Kotlin:检查lazy val是否已初始化

Question

有没有办法判断是否在Kotlin中初始化了懒惰的val而没有在此过程中初始化它？

例如,如果我有一个懒惰的val,查询它是否为null将实例化它

val messageBroker: MessageBroker by lazy { MessageBroker() }
if (messageBroker == null) {
    // oops
}

我可能会使用第二个变量,但这看起来很混乱.

private var isMessageBrokerInstantiated: Boolean = false
val messageBroker: MessageBroker by lazy {
    isMessageBrokerInstantiated = true
    MessageBroker()
}

...

if (!isMessageBrokerInstantiated) {
    // use case
}

是否有一些性感的方式来确定这一点,比如if (Lazy(messageBroker).isInstantiated())？

相关(但不相同):如何检查"lateinit"变量是否已初始化？

Answer 1

有一种方法,但您必须访问由lazy {}以下方式返回的委托对象:

val messageBrokerDelegate = lazy { MessageBroker() }
val messageBroker by messageBrokerDelegate

if(messageBrokerDelegate.isInitialized())
    ...

isInitialized是接口上的公共方法Lazy<T>,这里是文档.

Answer 2

从Kotlin 1.1开始,您可以直接使用属性委托.getDelegate().

您可以为属性引用编写扩展属性,以检查它是否Lazy具有已初始化的委托:

/**
 * Returns true if a lazy property reference has been initialized, or if the property is not lazy.
 */
val KProperty0<*>.isLazyInitialized: Boolean
    get() {
        if (this !is Lazy<*>) return true

        // Prevent IllegalAccessException from JVM access check on private properties.
        val originalAccessLevel = isAccessible
        isAccessible = true
        val isLazyInitialized = (getDelegate() as Lazy<*>).isInitialized()
        // Reset access level.
        isAccessible = originalAccessLevel
        return isLazyInitialized
    }

然后在使用网站:

val messageBroker: MessageBroker by lazy { MessageBroker() }

if (this::messageBroker.isLazyInitialized) {
    // ... do stuff here
}

^{此解决方案需要kotlin-reflect位于类路径上.使用Gradle,使用
compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"}

^{该isAccessible = true部分是必需的.getDelegate(),因为否则它无法访问存储委托引用的私有字段.}

Answer 3

在热键的解决方案的基础上,您可以创建一个isLazyInitialized属性(而不是函数),以与lateinit变量的isInitialized属性保持一致.

此外,没有必要处理null情况.

import kotlin.reflect.KProperty0,
import kotlin.reflect.jvm.isAccessible

val KProperty0<*>.isLazyInitialized: Boolean
    get() {
        // Prevent IllegalAccessException from JVM access check
        isAccessible = true
        return (getDelegate() as Lazy<*>).isInitialized()
    }