Kotlin 中的单例类

发布于08月14日

我想知道如何在Kotlin中创建一个Singleton类，以便我的Util类在每次执行应用程序时只实例化一次它.但是，当我将Java类转换为Kotlin时，生成了以下代码.

Is this correct?

companion object {
    private var utilProject: UtilProject? = null

    val instance: UtilProject
        get() {
            if (utilProject == null) utilProject = UtilProject()
            return utilProject!!
        }
}

我可以找到一个相关的100，但它有参数，我不能让它在没有参数的情况下转换.

推荐答案

Just

companion object {
    val instance = UtilProject()
}

will do the job because the companion object itself is a language-level singleton.
(The instance will be created when the companion object is first called.)

--更新--

If you need to control when the singleton object is initialized, you can create one object for each class.

class UtilProject {
    ....
    companion object {
        val instance = UtilProject()
    }
}

class AnotherClass {
    ...
    companion object {
        val instance = AnotherClass()
        const val abc = "ABC"
    }
}

fun main(args: Array<String>) {
    val a = UtilProject.instance // UtilProject.instance will be initialized here.
    val b = AnotherClass.abc // AnotherClass.instance will be initialized here because AnotherClass's companion object is instantiated.
    val c = AnotherClass.instance
}

在这里，AnotherClass.instance是在实际调用AnotherClass.instance之前初始化的.当AnotherClass的伴生对象被调用时，它被初始化.

class UtilProject {
    ....
    companion object {
        fun f() = ...
    }
}

class AnotherClass {
    ...
    companion object {
        const val abc = "ABC"
    }
}

object UtilProjectSingleton {
    val instance = UtilProject()
}

object AnotherClassSingleton {
    val instance = AnotherClass()
}

fun main(args: Array<String>) {
    UtilProject.f()
    println(AnotherClass.abc)

    val a = UtilProjectSingleton.instance // UtilProjectSingleton.instance will be initialized here.
    val b = AnotherClassSingleton.instance // AnotherClassSingleton.instance will be initialized here.

    val c = UtilProjectSingleton.instance // c is a.
}

如果您不关心每个单例是何时初始化的，您也可以这样使用它:

class UtilProject {
    ....
    companion object {
        fun f() = ...
    }
}

class AnotherClass {
    ...
    companion object {
        const val abc = "ABC"
    }
}

object Singletons {
    val utilProject = UtilProject()
    val anotherClass = AnotherClass()
}

fun main(args: Array<String>) {
    val a = Singletons.utilProject
    val b = Singletons.anotherClass 
}

In summary,
an object or a companion object is one singleton object in Kotlin.
You can assign variables in an object or objects, and then use the variables just like they were singletons.

object或companion object在首次使用时被实例化.