iOS developers can use GCD (Grand Central Dispatch) to monitor a folder for changes.
There is no notification to be gotten if the user adds or removes files to your app’s documents folder. The only way to update your list of files in that case is to monitor the folder for changes. There are several different approaches to achieve this, the traditional one being the File System Events API.
But since iOS 4 – together with GCD – Apple added a simpler method for monitoring a vnode, dispatch sources.
If you’re on a jailbroken device and want to monitor all filesystem changes, check out filemon for iOS.