Swift write to file (iOS)

Question

How can I write to an xml document in Swift? I have this code:

let content = "<tag>Example</tag>"
var error: NSError?
content.writeToFile(NSBundle.mainBundle().pathForResource("my_file", ofType: "xml")!, atomically: true, encoding: NSUTF8StringEncoding, error: &error)

But this does not update the file in my Xcode project or on the device. How can I get this to work?

As far as I know, bundles are read-only. You need to use a path in your app's doc or tmp folder. — Sergey Kalinichenko, Apr 29 '15 at 20:16
@GabrielJones [This Q&A](http://stackoverflow.com/a/24098149/335858) has an example. — Sergey Kalinichenko, Apr 29 '15 at 20:17
@dasblinkenlight And this would apply to iOS directories as well? — brimstone, Apr 29 '15 at 20:18
Yes. Usually you’d use NSSearchPathDomainMask.UserDomainMask instead of .AllDomainsMask, but either should work. — Noah Witherspoon, Apr 29 '15 at 20:20
@NoahWitherspoon Is there a way I could replace the whole document with new content? — brimstone, Apr 29 '15 at 20:30
Should be happening already—IIRC you have to do some additional work to append to a file instead of just overwriting it. Is that not what you’re seeing? — Noah Witherspoon, Apr 29 '15 at 20:39

Duncan C · Accepted Answer · 2015-04-29T21:00:24.307

To collect all the fragments provided in comments into one place:

The app bundle is read-only on iOS. You can't save to it. It isn't forced to be read-only on Mac OS, but you should still treat it as such. As a result of this difference, code that modifies the app bundle may work on the simulator, but fail on an actual device.

You should save your contents to a file in the user's documents directory or one of the other sandbox directories. That code might look like this:

let docsPath = NSSearchPathForDirectoriesInDomains( 
  NSSearchPathDirectory.DocumentDirectory,
  NSSearchPathDomainMask.UserDomainMask,
  true)[0] as! NSString
let filePath = docsPath.stringByAppendingPathComponent("myfile")
let content = "<tag>Example</tag>"
var error: NSError?
content.writeToFile(filePath, 
  ofType: "xml", 
  atomically: true, 
  encoding: NSUTF8StringEncoding, 
  error: &error)

Then you should add error-checking that checks the bool result of the writeToFile command. If the result is false, check the error to see what went wrong.

Note that there is built-in support in iOS for reading XML, and quite a few third party libraries for saving collections of objects in XML format. You should take a look at NSXMLParser for reading. I can't really recommend one of the XML writing libraries over the others.

If you aren't wedded to XML, you might take a look at JSON. It's more compact, easier to write, easier to read, less memory-intensive, and there is built-in system support for both reading and writing Cocoa data structures from/to JSON files.

Swift write to file (iOS)

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