Android Application Concepts: The Activity

Android applications can be constructed from four different types of components: activities, services, broadcast receivers, and content providers. Of the four, the only one that you must use at least one of in every application (and the only one that I've had to use so far) is the activity.

I'm not going to even try to give a full explanation of what an activity is: the Application Fundamentals page on the Android Developers site provides a thorough (and rather long) explanation of activies as well as the other three components. So I'm just going to list some aspects of activities to give you a conceptual overview of what an activity is:

• Each individual screen within an Android application is an activity, and users can perform actions by interacting with the visual components within an activity. Think of it as an individual web page within a modern web application, where the user can perform certain functions without leaving the page (using JavaScript), but must proceed to a different page in order to access a new set of tasks/functionality.
• Each activity operates independently of one another (again, similar to how individual pages in a web application are separate from one another).
• The visual objects within an activity (blocks of text, input boxes, buttons, etc.) are usually organized and instantiated by implementing instructions within a layout XML file.  They CAN be added individually with regular Java code, but the XML method is the recommended practice.
• The transistion from one activity to another is accomplished through the use of asynchronous messages called Intents. An intent can be used to pass data from one activity to another, and that data is structured using key/value pairs. It's similar to how data is passed when an HTML form is submitted.
• Each activity in an Android application is either a direct subclass of the Activity base class or a subclass of an Activity subclass that provides specialized functionality (such as the ListActivity class which contains methods specifically for the display of data in a touch-enabled list), and is represented in the application package as a single Java class.
• All of the activities in the application must be defined in the application's manifest file, an XML file that keeps track of all of the components, resources, and permissions that make up the application.
• All Android application components, including activities, have lifecycles. The current lifecycle state of a component provides part of the information the Android OS uses when it determines what components can be safely removed from memory in order to free up memory space. All activities inherit a set of lifecycle methods from the Activity base class that are executed when the corresponding lifecyle state of the activity is reached. These lifecycle methods are:
  • onCreate: the code within the onCreate lifecycle method is called when the activity is instantiated/loaded into memory. This is the method in which you want to set up the intial state of your variables, instantiate the visual objects within the activity via your layout file, and bind your event listeners. If you're a jQuery user, think of it as the equivalent of the document.ready function.
  • onStart: the method called when the activity becomes visible to the user.
  • onResume: the method called when the activity is moved to the foreground of the visible space, when the user is able to not only see the activity but interact with it. After the onResume method is called, the activity is considered to be "running."
  • onPause: called when the activity is no longer in the foreground, usually as the result of another activity being started.
  • onStop: called when the activity is no longer visible to the user in either the foreground or background.
  • onRestart: called when the activity is being restarted (made visible again) after executing onStop but not onDestroy.
  • onDestroy: called just before an activity is destroyed/unloaded from memory.
  
  
• Any activity that isn't currently "running" could potentially be removed from memory by the Android OS if available system memory is low. The more inactive an activity is, the better the chance of it being destroyed to reclaim memory, so an activity that is "stopped" is in more danger of being removed from memory than an activity that is "paused."
• Because even an activity that is "paused" can be destroyed under extremely low memory conditions, it is a best practice to run code to store/save any persistent data within the activity within the onPause method just in case (kind of like how the auto-save function works in Microsoft Office to save your work every few minutes).
• To preserve the current state of an activity (like filtered search results) rather than persistent data, you can use code in the onSaveInstanceState method to perserve that data in a Bundleobject (which, like an intent, stores data in key/value pairs).  The onSaveInstanceState method in the activity is called by the Android OS itself as soon as the activity has the potential to be removed from memory. However, onSaveInstanceState is NOT called when the user explicitly takes action to destroy an activity (usually by exiting the entire application).
• It is possible for one "application" to utilize activity objects from one or more different application packages (.apk files), given the proper planning and permissions.