An Android Intent is an abstract description of an operation to be performed. It can be used with startActivity to launch an Activity, broadcastIntent to send it to any interested BroadcastReceiver components, and startService(Intent) or bindService(Intent, ServiceConnection, int) to communicate with a background Service.
The intent itself, an Intent object, is a passive data structure holding an abstract description of an operation to be performed.
For example, let's assume that you have an Activity that needs to launch an email client and sends an email using your Android device. For this purpose, your Activity would send an ACTION_SEND along with appropriate chooser, to the Android Intent Resolver. The specified chooser gives the proper interface for the user to pick how to send your email data.
Intent email = new Intent(Intent.ACTION_SEND, Uri.parse("mailto:"));
email.putExtra(Intent.EXTRA_EMAIL, recipients);
email.putExtra(Intent.EXTRA_SUBJECT, subject.getText().toString());
email.putExtra(Intent.EXTRA_TEXT, body.getText().toString());
startActivity(Intent.createChooser(email, "Choose an email client from..."));
Above syntax is calling startActivity method to start an email activity and result should be as shown below −
Send Email
For example, assume that you have an Activity that needs to open URL in a web browser on your Android device. For this purpose, your Activity will send ACTION_WEB_SEARCH Intent to the Android Intent Resolver to open given URL in the web browser. The Intent Resolver parses through a list of Activities and chooses the one that would best match your Intent, in this case, the Web Browser Activity. The Intent Resolver then passes your web page to the web browser and starts the Web Browser Activity.
String q = "tutorialspoint";
Intent intent = new Intent(Intent.ACTION_WEB_SEARCH );
intent.putExtra(SearchManager.QUERY, q);
startActivity(intent);
Above example will search as tutorialspoint on android search engine and it gives the result of tutorialspoint in your an activity
There are separate mechanisms for delivering intents to each type of component − activities, services, and broadcast receivers.
Sr.No Method & Description
1
Context.startActivity()
The Intent object is passed to this method to launch a new activity or get an existing activity to do something new.
2
Context.startService()
The Intent object is passed to this method to initiate a service or deliver new instructions to an ongoing service.
3
Context.sendBroadcast()
The Intent object is passed to this method to deliver the message to all interested broadcast receivers.
Intent Objects
An Intent object is a bundle of information which is used by the component that receives the intent as well as information used by the Android system.
An Intent object can contain the following components based on what it is communicating or going to perform −
Action
This is mandatory part of the Intent object and is a string naming the action to be performed — or, in the case of broadcast intents, the action that took place and is being reported. The action largely determines how the rest of the intent object is structured . The Intent class defines a number of action constants corresponding to different intents. Here is a list of Android Intent Standard Actions
The action in an Intent object can be set by the setAction() method and read by getAction().
Data
Adds a data specification to an intent filter. The specification can be just a data type (the mimeType attribute), just a URI, or both a data type and a URI. A URI is specified by separate attributes for each of its parts −
These attributes that specify the URL format are optional, but also mutually dependent −
If a scheme is not specified for the intent filter, all the other URI attributes are ignored.
If a host is not specified for the filter, the port attribute and all the path attributes are ignored.
The setData() method specifies data only as a URI, setType() specifies it only as a MIME type, and setDataAndType() specifies it as both a URI and a MIME type. The URI is read by getData() and the type by getType().
Some examples of action/data pairs are −
Sr.No. Action/Data Pair & Description
1
ACTION_VIEW content://contacts/people/1
Display information about the person whose identifier is "1".
2
ACTION_DIAL content://contacts/people/1
Display the phone dialer with the person filled in.
3
ACTION_VIEW tel:123
Display the phone dialer with the given number filled in.
4
ACTION_DIAL tel:123
Display the phone dialer with the given number filled in.
5
ACTION_EDIT content://contacts/people/1
Edit information about the person whose identifier is "1".
6
ACTION_VIEW content://contacts/people/
Display a list of people, which the user can browse through.
7
ACTION_SET_WALLPAPER
Show settings for choosing wallpaper
8
ACTION_SYNC
It going to be synchronous the data,Constant Value is android.intent.action.SYNC
9
ACTION_SYSTEM_TUTORIAL
It will start the platform-defined tutorial(Default tutorial or start up tutorial)
10
ACTION_TIMEZONE_CHANGED
It intimates when time zone has changed
11
ACTION_UNINSTALL_PACKAGE
It is used to run default uninstaller
Category
The category is an optional part of Intent object and it's a string containing additional information about the kind of component that should handle the intent. The addCategory() method places a category in an Intent object, removeCategory() deletes a category previously added, and getCategories() gets the set of all categories currently in the object. Here is a list of Android Intent Standard Categories.
