如何在NSManagedObject Swift扩展中创建托管对象子类的实例？ [英] How can I create instances of managed object subclasses in a NSManagedObject Swift extension?

问题描述

在创建 NSManagedObject 的扩展帮助以创建新的托管对象子类时，swift提供 Self 类型mimic instancetype 这是伟大的，但我似乎不能类型从 AnyObject 。以下代码不会编译时出错AnyObject不能转换为自我

When creating an extension helper to NSManagedObject to create a new managed object subclass, swift provides the Self type to mimic instancetype which is great, but i can't seem to typecast from AnyObject. The below code does not compile with error 'AnyObject' is not convertible to 'Self'

帮助？

extension NSManagedObject
{
    class func createInContext(context:NSManagedObjectContext) -> Self {
        var classname = className()
        var object: AnyObject = NSEntityDescription.insertNewObjectForEntityForName(classname, inManagedObjectContext: context)
        return object
    }


    class func className() -> String {
        let classString = NSStringFromClass(self)
        //Remove Swift module name
        let range = classString.rangeOfString(".", options: NSStringCompareOptions.CaseInsensitiveSearch, range: Range<String.Index>(start:classString.startIndex, end: classString.endIndex), locale: nil)
        return classString.substringFromIndex(range!.endIndex)
    }

}

推荐答案

诀窍是使用一个泛型助手方法， self
。您的 className（）方法也可以稍微简化，更好的名称可以通过 entityName（）：

The trick is to use a generic helper method which infers the type of self from the context. Your className() method can also be simplified slightly, and a better name might by entityName():

extension NSManagedObject
{
    class func createInContext(context:NSManagedObjectContext) -> Self {
        return createInContextHelper(context)
    }

    private class func createInContextHelper<T>(context:NSManagedObjectContext) -> T {
        let classname = entityName()
        let object = NSEntityDescription.insertNewObjectForEntityForName(classname, inManagedObjectContext: context) as! T
        return object
    }

    class func entityName() -> String {
        let classString = NSStringFromClass(self)
        // The entity is the last component of dot-separated class name:
        let components = split(classString, { $0 == "." })
        return components.last ?? classString
    }
}

然后

let obj = YourEntity.createInContext(context)

工程，编译器将 obj 的类型正确推荐为 YourEntity 。

works and the compiler infers the type of obj correctly as YourEntity.

更新：使用如何使用泛型类型获取相同类型的对象，这也可以使用全局可重用函数而不是helper方法将返回值转换为适当的类型：

Update: Using the ideas from How to use generic types to get object with same type, this can also be done with a global reusable function instead of the helper method to cast the return value to the appropriate type:

func objcast<T>(obj: AnyObject) -> T {
    return obj as T
}

extension NSManagedObject
{
    class func createInContext(context:NSManagedObjectContext) -> Self {
        let classname = entityName()
        let object: AnyObject = NSEntityDescription.insertNewObjectForEntityForName(classname, inManagedObjectContext: context)
        return objcast(object)
    }

    class func entityName() -> String {
        let classString = NSStringFromClass(self)
        // The entity is the last component of dot-separated class name:
        let components = split(classString, { $0 == "." })
        return components.last ?? classString
    }
}

---

更新 Swift 1.2 / Xcode 6.3 和 Swift 2 / Xcode 7：

func objcast<T>(obj: AnyObject) -> T {
    return obj as! T
}

extension NSManagedObject
{
    class func createInContext(context:NSManagedObjectContext) -> Self {
        let classname = entityName()
        let object: AnyObject = NSEntityDescription.insertNewObjectForEntityForName(classname, inManagedObjectContext: context)
        return objcast(object)
    }

    class func entityName() -> String {
        let classString = NSStringFromClass(self)
        // The entity is the last component of dot-separated class name:
        let components = classString.componentsSeparatedByString(".")
        return components.last ?? classString
    }
}

或 unsafeBitCast 而不是注释中建议的辅助方法：

Or with unsafeBitCast instead of a helper method, as suggested in the comments:

extension NSManagedObject
{
    class func createInContext(context:NSManagedObjectContext) -> Self {
        let classname = entityName()
        let object: AnyObject = NSEntityDescription.insertNewObjectForEntityForName(classname, inManagedObjectContext: context)
        return unsafeBitCast(object, self)
    }

    class func entityName() -> String {
        let classString = NSStringFromClass(self)
        // The entity is the last component of dot-separated class name:
        let components = classString.componentsSeparatedByString(".")
        return components.last ?? classString
    }
}

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