将请求函数转换为泛型类型 [英] Convert request Function to Generic type

问题描述

我正在尝试将我的以下获取请求代码从核心数据转换为通用类型.

I am trying to convert my below fetch request code from core data to generic type.

let request = NSPredicate(format: "name == %@ AND password == %@ AND type == %@", "admin", "admin", "admin")
let fetchReq : NSFetchRequest = UserRegistration.fetchRequest()
fetchReq.predicate = request
let adminDetail :[UserRegistration] = DatabaseEngine.fetch(fetchRequest: fetchReq)!

到目前为止已转换:

extension UIViewController{
    class func getData<T: NSManagedObject>(req: NSPredicate) -> T{
        let fetchReq : NSFetchRequest = T.fetchRequest()
        fetchReq.predicate = req
        return DatabaseEngine.fetch(fetchRequest: fetchReq as! NSFetchRequest<NSManagedObject>)! as! T

    }
}

<小时>

DatabaseEngine.fetch 函数.

---

DatabaseEngine.fetch function.

static func fetch (fetchRequest: NSFetchRequest<T> = NSFetchRequest(), context:NSManagedObjectContext = kApplicationDelegate.managedObjectContext) -> [T]? {

    let entity =  NSEntityDescription.entity(forEntityName: typeName(some:T.self)
        , in:context)
    // Configure Fetch Request
    fetchRequest.entity = entity
    do {
        return try  context.fetch(fetchRequest as! NSFetchRequest<NSFetchRequestResult>) as? [T]
    } catch {

        //let fetchError = error as NSError
        // return nil

    }
    return nil
}

但是没有结果了.任何人都可以用几行解释来帮助我转换此代码.Ans 将不胜感激.

But no results any more. Anybody help me to convert this code with few explaining lines. Ans will be appreciated sure.

推荐答案

根据我的评论，我建议使用例如带有扩展名的协议

According to my comment I recommend to use a protocol with extension for example

protocol Fetchable
{
    associatedtype FetchableType: NSManagedObject = Self

    static var entityName : String { get }
    static var managedObjectContext : NSManagedObjectContext { get }
    static func objects(for predicate: NSPredicate?) throws -> [FetchableType]
}

extension Fetchable where Self : NSManagedObject, FetchableType == Self
{
    static var entityName : String {
        return NSStringFromClass(self).components(separatedBy: ".").last!
    }

    static var managedObjectContext : NSManagedObjectContext {
        return (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    }

    static func objects(for predicate: NSPredicate?) throws -> [FetchableType]
    {
        let request = NSFetchRequest<FetchableType>(entityName: entityName)
        request.predicate = predicate
        return try managedObjectContext.fetch(request)
    }
}

将 (UIApplication.shared.delegate as!AppDelegate).persistentContainer.viewContext 更改为对托管对象上下文的引用.

Change (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext to the reference to your managed object context.

使所有 NSManagedObject 子类都采用 Fetchable.子类中不需要额外的代码.

Make all NSManagedObject subclasses adopt Fetchable. There is no extra code needed in the subclasses.

现在你可以用

do {
   let predicate = NSPredicate(format: ...
   let objects = try MyEntity.objects(for: predicate)
} catch {
    print(error)
}

就是这样，objects 是 [MyEntity] 没有任何类型转换，并且在成功时总是非可选的.

That's all, objects are [MyEntity] without any type casting and always non-optional on success.

该协议可以通过默认的排序描述符、排序方向等轻松扩展

The protocol is easily extendable by default sorting descriptors, sorting directions etc.

这篇关于将请求函数转换为泛型类型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！