Today we'll look at a new library from Krzysztof Zabłocki called KZFileWatchers. It allows us to easily monitor local and remote files, running code when changes occur. Let's dive in.

Watching files for changes is a common technique used when creating "live" updating interfaces.

This is a popular technique among web developers. For example, it's now common to use a tool to "live reload" a web page whenever a CSS file is changed.

Live updating techniques can dramatically change how we work, allowing us to try out changes more rapidly.

KZFileWatchers helps us build this type of functionality by providing two simple tools:

FileWatcher.Local is for observing local file changes. (It can even cross sandbox boundaries for debug simulator builds, think "config files on the desktop").

FileWatcher.Remote is for observe remote file changes. We can place a file on a server somewhere, and KZFileWatchers will use the Etag headers and Last-Modified-Date. This even works with Dropbox. Neat.

Let's use a FileWatcher.Local to make a live updating label.

First we'll create a label:

let textLabel = UILabel()

Then we'll create our file if it doesn't already exist:

let filename = "label-content.txt"
let middle = FileWatcher.Local.simulatorOwnerUsername()
let path = "/Users/\(middle)/Desktop/\(filename)"

if FileManager.default.fileExists(atPath: path) == false {
  try! "Hello, world."
    .data(using: String.Encoding.utf8)?
    .write(to: NSURL.fileURL(withPath: path))
}

Finally, we'll wire up a watcher to our file's path, and update our label when the file changes:

let watcher = FileWatcher.Local(path: path)

try! watcher?.start {
  switch $0 {
  case .updated(let data):
    textLabel.text = String(
      data: data,
      encoding: String.Encoding.utf8
    )
  default: break
  }
}

We can use this technique to drive all sorts of systems. With this we can easily enable/disable features, change content of labels in our UI, or build a full fledged theming system that updates in "real time".

More info about KZFileWatchers can be found at git.io/filewatchers