Android Building Blocks

Android is designed to afford developers with the maximum amount of modularity possible. This

modularity makes it easy for developers to exchange functionality between their applications, which

is a central concept of the open source paradigm on which Android is based.

Application components are the essential building blocks of an Android application.

There are following five main components that can be used within an Android application:

Activities :

An Android activity contains a UI construct that accomplishes a given user-input task via the smartphone, tablet, e-reader, or iTV display screen.

An activity represents a single screen with a user interface. For example, an email application might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. If an application has more than one activity, then one of them should be marked as the activity that is presented when the application is launches.

Android applications can have more than one activity. In fact, more complex applications usually

have one activity for each UI screen implementation.

Intents :

Intents are asynchronous messages which allow application components to request functionality from other Android components. Intents allow you to interact with components from the own and other applications. 

For example an activity can start an external activity for taking a picture. Intents are objects of the android.content.Intent type.

Services : 

Unlike activities, services do not have any visual UI (that’s what an activity is for). Services handle

the processing or heavy lifting for your application.

A service is a component that runs in the background to perform long-running operations. For example, a service might play music in the background while the user is in a different application, or it might fetch data over the network without blocking user interaction with an activity.

Here are some examples of what service components can do:

•  Calculate complex numeric values
•  Process game logic in real time
•  Play new media elements such as video or audio files (local) or streams (remote)
•  Pull data from remote network locations (servers)
•  Transfer data between devices via Bluetooth

Broadcast Receivers :

Broadcast receivers are communication components that receive messages that are sent between

the Android operating system and other application components, or between Android application

components themselves.

For example, applications can also initiate broadcasts to let other applications know that some data has been downloaded to the device and is available for them to use, so this is broadcast receiver who will intercept this communication and will initiate appropriate action.

Content Providers :

Content providers in Android provide a way to make data available to your application and to other

applications, if that is desired. This can be data that is created in and for your own application, or it

can be data that can be accessed by your application, but that is created by other applications, or

even by the Android operating system utilities themselves to provide useful services to smartphone,

tablet, e-reader, and iTV users.The data may be stored in the file system, the database or somewhere else entirely.

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