property有三种，分别是stored property,computed property, type property. stored property可以用在class和structure中，computed property可以用在class、structure和enumeration中。

stored property

stored property 跟c++/c#里面的成员变量很相似。它可以用var和let两种修饰方式，分别表示变量和常量。对于变量，还可以提供lazy关键字来给property提供懒加载功能。例如：

struct FixedLengthRange {
    var firstValue: Int
    let length: Int
}
var rangeOfThreeItems = FixedLengthRange(firstValue: 0, length: 3)
// the range represents integer values 0, 1, and 2
rangeOfThreeItems.firstValue = 6
// the range now represents integer values 6, 7, and 8

摘录来自: Apple Inc. “The Swift Programming Language (Swift 3)”。 iBooks. 

关于lazy初始化property的例子：

class DataImporter {
    /*
     DataImporter is a class to import data from an external file.
     The class is assumed to take a non-trivial amount of time to initialize.
     */
    var fileName = "data.txt"
    // the DataImporter class would provide data importing functionality here
}
 
class DataManager {
    lazy var importer = DataImporter()
    var data = [String]()
    // the DataManager class would provide data management functionality here
}
 
let manager = DataManager()
manager.data.append("Some data")
manager.data.append("Some more data")
// the DataImporter instance for the importer property has not yet been created

摘录来自: Apple Inc. “The Swift Programming Language (Swift 3)”。 iBooks. 

computed property

computed property的意思就是本身并不直接存储值，但是可以通过get和set方法来间接获取或者改变其他property的值。看例子：

struct AlternativeRect {
    var origin = Point()
    var size = Size()
    var center: Point {
        get {
            let centerX = origin.x + (size.width / 2)
            let centerY = origin.y + (size.height / 2)
            return Point(x: centerX, y: centerY)
        }
        set {
            origin.x = newValue.x - (size.width / 2)
            origin.y = newValue.y - (size.height / 2)
        }
    }
}

摘录来自: Apple Inc. “The Swift Programming Language (Swift 3)”。 iBooks. 

在这里，set方法是可以有一个参数的，参数的值就是要设置的新值。如果没有参数，则可以用newValue来默认指代。
如果只有get方法，也就是说这个computed property 是一个只读property，则get方法和大括号都可以省略，例如：

struct AlternativeRect {
    var origin = Point()
    var size = Size()
    var center: Point {
        
        let centerX = origin.x + (size.width / 2)
        let centerY = origin.y + (size.height / 2)
        return Point(x: centerX, y: centerY)

    }
}

type property

type property 类似于c++/c#中的静态成员变量。对于struct和enumeration类型的用static定义，对于class类型的，可以使用class替代static，来让子类可以复写父类的实现。

struct SomeStructure {
    static var storedTypeProperty = "Some value."
    static var computedTypeProperty: Int {
        return 1
    }
}
enum SomeEnumeration {
    static var storedTypeProperty = "Some value."
    static var computedTypeProperty: Int {
        return 6
    }
}
class SomeClass {
    static var storedTypeProperty = "Some value."
    static var computedTypeProperty: Int {
        return 27
    }
    class var overrideableComputedTypeProperty: Int {
        return 107
    }
}

摘录来自: Apple Inc. “The Swift Programming Language (Swift 3)”。 iBooks. 

stored type property必须在定义的时候给一个默认值，因为在初始化的时候没有一个initializer可以给它赋值。stored type property会在初次被使用的时候懒加载，即使有多个线程同时访问它，也可以保证只被初始化一次，而且它们不需要加lazy关键字。

property observers

跟oc类似，property赋值前后有两个方法可以提前知晓和确认这种变化，willSet会在赋值前调用，didSet会在赋值后立即调用。

class StepCounter {
    var totalSteps: Int = 0 {
        willSet(newTotalSteps) {
            print("About to set totalSteps to \(newTotalSteps)")
        }
        didSet {
            if totalSteps > oldValue  {
                print("Added \(totalSteps - oldValue) steps")
            }
        }
    }
}
let stepCounter = StepCounter()
stepCounter.totalSteps = 200
// About to set totalSteps to 200
// Added 200 steps
stepCounter.totalSteps = 360
// About to set totalSteps to 360
// Added 160 steps
stepCounter.totalSteps = 896
// About to set totalSteps to 896
// Added 536 steps

摘录来自: Apple Inc. “The Swift Programming Language (Swift 3)”。 iBooks. 

参考

《The Swift Programming Language》