无法使用类型为（（NSFetchRequest <NSFetchRequestResult> ）”

Question

I have created a helper function to load NSManagedObject s using Swift 3, which supports iOS 9.0+.

class func loadContext(entityName: String, fetchConfiguration: ((NSFetchRequest<NSManagedObject>) -> Void)?) -> AnyObject? {
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    // Fetch requested data
    let dataFetchRequest = NSFetchRequest<NSManagedObject>(entityName: entityName)

    // Configure the fetch request with user parameters
    if fetchConfiguration != nil {
        fetchConfiguration!(dataFetchRequest)
    }

    do {
        return try appDelegate.managedObjectContext.fetch(dataFetchRequest as! NSFetchRequest<NSFetchRequestResult>)
    } catch {
        print("Failed to fetch feed data, critical error: \(error)")
    }

    return nil
}

However I get the following error:

Cannot invoke 'fetch' with an argument list of type '(NSFetchRequest<NSFetchRequestResult>)'

I have also tried several variants, such as having let dataFetchRequest = NSFetchRequest<NSFechRequestResult>(entityName: entityName) and they all run into the same error.

Answer 1

This has been answered on my question Swift 3/XCode 8 NSManagedObjectContext.fetch error .

The error message XCode is providing is incorrect. The correct fix is to cast your return and your method correctly.

managedObjectContext.fetch() returns [Any] . There are three changes you need to make.

1. class func loadContext(entityName: String, fetchConfiguration: ((NSFetchRequest<NSManagedObject>) -> Void)?) -> [NSManagedObject]? {
2. let dataFetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: entityName)
3. return try appDelegate.managedObjectContext.fetch(dataFetchRequest) as? [NSManagedObject]

I've submitted a bug report to Apple - https://openradar.appspot.com/radar?id=5049582866137088

Answer 2

The changes were a lot simpler, it seems as though xcode wrongfully inferences the "AnyObject" type when converting code from swift-2 to 3.0.

For reference this is the fixed function from @dmorrow's solution.

class func loadContext(entityName: String, fetchConfiguration: ((NSFetchRequest<NSManagedObject>) -> Void)?) -> [NSManagedObject]? {
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    // Fetch requested data
    let dataFetchRequest = NSFetchRequest<NSManagedObject>(entityName: entityName)

    // Configure the fetch request with user parameters
    fetchConfiguration?(dataFetchRequest)

    do {
        return try appDelegate.managedObjectContext.fetch(dataFetchRequest)
    } catch {
        print("Failed to fetch feed data, critical error: \(error)")
    }

    return nil
}