NSSearchPathForDirectoriesInDomains可在模拟器上运行，但不能在设备上运行

Question

I'm trying to save, and later load, an image from disk. The following works on the simulator, but when I test on the device, I get unexpectedly found nil while unwrapping an Optional value .

Saving image to disk:

let sketchData: NSData = UIImagePNGRepresentation(sketch)
let destinationFolder: String = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as String
let sketchPath: String = "\(destinationFolder)\(currentProjectID).png"
defaults.setObject(sketchPath, forKey: "sketchpath")
sketchData.writeToFile(sketchPath, atomically: true)

Loading image from disk:

let sketchPath: String = defaults.objectForKey("sketchpath") as String
let image = UIImage(contentsOfFile: sketchPath)

Here's an example path. It's identical in saving and loading, so the issue is not trying to load from a different file path.

/var/mobile/Containers/Data/Application/72EDBC3D-F9D8-4108-8A64-751A10FF1E71/Documents4.png

Does saving to an actual device require a different file path?

Answer 1

You just forgot to add "/" between the folder and the file's name:

let sketchPath: String = "\(destinationFolder)/\(currentProjectID).png"

You should use stringByAppendingPathComponent and stringByAppendingPathExtension:

let currentProjectID = "Test"
// you can also use NSFileManager's method URLsForDirectory to find the device's DocumentDirectory
let destinationFolder =  (NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first as NSURL).path!
let sketchPath = destinationFolder.stringByAppendingPathComponent(currentProjectID).stringByAppendingPathExtension("png")!

You are also saving it as NSData so you should load it as NSData

let imageData: NSData = UIImagePNGRepresentation(UIImage(data: NSData(contentsOfURL: NSURL(string:"http://i.stack.imgur.com/Xs4RX.jpg")!)! ))

imageData.writeToFile(sketchPath, atomically: true)

let loadedImage = UIImage(data: NSData(contentsOfFile: sketchPath)! )!!