There's already a lot of stuff about image resizing on iOS already available on the web with each discussing about potential pros and cons of the technique. There is also a good post on NSHipster summarizing some of the best available image resizing techniques.

With iOS 8, Apple has given us PhotoKit, a modern framework for managing photos without diving into the low level details and more performant than AssetsLibrary. Let's see how we can resize a simple image selected using the UIImagePickerController using PhotoKit.

imagePickerController.bk_didFinishPickingMediaBlock = {  
    (controller imagePickerController: UIImagePickerController!, dictionary: [NSObject:AnyObject]!) -> Void in
    imagePickerController.dismissViewControllerAnimated(true, completion: nil)

    guard let assetURL = dictionary[UIImagePickerControllerReferenceURL] as? NSURL else {
        if let image = dictionary[UIImagePickerControllerOriginalImage] as? UIImage {
            // TODO Image taken from the camera. Resize using one of the techniques discussed on http://nshipster.com/image-resizing/
        }
        return
    }

    // Fetch Photos Asset and resize image
    let fetchResult = PHAsset.fetchAssetsWithALAssetURLs([assetURL], options: nil)
    let options = PHImageRequestOptions()
    options.synchronous = false
    options.resizeMode = .Fast
    options.deliveryMode = .HighQualityFormat

    guard let asset = fetchResult.firstObject as? PHAsset else {
        if let image = dictionary[UIImagePickerControllerOriginalImage] as? UIImage {
            // TODO Cannot find the associated PhotoKit asset. Resize using one of the techniques discussed on http://nshipster.com/image-resizing/
        }
        return
    }

    // Request resized image
    PHImageManager.defaultManager().requestImageForAsset(asset, targetSize: CGSize(width: 1024, height: 1024), contentMode: .AspectFit, options: options, resultHandler: { (image, _) -> Void in
        // image is the resized image
    })
}

The code is pretty straightforward. We fetch the PHAsset from the asset url of the image and then use the PHImageManager to get the resized image. There are a few options to control the scaling mode.

  • deliveryMode : Controls the requested image quality and delivery priority.
    • Opportunistic: Photos automatically provides one or more results in order to balance image quality and responsiveness. The resultHandler block more than once with different quality images as they become available.
    • HighQualityFormat: Photos provides only the highest-quality image available, regardless of how much time it takes to load.
    • FastFormat: Photos provides only a fast-loading image, possibly sacrificing image quality.
  • resizeMode: A mode that specifies how to resize the requested image.
    • None: Photos does not resize the image asset.
    • Fast: Photos efficiently resizes the image to a size similar to, or slightly larger than, the target size.
    • Exact: Photos resizes the image to match the target size exactly.