When is applicationDidBecomeActive called?

Understand the states and transitions of an iOS

States

Non-running - The app is not running.

Inactive - The app is running in the foreground, but not receiving events. An iOS app can be placed into an inactive state, for example, when a call or SMS message is received.

Active - The app is running in the foreground, and receiving events.

Background - The app is running in the background, and executing code.

Suspended - The app is in the background, but no code is being executed.

The seven most important application delegate methods

The operating system calls specific methods within the application delegate to facilitate transitioning to and from various states. The seven most important application delegate methods a developer should handle are:

application:willFinishLaunchingWithOptions

Method called when the launch process is initiated. This is the first opportunity to execute any code within the app.

application:didFinishLaunchingWithOptions

Method called when the launch process is nearly complete. Since this method is called is before any of the app's windows are displayed, it is the last opportunity to prepare the interface and make any final adjustments.

applicationDidBecomeActive

Once the application has become active, the application delegate will receive a callback notification message via the method applicationDidBecomeActive.

This method is also called each time the app returns to an active state from a previous switch to inactive from a resulting phone call or SMS.

applicationWillResignActive

There are several conditions that will spawn the applicationWillResignActive method. Each time a temporary event, such as a phone call, happens this method gets called. It is also important to note that "quitting" an iOS app does not terminate the processes, but rather moves the app to the background.

applicationDidEnterBackground

This method is called when an iOS app is running, but no longer in the foreground. In other words, the user interface is not currently being displayed. According to Apple's UIApplicationDelegate Protocol Reference, the app has approximately five seconds to perform tasks and return. If the method does not return within five seconds, the application is terminated.

applicationWillEnterForeground

This method is called as an app is preparing to move from the background to the foreground. The app, however, is not moved into an active state without the applicationDidBecomeActive method being called. This method gives a developer the opportunity to re-establish the settings of the previous running state before the app becomes active.

applicationWillTerminate

This method notifies your application delegate when a termination event has been triggered. Hitting the home button no longer quits the application. Force quitting the iOS app, or shutting down the device triggers the applicationWillTerminate method. This is the opportunity to save the application configuration, settings, and user preferences.

need additional information ref this link1 or apple link2

When a user is using an application he is in an active state. The user switch to inactive state from a resulting phone call or when a pull down notification center is pulled or when the home screen is pressed(This is when the app is told to be in background state) and then the app is opened again(This is when the app is told to be back to foreground state).

So every time the user switches from inactive state to active state applicationDidBecomeActive this delegate is called