如何通过扩展函数编写自己的

How to write my own by extension function

我有一个这样的默认函数:

fun <T> makeDefault(): Animal<T> = Animal<Nothing>(
    name = "",
    size = 0,
    age = 0
)

我看到有 by 运算符,可以像这样用于视图模型:

val model: MyViewModel by viewModels() 

问题:如何为我的makeDefault()创建一个行为类似的函数?

我想做的事情:

val animal: Animal<Dog> by makeDefault()

您可以创建自己的委托,而不是扩展:

import kotlin.reflect.KProperty

class DefaultDelegate<T> {
    private var created: Animal<T> = Animal<Nothing>(
            name = "",
            size = 0,
            age = 0
        )

    operator fun getValue(thisRef: Any?, property: KProperty<*>): Animal<T> {
        return created
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Animal<T>) {
        created = value
    }
}

然后使用它:

val animal: Animal<Dog> by DefaultDelegate()

有关详细信息,请参阅 Delegated properties doc