Safari Web Extensions & NSXPCConnection

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I have a basic setup following WWDC 2020 on Safari Web Extensions and another one on XPC. The video even mentions that one can use UserDefaults or XPC to communicate with the host app. Here is my setup.

macOS 15.2, Xcode 16.2 A macOS app (all targets sandboxed, with an app group) with 3 targets:

SwiftUI Hello World
web extension
XPC Service

The web extension itself works and can update UserDefaults, which can then be read by SwiftUI app - everything works by the book.

The app can communicate to the XPC service via NSXPCConnection - again, everything works fine.

The problem is that the web extension does not communicate with XPC, and this is what I need so that I can avoid using UserDefaults for larger and more complex payloads.

Web Ext handler code:

 class SafariWebExtensionHandler: NSObject, NSExtensionRequestHandling {
    func beginRequest(with context: NSExtensionContext) {
        // Unpack the message from Safari Web Extension.
        let item = context.inputItems[0] as? NSExtensionItem
        let message = item?.userInfo?[SFExtensionMessageKey]
 
        // Update the value in UserDefaults.
        let defaults = UserDefaults(suiteName: "com.***.AppName.group")
        let messageDictionary = message as? [String: String]
        if messageDictionary?["message"] == "Word highlighted" {
            var currentValue = defaults?.integer(forKey: "WordHighlightedCount") ?? 0
            currentValue += 1
            defaults?.set(currentValue, forKey: "WordHighlightedCount")
        }
 
        let response = NSExtensionItem()
        response.userInfo = [ SFExtensionMessageKey: [ "Response to": message ] ]
 
        os_log(.default, "setting up XPC connection")
        let xpcConnection = NSXPCConnection(serviceName: "com.***.AppName.AppName-XPC-Service")
        
        xpcConnection.remoteObjectInterface = NSXPCInterface(with: AppName_XPC_ServiceProtocol.self)
        xpcConnection.resume()
 
        let service = xpcConnection.remoteObjectProxyWithErrorHandler { error in
            os_log(.default, "Received error:  %{public}@", error as CVarArg)
        } as? AppName_XPC_ServiceProtocol
        
        service?.performCalculation(firstNumber: 23, secondNumber: 19) { result in
            NSLog("Result of calculation XPC is: \(result)")
            os_log(.default, "Result of calculation XPC is: \(result)")
            
            context.completeRequest(returningItems: [response], completionHandler: nil)
        }
    }
}

The error I'm getting:

 Error Domain=NSCocoaErrorDomain Code=4099 "The connection to service named com.***.AppName.AppName-XPC-Service was invalidated: failed at lookup with error 3 - No such process."

What am I missing?

Answered by DTS Engineer in 831880022

Thanks for the clarifications.

So, yeah, that pretty much confirms what I was talking about in my previous post. You can’t use an XPC service for this task because there’s no way that the same XPC service can be visible to both your app and your appex. I see this a lot with folks building other extension types, like Finder Sync extensions.

How you get around that depends on your specific circumstances. What do you want to happen when the appex connects to the service when the container app isn’t running?

There are two standard answers to that:

The IPC is optional, so you want it to fail.
The IPC is critical, so you want it to launch the app.

I suspect it’s the latter, but that’s not really feasible because launching an app is a heavyweight operation. I talk about this whole issue in more detail in XPC and App-to-App Communication DevForums

The solution is for you to install something smaller than your app that gets launched on demand. Ideally that’d be an XPC service but, as I’ve noted above, that doesn’t work. However, there’s another option, namely a launchd agent.

These have a number of useful properties:

They’re relatively easy to install, using SMAppService.
They support XPC, via the MachServices property in the launchd property list.
And they support launch on demand, so the agent only need to runs when your appex (or app) calls upon its services.

Oh, and one thing to watch out for here. Your appex is sandboxed, and the app sandbox prevents it from connecting to arbitrary XPC named endpoints. To make this work the named endpoint must be ‘within’ an app group claimed by the appex. For example, you might have an app group ID like group.com.example.my-app.group and then set the XPC endpoint name to be group.com.example.my-app.group.xpc.

Note that app groups themselves have some sharp edges on macOS. See App Groups: macOS vs iOS: Fight!

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

Boost

Answer 1

DTS Engineer OP

Apple

6d

Written by ilia_saz in 778297021

The problem is that the web extension does not communicate with XPC

From an XPC perspective that not super surprising. An XPC service is only available to the app in which it’s embedded. Your XPC service is embedded in your app, and thus it’s visible to your app, but not to your appex. If moved it to your appex, the opposite would be true.

However, there’s a few web extension things I’d like to confirm. To start, how did you create your web extension? From the macOS > Safari Extension template?

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

0

Answer 2

ilia_saz OP

6d

I followed this WWDC talk to add a Safari Web Extension to my app. So I went to add a new target, selected Safari Extensions, then picked Web Extensions and select my app in "Embed in Application" dropdown.

Similarly, I added a new target for a XPC Service. And I tried with both high-level using NSXPCConnection and low-level using XPCSession.

When I build my app, I get both targets (Web Extension and XPC Service) embedded in my app's .app file - one under Plugins, and the other under XPCServices.

0

Answer 3

DTS Engineer OP

Apple

5d

Accepted Answer

Recommended

Thanks for the clarifications.

So, yeah, that pretty much confirms what I was talking about in my previous post. You can’t use an XPC service for this task because there’s no way that the same XPC service can be visible to both your app and your appex. I see this a lot with folks building other extension types, like Finder Sync extensions.

