Redth

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iOS7 Recipe: Background Fetching

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This is my entry into the Xamarin Recipe Cook-Off. Recipes, in Xamarin terms, are very simple demonstrations of how a single feature or piece of functionality is implemented. I thought background fetching would be useful to many developers, and it’s pretty easy to implement, especially after reading this recipe.

I’ve also included the recipe in this blog post:

Background Fetching Data

This recipe shows how to register your application to perform background fetching on intervals.

1. Recipe

In your application’s Info.plist file, add the value fetch to the UIBackgroundModes (Required background modes) property.

Next, in your AppDelegate class, in the FinishedLaunching override method, add the following code to register your application for background fetching:

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UIApplication.SharedApplication.SetMinimumBackgroundFetchInterval (UIApplication.BackgroundFetchIntervalMinimum);

Finally, in your AppDelegate class, override the PerformFetch method.

This method will be executed by the operating system when it sees best fit (eg: when the device is awake and connected already). You do not have complete control over how often or when fetching happens.

You should execute your own code to fetch new data in this method. It’s important to call the Action<UIBackgroundFetchResult> completionHandler parameter which is passed into this method with the appropriate result when you are done.

2. Sample PerformFetch

In this sample, a weather service is called by background fetching so that when the user opens the app, recent weather is available. The weather is cached locally after it’s fetched in the background, and the UI is also updated if there is new weather info.

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public override async void PerformFetch (UIApplication application, Action<UIBackgroundFetchResult> completionHandler)
{
  Console.WriteLine ("PerformFetch called...");

  //Return no new data by default
  var result = UIBackgroundFetchResult.NoData;

  try
  {
    //Get latest weather
    var w = await GetWeatherAsync("Windsor, Canada");

    if (w != null)
    {
      //Cache the weather locally
      CacheWeatherAsync(w);

      //Update the UI
      weatherViewController.UpdateWeather(w);

      //Indicate we have new data
      result = UIBackgroundFetchResult.NewData;
    }
  }
  catch
  {
    //Indicate a failed fetch if there was an exception
    result = UIBackgroundFetchResult.Failed;
  }
  finally
  {
    //We really should call the completion handler with our result
    completionHandler (result);
  }
}

3. Additional Information

  • Your PerformFetch has about 30 seconds to run before it’s killed
  • The operating system is more likely to grant more time (and more often) to your application for background fetching if you are efficient, which means executing quickly, and always calling completionHandler with an accurate result
  • You can tell the operating system the minimum time to sleep between waking up your application and calling its PerformFetch method if you know your app only updates at a certain interval, to avoid extra calls to PerformFetch and wasting battery life. You would specify the minimum time in seconds in the UIApplication.SharedApplication.SetMinimumBackgroundFetchInterval (double minimumBackgroundFetchInterval) method
  • You can actually make calls to update your UI from the PerformFetch method so that the next time the user launches the app, everything is up to date

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