Performance depends wether or not the data fits in RAM. If it does, then you should use NSData writeToURL with the atomically feature turned on, which is what you’re doing.
Apple’s notes about this being dangerous when “writing to a public directory” are completely irrelevant on iOS because there are no public directories. That section only applies to OS X. And frankly it’s not really important there either.
So, the code you’ve written is as efficient as possible as long as the video fits in RAM (about 100MB would be a safe limit).
For files that don’t fit in RAM, you need to use a stream or your app will crash while holding the video in memory. To download a large video from a server and write it to disk, you should use NSURLSessionDownloadTask.
In general, streaming (including NSURLSessionDownloadTask) will be orders of magnitude slower than NSData.writeToURL(). So don’t use a stream unless you need to. All operations on NSData are extremely fast, it is perfectly capable of dealing with files that are multiple terabytes in size with excellent performance on OS X (iOS obviously can’t have files that large, but it’s the same class with the same performance).
There are a few issues in your code.
This is wrong:
let filePath = NSTemporaryDirectory() + named
Instead always do:
let filePath = NSTemporaryDirectory().stringByAppendingPathComponent(named)
But that’s not ideal either, you should avoid using paths (they are buggy and slow). Instead use a URL like this:
let tmpDir = NSURL(fileURLWithPath: NSTemporaryDirectory())!
let fileURL = tmpDir.URLByAppendingPathComponent(named)
Also, you’re using a path to check if the file exists… don’t do this:
if NSFileManager.defaultManager().fileExistsAtPath( filePath ) {
Instead use NSURL to check if it exists:
if fileURL.checkResourceIsReachableAndReturnError(nil) {