APPLICATIONS COMPONENT

2.APPLICATIONS COMPONENT

Application components are the essential building blocks of an Android application. These components are loosely coupled by the application manifest file AndroidManifest.xml that describes each component of the application and how they interact.

Activities 

They dictate the UI and handle the user interaction tothe smartphone screen

Services 

They handle background processing associated with an application.

Broadcast Receivers 

They handle communication between Android OS and applications.

Content Providers 

They handle data and database management issues.

Activities

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 one for reading emails. If an application has more than one activity, then one of them should be marked asthe activity that is presented when the application is launched. An activity is implemented as a subclass of Activity class as follows:

import 

public class MainActivity extends Activity
{


}

Services

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.

A service is implemented as a subclass of Service class as follows:

public class MyService extends Service
{


}

Broadcast Receivers

Broadcast Receivers simply respond to broadcast messages from other applications or from the system. 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. A broadcast receiver is implemented as a subclass of BroadcastReceiver class and each message is broadcasted as an Intent object.

public class MyReciver extends RroadCastRecevier
{


}

Content Providers

A content provider component supplies data from one application to others on request. Such requests are handled by the methods of the ContentResolver class. The data may be stored in the file system, the database or somewhere else entirely. A content provider is implemented as a subclass of ContentProvider class and must implement a standard set of APIs that enable other applications to perform transactions.

public class MyContentProvider extends ContentProvider
{



}

Fragment

A Fragment is a piece of an activity which enable more modular activity design. It will not be wrong if we say, a fragment is a kind of sub-activity.

Following are important points about fragment −

• A fragment has its own layout and its own behaviour with its own life cycle callbacks.
• You can add or remove fragments in an activity while the activity is running.
• You can combine multiple fragments in a single activity to build a multi-pane UI.
• A fragment can be used in multiple activities.
• Fragment life cycle is closely related to the life cycle of its host activity which means when the activity is paused, all the fragments available in the activity will also be stopped.
• A fragment can implement a behaviour that has no user interface component.
• Fragments were added to the Android API in Honeycomb version of Android which API version 11.

You create fragments by extending Fragment class and You can insert a fragment into your activity layout by declaring the fragment in the activity's layout file, as a <fragment> element.

Prior to fragment introduction, we had a limitation because we can show only a single activity on the screen at one given point in time. So we were not able to divide device screen and control different parts separately. But with the introduction of fragment we got more flexibility and removed the limitation of having a single activity on the screen at a time. Now we can have a single activity but each activity can comprise of multiple fragments which will have their own layout, events and complete life cycle.

Following is a typical example of how two UI modules defined by fragments can be combined into one activity for a tablet design, but separated for a handset design.