Understanding iOS Application States and Lifecycle Transitions
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Chapter 1: Introduction to iOS Application States
When a user interacts with an app by tapping its icon, the application begins its lifecycle. The initial sequence of events typically unfolds as follows:
- The app is activated.
- Initialization routines may be executed (this is generally common).
- User interface elements are created.
- Optional API calls may occur.
Chapter 2: Understanding the Basic Lifecycle
As the application operates, its state transitions (UIApplication.State) occur. Here's how these transitions unfold:
- Inactive: The app is active in the foreground but not processing events. This transitional state arises during multitasking, such as when switching to another app or when the home button is pressed.
- Active: The app is fully engaged and receiving user inputs.
- Background: The app continues to run in the background and can respond to notifications or events. However, the operating system may terminate it after a certain duration (often around 10 to 180 seconds depending on the version of iOS).
If you need to manage background tasks, you can use beginBackgroundTask(expirationHandler:) or beginBackgroundTask(withName:expirationHandler:). The remaining background time can be checked using UIApplication.shared.backgroundTimeRemaining.
- Suspended: The app remains in memory but isn't executing any code. It will not drain battery life, but the OS may terminate it if memory is required.
- Not Running: This is the default state when the app is not active, whether because it has been terminated or not yet launched.
If your app requires specific lifecycle triggers, check for custom properties in push notifications that could initiate fetching or syncing tasks.
Chapter 3: Observing Lifecycle Method Triggers
Monitoring lifecycle methods is crucial, especially when the app transitions from background to active states. If you notice significant background time remaining, this indicates that termination has not yet been initiated by the OS.
Apple has revised the lifecycle states multiple times, leading to variances across iOS versions. Notably, iOS 13 subversions exhibit the most differences.
Subsection 3.1: Addressing Swizzling Issues
Swizzling is commonly employed by various third-party frameworks to override certain AppDelegate lifecycle methods. If multiple SDKs use swizzling, conflicts may arise. A potential resolution is to implement a CustomAppDelegate, as demonstrated in the example project, though outcomes can vary based on the specific third-party framework.
Chapter 4: Conclusion and Further Reading
Understanding the application lifecycle will enhance your ability to interpret logs effectively. Pay special attention to the application(_:didFinishLaunchingWithOptions:) method as it is activated during various states.
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The first video provides a detailed overview of the application lifecycle in iOS, with visual illustrations to enhance understanding.
The second video delves into the differences between App Delegate and Scene Delegate, particularly in the context of Swift 5, providing valuable insights for developers.