What Androids Are Made Of

When you write a desktop application, you are " master of your own domain." You launch your main window and any child windows—like dialog boxes—that are needed. From your standpoint, you are your own world, leveraging features supported by the operating system, but largely ignorant of any other program that may be running on the computer at the same time. If you do interact with other programs, it is typically through an API, such as using JDBC (or frameworks atop it) to communicate with MySQL or another database.

Android has similar concepts, but packaged differently, and structured to make phones more crash-resistant.

Activities

The building block of the user interface is the activity. You can think of an activity as being the Android analogue for the window or dialog in a desktop application.

While it is possible for activities to not have a user interface, most likely your "headless" code will be packaged in the form of content providers or services, like the following described.

Content Providers

Content providers provide a level of abstraction for any data stored on the device that is accessible by multiple applications. The Android development model encourages you to make your own data available to other applications, as well as your own—building a content provider lets you do that, while maintaining complete control over how your data gets accessed.

Intents

Intents are system messages, running around the inside of the device, notifying applications of various events, from hardware state changes (e.g., an SD card was inserted), to incoming data (e.g., an SMS message arrived), to application events (e.g., your activity was launched from the device's main menu). Not only can you respond to intents, but you can create your own, to launch other activities, or to let you know when specific situations arise (e.g., raise such-and-so intent when the user gets within 100 meters of this-and-such location).

Services

Activities, content providers, and intent receivers are all short-lived and can be shut down at any time. Services, on the other hand, are designed to keep running, if needed, independent of any activity. You might use a service for checking for updates to an RSS feed, or to play back music even if the controlling activity is no longer operating.

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