将 NSManagedObject 转换为子类返回 nil

Posted 2023-03-05

【中文标题】将 NSManagedObject 转换为子类返回 nil

【英文标题】：Casting NSManagedObject to subclass returns nil

【发布时间】：2015-01-14 16:18:35

【问题描述】：

当我在 Swift 中使用 RestKit 0.23.0 时尝试将 NSManagedObject 强制转换为我的模型类 (SCUser) 时，返回值为 nil。操作如下：

var urlString:String = SCApi.sharedInstance.apiUrl + "users/find_or_create?name=\(name)&email=\(email)"
var url = NSURL(string: urlString)!
var request = NSURLRequest(URL: url)

var responseDescriptor = RKResponseDescriptor(mapping: SCUser.entityMapping(), method: RKRequestMethod.Any, pathPattern: nil, keyPath: "user", statusCodes: nil)

var operation = RKManagedObjectRequestOperation(request: request, responseDescriptors: [responseDescriptor])
operation.managedObjectContext = RKManagedObjectStore.defaultStore().mainQueueManagedObjectContext
operation.managedObjectCache = RKManagedObjectStore.defaultStore().managedObjectCache

operation.setCompletionBlockWithSuccess(
    
        (operation:RKObjectRequestOperation!, result:RKMappingResult!) -> Void in

        println("result: \(result.firstObject())")
        var object:NSManagedObject = result.firstObject() as NSManagedObject
        if let user = (object as? SCUser) 
            println("success: \(user)")
         else 
            println("failure")
        
    ,
    failure: 
        (operation:RKObjectRequestOperation!, error:NSError!) -> Void in
        println("error: \(error)")
    
)
operation.start()

当它运行时，它表明result.firstObject() 是一个 SCUser，但不会将其转换为一个：

result: <NSManagedObject: 0x1740aee20> (entity: SCUser; id: 0xd000000000140000 <x-coredata://7CFB18B0-0E2D-4190-B8DF-20470425CFE3/SCUser/p5> ; data: 
    authenticationToken = Dt7tjbLtUzSAgBdzCyC3;
    name = g;
    email = "[e-mail]";
    userId = "9e99451d-ef9d-4876-bfd3-4a987c2fe316";
)
failure

我已将 SCUser 类和 RestKit 初始化 here 放入，以防万一。

【参考方案1】：

这个日志输出

result: <NSManagedObject: 0x1740aee20> (entity: SCUser; id: 0xd000000000140000 <x-coredata://7CFB18B0-0E2D-4190-B8DF-20470425CFE3/SCUser/p5> ; data: 
    authenticationToken = Dt7tjbLtUzSAgBdzCyC3;
    name = g;
    email = "[e-mail]";
    userId = "9e99451d-ef9d-4876-bfd3-4a987c2fe316";
)

显示该类实际上是NSManagedObject，即使实体是SCUser。这很可能是因为您没有在模型中设置类名，并且大概是您手动生成了SCUser 类代码文件？您需要设置类名（最好使用 Xcode 或 mogenerator 自动生成类文件）。