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Background Syncing

Sources

We encourange you to watch WWDC videos:

Implementation details

There are 4 different APIs and types of background tasks. Each one is specific and can be used for different scenarios. Synchronization of the blockchain data is time and memory consuming operation. Therefore the BGProcessingTask has been used. This type of task is designed to run for a longer time when certain conditions are met (watch Background execution demystified).

Steps to make it work

  1. Add a capability of background modes in the settings of the xcode project.
  2. Turn the Background Processing mode on in the new capability.
  3. Add Permitted background task scheduler identifiers to the info.plist.
  4. Create the ID for the background task in the newly created array.
  5. Register the BGTask in application.didFinishLaunchingWithOptions
BGTaskScheduler.shared.register(
    forTaskWithIdentifier: <ID>,
    using: DispatchQueue.main
) { task in
    // see the next steps
}

Note: The queue is an optional and most tutorials leave the parameter nil but Zashi requires main thread processing due to UI layer - therefore we pass DispatchQueue.main. 6. Call a method that schedules the task execution. 7. Start the synchronizer. 8. Set the expiration closure and stop the synchronizer inside it

task.expirationHandler = { 
    synchronizer.stop()
}
  1. The body of the registered task summarized:
BGTaskScheduler.shared.register(...) { task in
    scheduleTask() // 6
    
    synchronizer.start() // 7
    
    task.expirationHandler = { 
        synchronizer.stop() // 8
    }
}
  1. Call scheduleTask() when app goes to the background so there is the initial scheduling done, the next one will be handled by the closure of the registered task. The method usually consists of:
let request = BGProcessingTaskRequest(identifier: <ID>)

request.earliestBeginDate = <scheduledTime>
request.requiresExternalPower = true            // optional, we require the iPhone to be connected to the power
request.requiresNetworkConnectivity = true      // required

do {
    try BGTaskScheduler.shared.submit(request)
} catch { // handle error }
  1. Last step is to call .setTaskCompleted(success: <bool>) on the BGTask when the work is done. This is required by the system no matter what. We call it with true when the synchronizer finishes the work (up-to-date state) and with false for other or failed reasons (stopped state, error state, etc.).

You can see specific details of the Zashi implementation in:

  • Xcode project settings, steps 1-4.
  • AppDelegate.swift file, steps 5-9.
  • SecantApp.swift file, step 10.
  • RootInitialization.swift, step 11.

Gotchas

  • The requiresNetworkConnectivity flag doesn't specify or deal with the type of connectivity. It simply allows scheduling when the iPhone is connected to the internet. We deal with it when the task is triggered. The custom check wheather the wifi is or is not connected preceeds the start of the synchronizer.
  • When the app is killed by a user in the app switcher, the scheduled BGTask is deleted. So the BGTask is triggered at the scheduled time only when the app is suspended or killed by the system. Explicit termination of the app by a user leads to termination of any background processing.