iOS image loader effect

The stuff you are talking about is Progressive JPEG.

Progressive JPEG (PJPEG) is an image format that stores multiple, individual “scans” of a photo, each with an increasing level of detail. When put together, the scans create a full-quality image. The first scan gives a very low-quality representation of the image, and each following scan further increases the level of detail and quality. When images are downloaded using PJPEG, we can render the image as soon as we have the first scan. As later scans come through, we update the image and re-render it at higher and higher quality.

So, you have to make images loadable in this modern progressive format.You can follow a approach where every image before saving on server just convert it into appropriate progressive format.There are several tools avaiable for different type of server. E.g jpegtran

To check whether image is progressive or not you can use this tool or tool2.You can search for online tool a lot of tool is available.

Now for iOS

You can follow this tutorial

Some library
Concorde
DFImageManager

Assuming you are in control of the hosting of the images you can host lower resolution images and make 2 fetches, one for the lower resolution, and one for the higher. If you make the lower resolution image significantly smaller in file size the fetch for it will finish before the fetch for the full image, and you populate the UIImageView with this image until such a time as you can replace it.

Purely example code would look like this:

let imageView = UIImageView()

let lowResOperation = NSBlockOperation {
    guard let imageData = NSData(contentsOfURL: NSURL(string: "https://myhost.org/low-res-image")!),
        image = UIImage(data: imageData) else { return }

    if imageView.image == nil {
        imageView.image = image
    }
}

let highResOperation = NSBlockOperation {
    guard let imageData = NSData(contentsOfURL: NSURL(string: "https://myhost.org/high-res-image")!),
        image = UIImage(data: imageData) else { return }

    imageView.image = image
}

let backgroundQueue = NSOperationQueue()
backgroundQueue.addOperations([lowResOperation, highResOperation], waitUntilFinished: true)