WKWatchConnectivityRefreshBackgroundTask is never triggered in background, but WKSnapshotRefreshBackgroundTask

Question

I want to update my watch app state in background from iPhone, using session.updateApplicationContext(applicationContext).

Sending an application contact while the app on the watch is active does work properly.
When I activate the home button on the watch, the watch app goes to the background, handle(_ backgroundTasks: Set<WKRefreshBackgroundTask>) is called, and a WKSnapshotRefreshBackgroundTask is provided.

So I don’t understand why a WKSnapshotRefreshBackgroundTask is triggered properly, but not a WKWatchConnectivityRefreshBackgroundTask.

Apple’s docs say „When you receive background data from the paired iPhone, the system launches your app in the background, instantiates a WKWatchConnectivityRefreshBackgroundTask object, and passes the task object to your extension delegate’s handleBackgroundTasks: method.“.

But this does not happen, neither on a device, nor on the simulator. What could be wrong?

Edit:

To check what might be wrong, I downloaded Apple’s demo project „QuickSwitch“ that can be downloaded here. Here is the code that should handle background tasks:

func handle(_ backgroundTasks: Set<WKRefreshBackgroundTask>) {
    for backgroundTask in backgroundTasks {
        if let wcBackgroundTask = backgroundTask as? WKWatchConnectivityRefreshBackgroundTask {
            // store a reference to the task objects as we might have to wait to complete them
            self.wcBackgroundTasks.append(wcBackgroundTask)
        } else {
            // immediately complete all other task types as we have not added support for them
            backgroundTask.setTaskCompleted()
        }
    }
    completeAllTasksIfReady()
}

There, the same happens:
I did set a breakponint in the line of the if statement and executed the app.
When the home button on the watch simulator is pressed, the breakpoint is reached with a WKSnapshotRefreshBackgroundTask. This is OK (see above).
However, if a different line is selected on the iPhone simulator, watchOS does not schedule a WKWatchConnectivityRefreshBackgroundTask, as expected. After all, this demo project should demo exactly this point.
Maybe somebody could try the demo project and confirm this problem or not.

What is wrong?

Answer 1

Update my answer

---

Conclusion first

Currently WKWatchConnectivityRefreshBackgroundTask is only called for sure on a watchOS Simulator when the watchOS extension's WCSession is in notActivated state and the extension is not running in foreground (in background or terminated).

In real devices, it won't be called in my tests. But Apple docs says it may. So you shouldn't rely on it won't be called until Apple changes its docs.

WCSession Cores

For WCSession, when it is activated, you can transfer userInfo, and when the counterpart is active, it can get the userInfo. The counterpart won't need to be in foreground to be activated, it can be in a high priority background.

Testing Results

Here are my testing results.

How to make WCSession notActivated?

1. Using Xcode terminate your watchOS Extension. Xcode will send a kill signal to your WKExtension.
2. Or don't run WCSession.activate() in your code on watchOS Extension side. As WCSession is notActivated by default.

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-------------below are old post, you can ignore safely if you don't want to read.-------------------

Theory

Please watch the picture first then I will explain.

Because of the history of watchOS, there are both WCSessionDelegate receiving functions (start from watchOS 2.0) and WKExtensionDelegate.handle(_:) function (start from watchOS 3.0).

Although they all claims to be background dealing, the former only works immediately when your app is in foreground. The data will be queued if your app is not in foreground (in background or being terminated) and executed immediately later when your app becomes in foreground again.

WKExtensionDelegate.handle(_:) is really working in background. However, the WKExtensionDelegate.handle(_:) is optional although it is recommended and well-prepared if you use Xcode.

If you don't implement WKExtensionDelegate.handle(_:) by commenting it. You app works in a watchOS 2.0 way.

If you implement WKExtensionDelegate.handle(_:) but you don't have a WCSession in your watchOS app. The result is tricky. You won't get any data when you watchOS app is in foreground, as you don't has a WCSession. When your app is in background, it will be waken when data comes, but you can't get the data as you don't have a session.

If you implemented them both, which are in most situations, data comes will be dealt depending on the state of your watchOS app and never be queued.

---

How to proving it?

Create a new watchOS project. In iOS part, add a button, each time you clicked the button, send a userInfo to watchOS

session.transferUserInfo(["send test":""])

In your watchOS app, add a label in interface.storyboard, and drag it to viewController as @IBOutlet var label: WKInterfaceLabel!, and implement both WKExtensionDelegate.handle(_:) and func session(WCSession, didReceiveUserInfo: [String : Any] = [:]) appDelegate.

var total = 0

func handle(_ backgroundTasks: Set<WKRefreshBackgroundTask>) {
    // Sent when the system needs to launch the application in the background to process tasks. Tasks arrive in a set, so loop through and process each one.
    for task in backgroundTasks {
        // Use a switch statement to check the task type
        switch task {
        case let backgroundTask as WKApplicationRefreshBackgroundTask:
            // Be sure to complete the background task once you’re done.
            backgroundTask.setTaskCompleted()
        case let snapshotTask as WKSnapshotRefreshBackgroundTask:
            // Snapshot tasks have a unique completion call, make sure to set your expiration date
            snapshotTask.setTaskCompleted(restoredDefaultState: true, estimatedSnapshotExpiration: Date.distantFuture, userInfo: nil)
        case let connectivityTask as WKWatchConnectivityRefreshBackgroundTask:
            // Be sure to complete the connectivity task once you’re done.
            total += 1
            DispatchQueue.main.async {
                if let viewController = WKExtension.shared().rootInterfaceController as? InterfaceController {
                    viewController.label.setText(String(self.total))
                }
            }

            connectivityTask.setTaskCompleted()
        case let urlSessionTask as WKURLSessionRefreshBackgroundTask:
            // Be sure to complete the URL session task once you’re done.
            urlSessionTask.setTaskCompleted()
        default:
            // make sure to complete unhandled task types
            task.setTaskCompleted()
        }
    }
}

public func session(_ session: WCSession, didReceiveUserInfo userInfo: [String : Any] = [:]) {
    total += 4
    DispatchQueue.main.async {
        if let viewController = WKExtension.shared().rootInterfaceController as? InterfaceController {
            viewController.label.setText(String(self.total))
        }
    }
}

If WKExtensionDelegate.handle(_:) runs, we add total by 1. If func session(WCSession, didReceiveUserInfo: [String : Any] = [:]) runs, we add total by 4.

Debug

In Xcode, choose product->scheme as WatchKit app so we can terminate the watchOS app in Xcode.

1. run the project.
2. when watchOS app shows, open iOS app manually.
3. clicked the button in iOS app. You can see the label in watchOS changes by 4.
4. in Xcode, click product->stop(or cmd+.). watchOS app will disappear.
5. click one or more times on iOS app's button. Then manually open the watchOS app. You will see this time the label changes by 1 multiply your clicks.
6. The step will be 4 again when watchOS app is in foreground.