Swift:返回 self 类型的数组 [英] Swift: return Array of type self

问题描述

我想写一个类函数，它会返回一个类类型的数组.据我所知，我可以使用 Self 作为 objective-c instanceType.我的目标是使用 fetchObjects 方法为 NSManagedObject 创建一个 extension.此方法将返回 NSManagedObject 子类的数组.这是我无法编译的伪代码示例:

I would like to write a class function, which will return an array of the class type. As far as I understood, I can use Self for the objective-c instanceType. My goal is to create an extension for a NSManagedObject with a fetchObjects method. This method will than return an array of NSManagedObject subclasses. Here is a example of my pseudo code which does not compile:

extension NSManagedObject {

    class func fetchObjects(entity: String, context: NSManagedObjectContext, predicate: NSPredicate?, sortDescriptors: NSSortDescriptor[]?) -> Self[] {
        // can't define return type of an array with type Self
        // also var declaration does not work
        var objects : Self[]?

        return objects
    }
}

知道如何定义 Self 类型的数组吗?

Any idea how i can define an array of type Self?

感谢您的帮助！

推荐答案

这是我用于类似功能的 jist，注意它是对 NSManagedObjectContext 而不是 NSManagedObject 的扩展.类似的事情可能可以在 NSManagedObject

This is the jist of what I use for a similar function, note that it's an extension on NSManagedObjectContext rather than NSManagedObject. Something similar could probably be done on NSManagedObject

protocol NamedManagedObject {

    class func entityName() -> String;

}

extension NSManagedObjectContext {

    func fetchObjects<T:NSManagedObject where T:NamedManagedObject>(entity:T.Type, predicate:NSPredicate? = nil, sortDescriptors:NSSortDescriptor[]? = nil) -> T[]? {
        let request = NSFetchRequest(entityName: entity.entityName())

        request.predicate = predicate
        request.sortDescriptors = sortDescriptors

        var error:NSError? = nil
        let results = self.executeFetchRequest(request, error: &error) as? T[]

        assert(error == nil)

        return results
    }

}

extension MyObjectClass : NamedManagedObject {
    class func entityName() -> String {
        return "MyObjectClass"
    }
}

那么使用就这么简单:

let objects = managedObjectContext.fetchObjects(MyObjectClass)

请注意，您还可以通过以下方式为所有 NSManagedObject 实现 NamedManagedObject:

Note that you can also implement NamedManagedObject for all NSManagedObjects with:

extension NSManagedObject : NamedManagedObject {
    class func entityName() -> String {
        return NSStringFromClass(self)
    }
}

如果您还确保所有类都明确指定了 Objective-C 友好名称:

If you also insure that all your classes are explicitly given Objective-C friendly names:

@objc(MyManagedObject)
class MyManagedObject : NSManagedObject { ... }

这篇关于Swift:返回 self 类型的数组的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！