Interview Questions for Xamarin Android

Activity LifeCycle?
The following diagram illustrates the states an Activity can go through during its lifetime:

Activity states diagram
The following diagram illustrates these methods in relationship to the Activity Lifecycle:
Activity Lifecycle flowchart
OnCreate
This is the first method to be called when an activity is created. OnCreate is always overridden to perform any startup initializations that may be required by an Activity such as:
  • Creating views
  • Initializing variables
  • Binding static data to lists
OnStart

This method is always called by the system after OnCreate is finished. Activities may override this method if they need to perform any specific tasks right before an activity becomes visible such as refreshing current values of views within the activity. Android will call OnResume immediately after this method.

OnResume

The system calls this method when the Activity is ready to start interacting with the user. Activities should override this method to perform tasks such as:
  • Ramping up frame rates (a common task in game building)
  • Starting animations
  • Listening for GPS updates
  • Display any relevant alerts or dialogs
  • Wire up external event handlers
OnPause

This method is called when the system is about to put the activity into the background or when the activity becomes partially obscured. Activities should override this method if they need to:
  • Commit unsaved changes to persistent data
  • Destroy or clean up other objects consuming resources
  • Ramp down frame rates and pausing animations
  • Unregister external event handlers or notification handlers (i.e. those that are tied to a service). This must be done to prevent Activity memory leaks.
  • Likewise, if the Activity has displayed any dialogs or alerts, they must be cleaned up with the .Dismiss() method.
There are two possible lifecycle methods that will be called after OnPause:
  1. OnResume will be called if the Activity is to be returned to the foreground.
  2. OnStop will be called if the Activity is being placed in the background.
OnStop

This method is called when the activity is no longer visible to the user. This happens when one of the following occurs:
  • A new activity is being started and is covering up this activity.
  • An existing activity is being brought to the foreground.
  • The activity is being destroyed.
OnDestroy

This is the final method that is called on an Activity instance before it's destroyed and completely removed from memory. There will be no lifecycle methods called after the Activity has been destroyed.

OnRestart

This method is called after your activity has been stopped, prior to it being started again. A good example of this would be when the user presses the home button while on an activity in the application. When this happens OnPause and then OnStopmethods are called, and the Activity is moved to the background but is not destroyed. If the user were then to restore the application by using the task manager or a similar application, Android will call the OnRestart method of the activity.

Explain the Activity lifecycle when it goes from Activity A to Activity B and vice versa

When the app first starts, the output window displays the state changes of Activity A:
[ActivityLifecycle.MainActivity] Activity A - OnCreate
[ActivityLifecycle.MainActivity] Activity A - OnStart
[ActivityLifecycle.MainActivity] Activity A - OnResume
When we click the Start Activity B button, we see Activity A pause and stop while Activity B goes through its state changes:
[ActivityLifecycle.MainActivity] Activity A - OnPause
[ActivityLifecycle.SecondActivity] Activity B - OnCreate
[ActivityLifecycle.SecondActivity] Activity B - OnStart
[ActivityLifecycle.SecondActivity] Activity B - OnResume
[ActivityLifecycle.MainActivity] Activity A - OnStop
When we click the Back button, Activity B is destroyed and Activity A is resumed:
[ActivityLifecycle.SecondActivity] Activity B - OnPause
[ActivityLifecycle.MainActivity] Activity A - OnRestart
[ActivityLifecycle.MainActivity] Activity A - OnStart
[ActivityLifecycle.MainActivity] Activity A - OnResume
[ActivityLifecycle.SecondActivity] Activity B - OnStop
[ActivityLifecycle.SecondActivity] Activity B - OnDestroy

What is the purpose of WebView?

WebView allows you to create your own window for viewing web pages (or even develop a complete browser).
Note : I'll be putting some more questions in few days.

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