This is a fully-working Xcode project to create a generic menu bar app (aka agent) located at the top right corner of the screen in Mac OS X.
The main components in this template are an AppDelegate and a .xib file containing the interface of the agent, which includes a contextual menu containing the "quit" option.
When compiled with Xcode, this project works out of the box. AppDelegate.swift contains a function called update() which can be fully customized to achieve the desired functionality of the agent (currently, it simply displays the time as a proof of concept). The function update() is triggered by the applicationDidFinishLaunching() method, where you can also specify the timeInterval in seconds between each call to the function:
Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(AppDelegate.update), userInfo: nil, repeats: true)
Feel free to use this template to create your own menu bar agent. For instance, I built a simple app to bring back the battery time left in my macbook pro since the latest update of Mac OS X removed that feature and I was missing it.