Testing and Debugging XPC Code With an Anonymous Listener

IMPORTANT This post is now retired in favour of official documentation, namely TN3113 Testing and Debugging XPC Code With an Anonymous Listener. I’m leaving the original post here just for the record.

Testing and debugging XPC code can be tricky because there are two processes involved. Imagine you might have an app with an embedded XPC Service. To debug this you have to run two instances of the debugger, one connected to the app and another to the service, and then bounce between them. There is, however, an easier way, a way that allows you to test and debug all of your XPC code in a single process.

The trick is to set up an anonymous XPC listener. To start, tweak your XPC listener abstraction to accept an NSXPCListener instance. For example, say you have a MyListener class like this:

class MyListener {

    init() {
        self.listener = NSXPCListener.service()
    }

    let listener: NSXPCListener
    
    … more code …
}

Change the initialiser to look like this:

init(listener: NSXPCListener = .service()) {
    self.listener = listener
}

This uses the XPC Service’s listener by default, but allows you to override that by passing a value to the listener parameter.

Now, in your test environment, call the anonymousListener method to create a anonymous listener and pass that to your listener abstraction:

let myListener = MyListener(listener: .anonymous())

On the client side, tweak your XPC connection abstraction to accept an NSXPCConnection instance. For example, say you have a MyConnection class like this:

class MyConnection {

    init() {
        self.connection = NSXPCConnection(serviceName: "com.example.MyService")
    }

    let connection: NSXPCConnection
}

Change the initialiser to look like this:

init(connection: NSXPCConnection = .init(serviceName: "com.example.MyService")) {
    self.connection = connection
}

This sets up a connection to the XPC Service’s listener by default, but allows you to override that by passing a value to the connection parameter.

Finally, in your test environment, use the init(listenerEndpoint:) initialiser to create a connection to your anonymous listener:

let connection = NSXPCConnection(listenerEndpoint: myListener.listener.endpoint)
let myConnection = MyConnection(connection: connection)

You now have a connection connected to your listener, both running in the same process. This makes it much easier to debug your XPC code. It’s also perfect for unit tests.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"