Designing the user interface

 Android Intent is the message that is passed between components such as activities, content providers, broadcast receivers, services etc.

It is generally used with startActivity() method to invoke activity, broadcast receivers etc.

The dictionary meaning of intent is intention or purpose. So, it can be described as the intention to do action.

The LabeledIntent is the subclass of android.content.Intent class.

Android intents are mainly used to:

o Start the service

o Launch an activity

o Display a web page

o Display a list of contacts

o Broadcast a message

o Dial a phone call etc.

Types of Android Intents

1) Implicit Intent

Implicit Intent doesn't specify the component. In such case, intent provides information of available components provided by the system that is to be invoked.

For example, you may write the following code to view the webpage.

Intent intent=new Intent(Intent.ACTION_VIEW);  

                intent.setData(Uri.parse("http://www.javatpoint.com"));  

                startActivity(intent);  

2) Explicit Intent

Explicit Intent specifies the component. In such case, intent provides the external class to be invoked.

Intent i = new Intent(getApplicationContext(), ActivityTwo.class);  

startActivity(i);  

Activity Lifecycle

An activity represents a single screen with a user interface just like window or frame of Java.Android activity is the subclass of ContextThemeWrapper class.

If you have worked with C, C++ or Java programming language then you must have seen that your program starts from main() function. Very similar way, Android system initiates its program with in an Activity starting with a call on onCreate() callback method. There is a sequence of callback methods that start up an activity and a sequence of callback methods that tear down an activity as shown in the below Activity life cycle diagram: (image courtesy : android.com )

                

The Activity class defines the following call backs i.e. events. You don't need to implement all the callbacks methods. However, it's important that you understand each one and implement those that ensure your app behaves the way users expect.

Sr.No Callback & Description

1 onCreate()

This is the first callback and called when the activity is first created.

2 onStart()

This callback is called when the activity becomes visible to the user.

3 onResume()

This is called when the user starts interacting with the application.

4 onPause()

The paused activity does not receive user input and cannot execute any code and called when the current activity is being paused and the previous activity is being resumed.

5 onStop()

This callback is called when the activity is no longer visible.

6 onDestroy()

This callback is called before the activity is destroyed by the system.

7 onRestart()

This callback is called when the activity restarts after stopping it.

Activity manifest

The AndroidManifest.xml file contains information of your package, including components of the application such as activities, services, broadcast receivers, content providers etc.

It performs some other tasks also:

o It is responsible to protect the application to access any protected parts by providing the permissions.

o It also declares the android api that the application is going to use.

o It lists the instrumentation classes. The instrumentation classes provide profiling and other information’s. This information’s are removed just before the application is published etc.

This is the required xml file for all the android application and located inside the root directory.

Elements of the AndroidManifest.xml file

The elements used in the above xml file are described below.

<manifest>

manifest is the root element of the AndroidManifest.xml file. It has package attribute that describes the package name of the activity class.

<application>

application is the subelement of the manifest. It includes the namespace declaration. This element contains several subelements that declares the application component such as activity etc.

The commonly used attributes are of this element are icon, label, theme etc.

android:icon represents the icon for all the android application components.

android:label works as the default label for all the application components.

android:theme represents a common theme for all the android activities.

<activity>

activity is the subelement of application and represents an activity that must be defined in the AndroidManifest.xml file. It has many attributes such as label, name, theme, launchMode etc.

android:label represents a label i.e. displayed on the screen.

android:name represents a name for the activity class. It is required attribute.

<intent-filter>

intent-filter is the sub-element of activity that describes the type of intent to which activity, service or broadcast receiver can respond to.

<action>

It adds an action for the intent-filter. The intent-filter must have at least one action element.

<category>

It adds a category name to an intent-filter.

2.2 Views in Android