Application Lifecycle Demo

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last updated: 2017-03

In this section, we are going to examine an application that demonstrates the four Application States, and the role of the AppDelegate methods in notifying the application of when states get changed. The application will print updates to the console whenever the app changes state:

Walkthrough

  1. Open the Lifecycle project in the LifecycleDemo solution.
  2. Open up the AppDelegate class. Note that we have added logging to the lifecycle methods to let us know when the application has changed state:
public override void OnActivated(UIApplication application)
        {
          Console.WriteLine("OnActivated called, App is active.");
        }
        public override void WillEnterForeground(UIApplication application)
        {
          Console.WriteLine("App will enter foreground");
        }
        public override void OnResignActivation(UIApplication application)
        {
          Console.WriteLine("OnResignActivation called, App moving to inactive state.");
        }
        public override void DidEnterBackground(UIApplication application)
        {
          Console.WriteLine("App entering background state.");
        }
        // not guaranteed that this will run
        public override void WillTerminate(UIApplication application)
        {
          Console.WriteLine("App is terminating.");
        }
  1. Launch the application in the simulator or on the device. OnActivated will be called when the app launches. The application is now in the Active state.
  2. Hit the Home button on the simulator or device to bring the application to the background. OnResignActivation and DidEnterBackground will be called as the app transitions from Active to Inactive and into the Backgrounded state. Since we have not given our application any code to execute in the background, the application is considered Suspended in memory.
  3. Navigate back to the app to bring it back into the foreground. WillEnterForeground and OnActivated will both be called:

    Note that we added the following line of code to our view controller notifying us that the application has entered the foreground from the background:

UIApplication.Notifications.ObserveWillEnterForeground ((sender, args) => {
                label.Text = "Welcome back!";
            });
  1. Press the Home button to put the application into the background. Then, double-tap the Home button to bring up the application switcher:

  2. Locate the application in the App Switcher, and swipe up to remove it:

iOS will terminate the application. Keep in mind that WillTerminate is not called because we are terminating an application that is Suspended in the background.

Now that we understand iOS application states and transitions, we'll take a look at the different options available for backgrounding in iOS.

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