watchOS: Complications

Feb 7 2023 · Swift 5.6, watchOS 8.5, Xcode 13

Part 1: Introduction to Complications

07. Update with Background URL Downloads

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Previous episode: 06. Update with Background Tasks Next episode: 08. Update with Push Notifications

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Downloading data from the network follows the same general pattern as background tasks.

URLSession setup and configuration

To start, create a file named UrlDownloader.swift

final class UrlDownloader: NSObject {

}

  let identifier: String

  init(identifier: String) {
    self.identifier = identifier
  }

  private lazy var backgroundUrlSession: URLSession = {
  
  }()

    let config = URLSessionConfiguration.background(
      withIdentifier: identifier
    )

    config.isDiscretionary = false

    config.sessionSendsLaunchEvents = true

    return .init(
      configuration: config,
      delegate: self,
      delegateQueue: nil
    )

extension UrlDownloader: URLSessionDownloadDelegate {
  func urlSession(
    _ session: URLSession,
    downloadTask: URLSessionDownloadTask,
    didFinishDownloadingTo location: URL
  ) {
  }
}

Scheduling a network download

Now, similar to how we created a background task in the previous episode, we can create a URL Sessions Download Task.

private var backgroundTask: URLSessionDownloadTask?

func schedule(firstTime: Bool = false) {
  let minutes = firstTime ? 1 : 15

  let when = Calendar.current.date(
    byAdding: .minute,
    value: minutes,
    to: Date.now
  )!

  let url = URL(
    string: "https://api.weather.gov/gridpoints/TOP/31,80/forecast"
  )!
  let task = backgroundUrlSession.downloadTask(with: url)

  task.earliestBeginDate = when

  task.countOfBytesClientExpectsToSend = 100
  task.countOfBytesClientExpectsToReceive = 12_000

  task.resume()

  backgroundTask = task
}

URLSessionDownloadDelegate

When the backgroundTask finishes downloading, watchOS will call this urlSession(_:downloadTask:didFinishDownloadingTo:) method we left empty earlier.

let decoder = JSONDecoder()

guard

else {
  return
}

guard
  location.isFileURL,
else {
  return
}

  let data = try? Data(contentsOf: location),

  let decoded = try? decoder.decode(Weather.self, from: data),

  let temperature = decoded.properties.periods.first?.temperature
else {
  return
}

UserDefaults.standard.set(temperature, forKey: "temperature")

func urlSession(
  _ session: URLSession,
  task: URLSessionTask,
  didCompleteWithError error: Error?
) {

}

  backgroundTask = nil

  DispatchQueue.main.async {
    self.completionHandler?(error == nil)
    self.completionHandler = nil
  }
}

Preparing for download

So let’s add one at the top of UrlDownloader:

private var completionHandler: ((Bool) -> Void)?

public func perform(_ completionHandler: @escaping (Bool) -> Void) {
  self.completionHandler = completionHandler
  _ = backgroundUrlSession
}

Alright, let’s tie this all together in back in ExtensionDelegate.swift

private var downloads: [String: UrlDownloader] = [:]

private func downloader(for identifier: String) -> UrlDownloader { 

}

  guard let download = downloads[identifier] else {
    let downloader = UrlDownloader(identifier: identifier)
    downloads[identifier] = downloader
    return downloader
  }

  return download

case let task as WKURLSessionRefreshBackgroundTask:

  let downloader = downloader(for: task.sessionIdentifier)

  downloader.perform { updateComplications in
  
  }

    if updateComplications {
      Self.updateActiveComplications()
    }
    
    downloader.schedule()
    task.setTaskCompletedWithSnapshot(false)
  }

Updating ContentView

In ContentView.swift, add a URLDownloader.

@State private var downloader = UrlDownloader(identifier: "ContentView")

Button {
  downloader.schedule(firstTime: true)
} label: {
  Text("Download")
}