A simple Swift event source for fun and profit

EventSource is based on the EventSource Web API to enable Server Sent Events.

If you're unfamiliar with this one-way streaming protocol - Start Here.

Under the hood, EventSource is built on top of NSURLSession and has zero third-party dependencies.

Enjoy!

An Event looks like this

struct Event {

  let readyState: EventSourceState // The EventSourceState at the time of the event's creation

  let id: String?
  let name: String?
  let data: String?
  let error: NSError?
}

You create an EventSource with an NSURL

import EventSource

let url = NSURL(string: "http://localhost:8000/stream")!
        
let eventSource = EventSource(url: url)

eventSource.open()
eventSource.close()

eventSource.onOpen { event in
  debugPrint(event)
}

eventSource.onMessage { event in
  debugPrint(event)
}

eventSource.onClose { event in
  debugPrint(event)
}

eventSource.onError { event in
  debugPrint(event)
}

eventSource.addHandler("tweet.create") { event in
  debugPrint(event.data)
}

In the Example directory, you'll find the Server and EventSourceExample directories. The Server directory contains a simple python server that sends events to any connected clients, and the EventSourceExample directory contains a simple iOS app to display recent events from that server.

The server uses Redis to setup pub / sub channels, and it uses Flask deployed with Gunicorn to serve events to connected clients.

Install the following packages to run the simple python server

brew install redis
pip install flask redis gevent gunicorn

Start redis and deploy the server (in two separate terminal tabs)

redis-server
gunicorn --worker-class=gevent -b 0.0.0.0:8000 app:app

Open the EventSourceExample Xcode project and run the app in the simulator
Tap the "Open" button in the app to open a connection to the server

Now you can visit http://localhost:8000/publish in your browser to start sending events

If all goes well, you should get a nice stream of events in your simulator

EventSource deviates slightly from the Web API where it made sense for a better iOS API. For example, an Event has a name property so you can subscribe to specific, named events like tweet.create. This is in lieu of the Web API's event property of an Event (because who wants to write let event = event.event? Not me... 😞).

An EventSource will automatically reconnect to the server if it enters an Error state, and based on the protocol, a server can send a retry event with an interval indicating how frequently the EventSource should retry the connection after encountering an error. Be warned: an EventSource expects this interval to be in seconds - not milliseconds as described by the Web API.

Add the following line to your Cartfile.

github "christianbator/EventSource"

Then run

carthage update --platform iOS