iOS: UIImagePNGRepresentation().writeToFile not writing to intended directory

Question

Using Swift I am trying to download a JPG from a URL and then saving that image to a file, but when I try to save it to a different directory it won't. It will download to the app's Documents folder, but not when I try to set the path to another subfolder.

let dir = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0].stringByAppendingPathComponent("SubDirectory") as String
let filepath = dir.stringByAppendingPathComponent("test.jpg")
UIImagePNGRepresentation(UIImage(data: data)).writeToFile(filepath, atomically: true)

When I run this, it doesn't save the image to it? Why is this happening? Do I need to create the subfolder beforehand?

Answer 1

A couple of thoughts:

1. Does the subdirectory folder already exist? If not, you have to create it first. And it's now advisable to use NSURL instead of path strings. So that yields:

    let filename = "test.jpg" let subfolder = "SubDirectory" do { let fileManager = NSFileManager.defaultManager() let documentsURL = try fileManager.URLForDirectory(.DocumentDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: false) let folderURL = documentsURL.URLByAppendingPathComponent(subfolder) if !folderURL.checkPromisedItemIsReachableAndReturnError(nil) { try fileManager.createDirectoryAtURL(folderURL, withIntermediateDirectories: true, attributes: nil) } let fileURL = folderURL.URLByAppendingPathComponent(filename) try imageData.writeToURL(fileURL, options: .AtomicWrite) } catch { print(error) } 

2. I would advise against converting the NSData to a UIImage , and then converting it back to a NSData . You can just write the original NSData object directly if you want.

   The process of round-tripping this through a UIImage can make lose quality and/or metadata, potentially making the resulting asset larger, etc. One should generally use the original NSData where possible.