Why can't I append a String to a NSURL? - swift

Question

Appending the .txt file component to the URL path doesn't work:
var error:NSError?
let manager = NSFileManager.defaultManager()
let docURL = manager.URLForDirectory(.DocumentDirectory, inDomain:.UserDomainMask, appropriateForURL:nil, create:true, error:&error)
docURL.URLByAppendingPathComponent("/RicFile.txt") <-- doesn't work
via debugger:
file:///Users/Ric/Library/Developer/CoreSimulator/Devices/
<device id>/data/Containers/Data/Application/<app id>/Documents/
Writing a String using docURL to a file doesn't work because of the missing file name.
Reason (via error):
"The operation couldn’t be completed. Is a directory"
So Question: Why doesn't the following work?
docURL.URLByAppendingPathComponent("/RicFile.txt")

Answer 1

URLByAppendingPathComponent: doesn't mutate the existing NSURL, it creates a new one. From the documentation:
URLByAppendingPathComponent: Returns a new URL made by appending a
path component to the original URL.
You'll need to assign the return value of the method to something. For example:
let directoryURL = manager.URLForDirectory(.DocumentDirectory, inDomain:.UserDomainMask, appropriateForURL:nil, create:true, error:&error)
let docURL = directoryURL.URLByAppendingPathComponent("/RicFile.txt")
Even better would be to use NSURL(string:String, relativeTo:NSURL):
let docURL = NSURL(string:"RicFile.txt", relativeTo:directoryURL)

Answer 2

With the update to the Swift language, the suggested call to manager.URLForDirectory(...) no longer works because the call can throw (an exception). The specific error is "Call can throw, but it is not marked with 'try' and the error is not handled". The throw can be handled with the following code:
let directoryURL: NSURL?
do
{
directoryURL = try manager.URLForDirectory(.DocumentationDirectory,
inDomain: .UserDomainMask, appropriateForURL: nil, create: true)
}
catch _
{
print("Error: call to manager.URLForDirectory(...) threw an exception")
}