iOS Swift CoreData图像数据的异步下载

Question

As someone who is quite new to Swift and CoreData, I am sure I am going about this the wrong way and am hoping someone might be able to help.

Background: I am downloading JSON over an API, and caching the content locally on an iPad in CoreData. As part of the process, I need to download a small image thumbnail, which I am also storing in CoreData (as a Transformable ).

I was doing: My original implementation downloaded images and saved them to CoreData, but although it is being triggered from a background thread (a callback following the API call), the actual downloading of the images seems to cause the app to hang:

let data = NSData(contentsOfURL: imgURL)
if let imageData = data {
    coreDataEntity.thumbnail = imageData
}

What I am now doing: I have since updated my code to the following:

func downloadImage(url: NSURL, cdEntity: CoreDataEntity, moc: NSManagedObjectContext, handler: ((image: UIImage?, cdEntity: CoreDataEntity, moc: NSManagedObjectContext, NSError!) -> Void))
{
    var imageRequest: NSURLRequest = NSURLRequest(URL: url)
    NSURLConnection.sendAsynchronousRequest(imageRequest,
        queue: NSOperationQueue.mainQueue(),
        completionHandler: {response, data, error in
             handler(image: UIImage(data: data), cdEntity: cdEntity, moc: moc, error)
    })
}

func setImage(image: UIImage?, cdEntity: CoreDataEntity, moc: NSManagedObjectContext, error: NSError?) {
    cdEntity.thumbnail = image
    save(moc)
}

I then call the code thus:

downloadImage(imgURL, coreDataEntity, moc, { (image, cdEntity, moc, error) -> Void in
    setImage(image, cdEntity, moc, error)
})

Doing this, I don't get any blocks on the main thread that I notice. Inspecting the sqlite file from the device, it looks as though the data is getting set properly, but the images are not showing up in the UI.

Is my method completely wrong? If not, how can I inform the UI that the core data model has been updated in such a way that will cause a refresh? What function can be called to update a specific row/cell of a table/uicollectionview?

Answer 1

1. Your NSURLConnection's callbacks are happening on the main queue. This is one reason your application is locking up.

2. Your managed object context, at least in the code you have posted, is not following the Core Data concurrency rules. You have a choice between thread confinement (unfortunately, the default, which is obsolete), and queue confinement.

With thread confinement, you can only use a managed object context from the thread where it was created. If you created your managed object context on the main thread (or queue), you can only use it from there - so any Core Data operation you perform on that context will block the main thread, and in turn the UI.

Using queue confinement (almost) any operation on the managed object context must go through either the performBlock: or performBlockAndWait: methods. The enqueued block is executed on the context's serial queue. This is far less failure prone than thread confinement and has been the recommended practice for concurrency since iOS 5.

If not, how can I inform the UI that the core data model has been updated in such a way that will cause a refresh? What function can be called to update a specific row/cell of a table/uicollectionview?

Typically this would be done using an NSFetchedResultsController , which observes a managed object context for changes relevant to it's fetch request. Once the initial fetch populates the controller it will listen for changes to the context that affect the objects specified by the fetch request. When relevant changes occur the controller informs it's delegate through callbacks.