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How to save a UIImage to a file using UIImagePNGRepresentation

Written by Paul Hudson    @twostraws

If you've generated an image using Core Graphics, or perhaps rendered part of your layout, you might want to save that out as either a PNG or a JPEG. Both are easy thanks to two functions: UIImagePNGRepresentation() and UIImageJPEGRepresentation(), both of which convert a UIImage into a Data so you can write it out.

Here's an example:

if let image = UIImage(named: "example.png") {
    if let data = UIImagePNGRepresentation(image) {
        let filename = getDocumentsDirectory().appendingPathComponent("copy.png")
        try? data.write(to: filename)
    }
}

That call to getDocumentsDirectory() is a little helper function I include in most of my projects, because it makes it easy to locate the user's documents directory where you can save app files. Here it is:

func getDocumentsDirectory() -> URL {
    let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
    return paths[0]
}

If you want to save your image as a JPEG rather than a PNG, use this code instead:

if let image = UIImage(named: "example.png") {
    if let data = UIImageJPEGRepresentation(image, 0.8) {
        let filename = getDocumentsDirectory().appendingPathComponent("copy.png")
        try? data.write(to: filename)
    }
}

The second parameter to UIImageJPEGRepresentation() is a float that represents JPEG quality, where 1.0 is highest and 0.0 is lowest.

Available from iOS 2.0 – see Hacking with Swift tutorial 10

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