of tasks. As you consider the data constructs and functionality that
you need for a project, decide whether each data construct
should be defined as a class or as a structure.
As a general guideline, consider creating a structure when
one or more of these conditions apply:
- The structure’s primary purpose is to encapsulate a few
relatively simple data values.
- It is reasonable to expect that the encapsulated values
will be copied rather than referenced when you assign or
pass around an instance of that structure.
- Any properties stored by the structure are themselves
value types, which would also be expected to be copied rather than referenced.
- The structure does not need to inherit properties or behavior from another existing type.
Examples of good candidates for structures include:
- The size of a geometric shape, perhaps encapsulating a width property and a height property, both of type Double.
- A way to refer to ranges within a series, perhaps encapsulating a start property and a length property, both of type Int.
- A point in a 3D coordinate system, perhaps encapsulating x, y and z properties, each of type Double.
In all other cases, define a class, and create instances
of that class to be managed and passed by reference. In
practice, this means that most custom data constructs
should be classes, not structures.
按照通用的準(zhǔn)則帽蝶，當(dāng)符合一條或多條以下條件時(shí)赦肋，請(qǐng)考慮構(gòu)建結(jié)構(gòu)體：
- 結(jié)構(gòu)體的主要目的是用來封裝少量相關(guān)簡單數(shù)據(jù)值。
- 有理由預(yù)計(jì)一個(gè)結(jié)構(gòu)體實(shí)例在賦值或傳遞時(shí)，封裝的數(shù)據(jù)將會(huì)被拷貝而不是被引用金砍。
- 任何在結(jié)構(gòu)體中儲(chǔ)存的值類型屬性局蚀，也將會(huì)被拷貝，而不是被引用恕稠。
- 結(jié)構(gòu)體不需要去繼承另一個(gè)已存在類型的屬性或者行為。
合適的結(jié)構(gòu)體候選者包括：
- 幾何形狀的大小扶欣，封裝一個(gè)width屬性和height屬性鹅巍，兩者均為Double類型。
- 一定范圍內(nèi)的路徑料祠，封裝一個(gè)start屬性和length屬性骆捧，兩者均為Int類型。
- 三維坐標(biāo)系內(nèi)一點(diǎn)髓绽，封裝x敛苇，y和z屬性，三者均為Double類型顺呕。
字符串與集合類型的賦值和拷貝行為(Assignment and Copy Behavior for Strings, Arrays,and Dictionaries)
In Swift, many basic data types such as String, Array, and
Dictionary are implemented as structures. This means that
data such as strings, arrays, and dictionaries are copied
when they are assigned to a new constant or variable, or
when they are passed to a function or method.
- 在Swift中String,Array,Dictionary都是以結(jié)構(gòu)體的方式實(shí)現(xiàn)的枫攀。因此當(dāng)使用它們賦值或者傳入函數(shù)作為參數(shù)時(shí)都是以拷貝的方式傳入。
例子：
var array:[Int] = [1,2,3]
var otherArray = array
otherArray.append(4)
print("array:\(array),otherArray:\(otherArray)")
執(zhí)行結(jié)果:
array:[1, 2, 3],otherArray:[1, 2, 3, 4]
