Kotlin-了解getter和setter

Question

Kotlin自动生成它的getters和settings，但是我从来没有引用过它们？另外，用Kotlin编写自定义getter/setter的正确方法是什么？当我说< code>myObj.myVar = 99时，我感觉< code>myVar是我正在直接访问的< code>myObj的公共字段。这里到底发生了什么？

Answer 1

以下是一些自定义getter和setter的真实示例。您可以在此处查看更多信息。

// Custom getter example
val friendlyDescription get(): String {
    val isNeighborhood = district != null
    var description = if (isNeighborhood) "Neighborhood" else "City"
    description += " in"
    if (isNeighborhood) {
        description += " $city,"
    }
    province?.let {
        if (it.isNotEmpty()) {
            description += " $it,"
        }
    }
    description += " $country"
    return description
}

print(myLocation.friendlyDescription) // "Neighborhood in Denver, Colorado, United States"

// Custom setter example
enum class SearchResultType {
    HISTORY, SAVED, BASIC
}

private lateinit var resultTypeString: String

var resultType: SearchResultType
    get() {
        return enumValueOf(resultTypeString)
    }
    set(value) {
        resultTypeString = value.toString()
    }

result.resultType = SearchResultType.HISTORY
print(result.resultTypeString) // "HISTORY"

Answer 2

这已经在几个地方得到了回答，但我想我会分享一个具体的例子，供人们从Java /C#/C/C过渡到静态编程语言，他们有和我一样的问题：

我很难理解 getter 和 setters 在 Kotlin 中是如何工作的，特别是因为它们从未被明确调用（就像在 Java 中一样）。正因为如此，我感到不舒服，因为看起来我们只是直接将vars/vals称为字段。因此，我进行了一个小实验来证明事实并非如此，实际上，当您访问变量/值时，它是在 Kotlin 中调用的隐式（自动生成）或显式 getter/setter。不同之处在于，您没有明确要求默认的 getter/setter。

从文档中 - 声明属性的完整语法是：

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

我的例子是

class modifiersEg {

/** this will not compile unless:
 *      - we assign a default here
 *      - init it in the (or all, if multiple) constructor
 *      - insert the lateinit keyword    */
var someNum: Int?
var someStr0: String = "hello"
var someStr1: String = "hello"
    get() = field  // field is actually this.someStr1, and 'this' is your class/obj instance
    set(value) { field = value }

// kotlin actually creates the same setters and getters for someStr0
// as we explicitly created for someStr1

var someStr2: String? = "inital val"
    set(value) { field = "ignore you" }

var someStr3: String = "inital val"
    get() = "you'll never know what this var actually contains"

init {
    someNum = 0

    println(someStr2) // should print "inital val"

    someStr2 = "blah blah blah"
    println(someStr2) // should print "ignore you"

    println(someStr3) // should print "you'll never know what this var actually contains"
}

我希望这能帮助其他人把这一切结合起来？