D. Android App Activities

• Activity


  A component of an application that provides a single screen with a user interface
• Applications


  • Contain multiple activities
  • Each activity has at least one connection with the others
  • Activities can start other activities to perform various actions
• Three states of an activity

  • Running/Resumed
  • Paused
  • Stopped
• Running/Resumed


  The activity is in the foreground of the screen and has user focus
• Paused


  Another activity is in the foreground and has focus but is still visible
• Stopped


  The activity is in the background and is no longer available to the user
• Callback methods


  • Methods involved in the life cycle of an activity
  • Allow monitoring of activities throughout their life cycles
• Three nested loops of callback methods

  • Entire lifetime
  • Visible lifetime
  • Foreground lifetime
• Entire lifetime


  • Happens between the call to onCreate() and the call to onDestroy()
  • Setting up of "global" state such as defining layout is performed in onCreate()
  • All remaining resources are released in onDestroy()
• Visible lifetime


  • Happens between the call to onStart() and the call to onStop()
  • The user can see and interact with the activity
• Foreground lifetime


  • Happens between the onResume() and the call to onPause()
  • The activity is in front of all other activities and is in focus
• Fragments


  • Represent a portion or a behavior of a user in an activity
  • Considered a modular section of an activity that has its own life cycle and receives its own input events
• Three essential callback methods for fragments

  • onCreate()
  • onCreateView()
  • onPause()
• onCreate()


  • Called when creating the fragment
  • Essential components of the fragment that need to be retained must be initialized within the implementation
• onCreateView()


  • Called when the fragment's interface is about to be drawn for the first time
  • A null value is returned if the fragment does not provide a user interface
• onPause()


  Called when the user is leaving the fragment