带有异步代码的UInavigationController中的弱无用用法

Question

I have a classic master detail view application. In detail view, I added pull to refresh capability to refresh data from network. Even though I can't replicate on my end, I get crash reports related to this refresh function. I tried to throttle my connection, refreshed many times, go back and forth between view controllers, still couldn't reproduce it. I think I should use weak or unowned self in here but I am not sure which one to choose. User is a Core Data object that I pass from Master View. My crash reports indicate during write operation to User, program crashes. Crashlytics reports that User.detail.count is 0 which seems to indicate that User.detail is deallocated? I appreciate if you can help me to solve this.

class User:NSManagedObject {
    @NSManaged var detail: NSOrderedSet
}

class UserDetail:NSManagedObject {
    @NSManaged var message:String?

}
class APIService {

    func getData(date:NSDate?,success: (User) -> Void, failure: (NSHTTPURLResponse?, AnyObject?, ErrorType) -> Void) {

    }
}

class ViewController {
var myUser:User!
typealias UserCompletion = (error: ErrorType?) -> Void

    func deleteDetails() {

    }

    func updateDetails() {

    }

func refreshList(completion: UserCompletion) {
    let downloadGroup = dispatch_group_create()
    var storedError: ErrorType?


    let service = APIService()
    dispatch_group_enter(downloadGroup)

    service.getData(nil,
        success: { user in
            var isitNew = false

            //first check if the amount of details changed.
            if user.detail.count == self.myUser.detail.count {
                let oldDetail = self.myUser.detail.lastObject as! UserDetail
                let newDetail = user.detail.lastObject as! UserDetail
                if oldDetail.message != newDetail.message {

                    isitNew = true
                }

            }
            if isitNew || (user.detail.count > 0 && user.detail.count != self.myUser.detail.count) { //something is changed

                //Following log function logs detail count as 0 "Something is new count: 0"
                //CLSLogv("Something is new count: %d", getVaList([self.myUser.detail.count]))

                //Crash after this log
                // delete old details and update
                self.deleteDetails()
                self.updateDetails()

            }
            dispatch_group_leave(downloadGroup)

        },
        failure: { response, document, error in
            storedError = error
            dispatch_group_leave(downloadGroup)

    })
    // Exit from group
    dispatch_group_notify(downloadGroup, dispatch_get_main_queue()) { // 2
        completion(error: storedError)
        }
    }
}

Answer 1

EDIT

Oh darn - I think I misread your post a bit (having had the same issue as the one I originally answered). Use [weak self] in the closure to get a weak reference:

success: { [weak self] user in
        var isitNew = false

        //first check if the amount of details changed.
        if user.detail.count == self?.myUser.detail.count {
            let oldDetail = self?.myUser.detail.lastObject as! UserDetail
            let newDetail = user.detail.lastObject as! UserDetail
            if oldDetail.message != newDetail.message {

                isitNew = true
            }

        }
        if isitNew || (user.detail.count > 0 && user.detail.count != self?.myUser.detail.count) { //something is changed

            //Following log function logs detail count as 0 "Something is new count: 0"
            //CLSLogv("Something is new count: %d", getVaList([self?.myUser.detail.count]))

            //Crash after this log
            // delete old details and update
            self?.deleteDetails()
            self?.updateDetails()

        }
        dispatch_group_leave(downloadGroup)

    }

ORIGINAL RESPONSE (leaving this in for now - might still be the cause of your crash)

I think your problem might be that the detail view controller is keeping a reference to an NSManagedObject whose underlying data is removed from the datastore by the network call ( similar issue )

One suggestion is to not keep a reference to the NSManagedObject itself in the detail but rather keep the key properties that you are comparing with.

Another idea would be to let your getData return the "isitNew" value. That function would know if a new core data entry was created or not.

But I am keeping the best solution (in my opinion) to last. You can use an NSFetchedResultsController to detect changes. Set your detail-view up as an NSFetchedResultsControllerDelegate . The apple documentation actually contains a good example of how to do this.

The advantage of the last solution is that you can get rid of the closure and decouple the detail from whatever changes the data. This may be useful one day when you need to expand the app and another source of changes are introduced. This source might not know of your detail view controller and possibly shouldn't know of it either.