How you get around that depends on your specific circumstances. What do you want to happen when the appex connects to the service when the container app isn’t running?

There are two standard answers to that:

The IPC is optional, so you want it to fail.
The IPC is critical, so you want it to launch the app.

I suspect it’s the latter, but that’s not really feasible because launching an app is a heavyweight operation. I talk about this whole issue in more detail in XPC and App-to-App Communication DevForums

The solution is for you to install something smaller than your app that gets launched on demand. Ideally that’d be an XPC service but, as I’ve noted above, that doesn’t work. However, there’s another option, namely a launchd agent.

These have a number of useful properties:

They’re relatively easy to install, using SMAppService.
They support XPC, via the MachServices property in the launchd property list.
And they support launch on demand, so the agent only need to runs when your appex (or app) calls upon its services.

Oh, and one thing to watch out for here. Your appex is sandboxed, and the app sandbox prevents it from connecting to arbitrary XPC named endpoints. To make this work the named endpoint must be ‘within’ an app group claimed by the appex. For example, you might have an app group ID like group.com.example.my-app.group and then set the XPC endpoint name to be group.com.example.my-app.group.xpc.

Note that app groups themselves have some sharp edges on macOS. See App Groups: macOS vs iOS: Fight!

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

1

Answer 4

ilia_saz OP

5d

Thank you for the very detailed answer!

I get the idea, and I have a few follow-up questions. And let me give a bit more context about what I'm trying to achieve. The Web Extension captures quite a bit of data from an active page and sends it to the app for background processing - doing things like parsing, storing it in a DB, some compute intensive work, REST calls, and etc. The user can see the progress and the results in the app whenever they would like but usually this will happen some time later. I don't want to distract or impact the user's browsing activity in any way, it should be "press a button and forget" kind of experience, and hence your suggestion of implementing this with launchd makes perfect sense.

While I'm in the early development phase, it might be ok to just fail if the app is not running. How does this assumption make things any simpler if web ext still can't talk to the app? Or do you suggest using a socket or even an embedded web server rather than XPC?
If I go with launchd approach, how will this work on iOS?

0

Answer 5

DTS Engineer OP

Apple

2d

Written by ilia_saz in 831874022

If I go with launchd approach, how will this work on iOS?

It won’t )-:

While iOS does support XPC, that support is very limited. It certainly doesn’t include anything like a launchd agent.

The key problem on iOS is that your container app can’t reliably run in the background. It might not be running at all but, even if it’s running, it might be suspended. And there’s no mechanism for you to resume it.

The standard approach for this sort of thing on iOS is for the appex to drop information about its state into an app group container and then post a Darwin notification. If the container app happens to be running, it’ll see the notification and react promptly. If not, the container app should check the app group container when it next runs.

If you want to minimise this latency you can use the background processing task mechanism. See iOS Background Execution Limits for links to docs and more context.

Written by ilia_saz in 831874022

While I'm in the early development phase, it might be ok to just fail if the app is not running.

Sure. But…

Written by ilia_saz in 831874022

Or do you suggest using a socket or even an embedded web server rather than XPC?

I generally recommend that you avoid using the networking stack for IPC. It’ll work, but then you have to start worrying about folks connecting from afar.

Using a Unix domain socket is fine. The socket has to be in an app group container so that both your app and appex can access it.

There are a few drawbacks here:

XPC is a lot more full-features than Unix domain socket.
You traditionally access Unix domain sockets via the BSD Sockets API, which is un-fun to use from Swift. You can get around that by using Network framework.
If you eventually decide to switch to XPC, you’ll end up discarding all this code.

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

0

Answer 6

ilia_saz OP

2d

Thank you very much for your detailed responses. Also, the links you provided are super helpful!

0

	class SafariWebExtensionHandler: NSObject, NSExtensionRequestHandling {
	func beginRequest(with context: NSExtensionContext) {
	// Unpack the message from Safari Web Extension.
	let item = context.inputItems[0] as? NSExtensionItem
	let message = item?.userInfo?[SFExtensionMessageKey]

	// Update the value in UserDefaults.
	let defaults = UserDefaults(suiteName: "com.***.AppName.group")
	let messageDictionary = message as? [String: String]
	if messageDictionary?["message"] == "Word highlighted" {
	var currentValue = defaults?.integer(forKey: "WordHighlightedCount") ?? 0
	currentValue += 1
	defaults?.set(currentValue, forKey: "WordHighlightedCount")
	}

	let response = NSExtensionItem()
	response.userInfo = [ SFExtensionMessageKey: [ "Response to": message ] ]

	os_log(.default, "setting up XPC connection")
	let xpcConnection = NSXPCConnection(serviceName: "com.***.AppName.AppName-XPC-Service")

	xpcConnection.remoteObjectInterface = NSXPCInterface(with: AppName_XPC_ServiceProtocol.self)
	xpcConnection.resume()

	let service = xpcConnection.remoteObjectProxyWithErrorHandler { error in
	os_log(.default, "Received error: %{public}@", error as CVarArg)
	} as? AppName_XPC_ServiceProtocol

	service?.performCalculation(firstNumber: 23, secondNumber: 19) { result in
	NSLog("Result of calculation XPC is: \(result)")
	os_log(.default, "Result of calculation XPC is: \(result)")

	context.completeRequest(returningItems: [response], completionHandler: nil)
	}
	}
	}

	Error Domain=NSCocoaErrorDomain Code=4099 "The connection to service named com.***.AppName.AppName-XPC-Service was invalidated: failed at lookup with error 3 - No such process."