reflection - 在没有实例的情况下访问 Kotlin 委托(delegate)类型

Question

我已阅读 Access property delegate in Kotlin这是关于从实例访问委托(delegate)。可以使用 KProperty::getDelegate从 Kotlin 1.1 开始，但是这将返回委托(delegate)的实例，因此首先需要一个类的实例。

现在我想在没有类实例的情况下获取委托(delegate)的类型。考虑具有自定义委托(delegate)类型的库 CustomDelegate想要获取委托(delegate)给 CustomDelegate 实例的类的所有属性:

class Example
{
    var nonDelegatedProperty = "I don't care about this property"
    var delegatedProperty1 by lazy { "I don't care about this too" }
    var delegatedProperty2 by CustomDelegate("I care about this one")
}

鉴于我有KClass<Example>，我怎么能, 但不是 Example 的实例, 获取委托(delegate)给 CustomDelegate 的所有属性？

Answer 1

How can I, given I have KClass<Example>, but not an instance of Example, get all properties delegated to CustomDelegate?

您可以根据需要以两种方式完成此操作。

首先，您必须在 build.gradle 文件中包含 kotlin-reflect 依赖项:

compile "org.jetbrains.kotlin:kotlin-reflect:1.1.51"

在我看来，如果可以的话，你应该使用第一个解决方案，因为它是最清晰和优化的。相反，第二个解决方案可以处理第一个解决方案无法处理的情况。

首先

您可以循环声明的属性并检查属性的类型或委托(delegate)的类型是否为 CustomDelegate。

// Loop on the properties of this class.
Example::class.declaredMemberProperties.filter { property ->
    // If the type of field is CustomDelegate or the delegate is an instance of CustomDelegate,
    // it will return true.
    CustomDelegate::class.java == property.javaField?.type
}

此解决方案只有一个问题，您还将获得类型为 CustomDelegate 的字段，因此，在这个示例中:

class Example {
    var nonDelegatedProperty = "I don't care about this property"
    val delegatedProperty1 by lazy { "I don't care about this too" }
    val delegatedProperty2 by CustomDelegate("I care about this one")
    val customDelegate = CustomDelegate("jdo")
}

您将获得 delegatedProperty2 和 customDelegate。如果您只想获得 delegatedProperty2，我发现了一个可怕的解决方案，如果您需要管理这种情况，您可以使用它。

第二

如果您查看KPropertyImpl 的源代码，您可以看到委托(delegate)是如何实现的。所以，你可以这样做:

// Loop on the properties of this class.
Example::class.declaredMemberProperties.filter { property ->
    // You must check in all superclasses till you find the right method.
    property::class.allSuperclasses.find {
        val computeField = try {
            // Find the protected method "computeDelegateField".
            it.declaredFunctions.find { it.name == "computeDelegateField" } ?: return@find false
        } catch (t: Throwable) {
            // Catch KotlinReflectionInternalError.
            return@find false
        }

        // Get the delegate or null if the delegate is not present.
        val delegateField = computeField.call(property) as? Field
        // If the delegate was null or the type is different from CustomDelegate, it will return false.
        CustomDelegate::class.java == delegateField?.type
    } != null
}

在这种情况下，您只会得到 delegatedProperty2 作为结果。