Android(安卓)Kotlin入门-属性和字段

正文：

类可以有属性，你可以定义可变的属性或者不可变的属性（var可变，val不可变）。形如：

public class Address {   public var name: String = ...  public var street: String = ...  public var city: String = ...  public var state: String? = ...  public var zip: String = ...}

使用一个属性，往往是访问其名字。形如:

fun copyAddress(address: Address): Address {  val result = Address() // there's no 'new' keyword in Kotlin  result.name = address.name // accessors are called  result.street = address.street  // ...  return result}

Getters and Setters

语法声明的形式：

var <propertyName>: <PropertyType> [= <property_initializer>]  [<getter>]  [<setter>]

初始化getter和setter是可选的，如果属性的类型如果可以从初始化或从基类成员被覆盖推断出来就可以省略。形如：

var allByDefault: Int? // error: explicit initializer required, default getter and setter impliedvar initialized = 1 // has type Int, default getter and setter

var是可变的可以有getter和setter方法，而val是不可变的不能有setter方法。形如：

val inferredType = 1 // has type Int and a default getter

我们可以编写自定义的getter方法。形如：

val isEmpty: Boolean  get() = this.size == 0

自定义的setter方法。形如：

var stringRepresentation: String  get() = this.toString()  set(value) {    setDataFromString(value) // parses the string and assigns values to other properties  }

如果你只是想改变getter和setter的可见，就可以不去定义它的函数体，形如：

 var setterVisibility: String = "abc" // Initializer required, not a nullable type            private set // the setter is private and has the default implementation

Backing Fields

var counter = 0 // the initializer value is written directly to the backing field            set(value) {                if (value >= 0)                    field = value//这里的field指代的是counter            }

Backing Properties

你也可以将你的set方法返回给其它的属性，形如：

private var _table: Map<String, Int>? = nullpublic val table: Map<String, Int>  get() {    if (_table == null)      _table = HashMap() // Type parameters are inferred    return _table ?: throw AssertionError("Set to null by another thread")  }

Getters and Setters

Backing Fields

Backing Properties

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