You can check detail on Intent Filters in below section to understand how do we use categories to choose appropriate activity corresponding to an Intent.
Extras
This will be in key-value pairs for additional information that should be delivered to the component handling the intent. The extras can be set and read using the putExtras() and getExtras() methods respectively. Here is a list of Android Intent Standard Extra Data
Flags
These flags are optional part of Intent object and instruct the Android system how to launch an activity, and how to treat it after it's launched etc.
Sr.No Flags & Description
1
FLAG_ACTIVITY_CLEAR_TASK
If set in an Intent passed to Context.startActivity(), this flag will cause any existing task that would be associated with the activity to be cleared before the activity is started. That is, the activity becomes the new root of an otherwise empty task, and any old activities are finished. This can only be used in conjunction with FLAG_ACTIVITY_NEW_TASK.
2
FLAG_ACTIVITY_CLEAR_TOP
If set, and the activity being launched is already running in the current task, then instead of launching a new instance of that activity, all of the other activities on top of it will be closed and this Intent will be delivered to the (now on top) old activity as a new Intent.
3
FLAG_ACTIVITY_NEW_TASK
This flag is generally used by activities that want to present a "launcher" style behavior: they give the user a list of separate things that can be done, which otherwise run completely independently of the activity launching them.
Component Name
This optional field is an android ComponentName object representing either Activity, Service or BroadcastReceiver class. If it is set, the Intent object is delivered to an instance of the designated class otherwise Android uses other information in the Intent object to locate a suitable target.
The component name is set by setComponent(), setClass(), or setClassName() and read by getComponent().
Types of Intents
There are following two types of intents supported by Android
Intent
Explicit Intents
Explicit intent going to be connected internal world of application,suppose if you wants to connect one activity to another activity, we can do this quote by explicit intent, below image is connecting first activity to second activity by clicking button.
Explicit Intents
These intents designate the target component by its name and they are typically used for application-internal messages - such as an activity starting a subordinate service or launching a sister activity. For example −
// Explicit Intent by specifying its class name
Intent i = new Intent(FirstActivity.this, SecondActivity.class);
// Starts TargetActivity
startActivity(i);
Implicit Intents
These intents do not name a target and the field for the component name is left blank. Implicit intents are often used to activate components in other applications. For example −
Intent read1=new Intent();
read1.setAction(android.content.Intent.ACTION_VIEW);
read1.setData(ContactsContract.Contacts.CONTENT_URI);
startActivity(read1);
Above code will give result as shown below
Intent
The target component which receives the intent can use the getExtras() method to get the extra data sent by the source component. For example −
// Get bundle object at appropriate place in your code
Bundle extras = getIntent().getExtras();
// Extract data using passed keys
String value1 = extras.getString("Key1");
String value2 = extras.getString("Key2");
Example
Following example shows the functionality of a Android Intent to launch various Android built-in applications.
Step Description
1 You will use Android studio IDE to create an Android application and name it as My Application under a package com.example.saira_000.myapplication.
2 Modify src/main/java/MainActivity.java file and add the code to define two listeners corresponding two buttons ie. Start Browser and Start Phone.
3 Modify layout XML file res/layout/activity_main.xml to add three buttons in linear layout.
4 Run the application to launch Android emulator and verify the result of the changes done in the application.
Following is the content of the modified main activity file src/com.example.My Application/MainActivity.java.
package com.example.saira_000.myapplication;
import android.content.Intent;
import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
Button b1,b2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
b1=(Button)findViewById(R.id.button);
b1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent i = new Intent(android.content.Intent.ACTION_VIEW,
Uri.parse("http://www.example.com"));
startActivity(i);
}
});
b2=(Button)findViewById(R.id.button2);
b2.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent i = new Intent(android.content.Intent.ACTION_VIEW,
Uri.parse("tel:9510300000"));
startActivity(i);
}
});
}
}
Following will be the content of res/layout/activity_main.xml file −
Following will be the content of res/values/strings.xml to define two new constants −
My Applicaiton
Following is the default content of AndroidManifest.xml −
Let's try to run your My Application application. I assume you had created your AVD while doing environment setup. To run the app from Android Studio, open one of your project's activity files and click Run Eclipse Run Icon icon from the toolbar.Android Studio installs the app on your AVD and starts it and if everything is fine with your setup and application, it will display following Emulator window −
Android Intent Screen
Now click on Start Browser button, which will start a browser configured and display http://www.example.com as shown below −
Android Intent Browser
Similar way you can launch phone interface using Start Phone button, which will allow you to dial already given phone number.
Intent Filters
You have seen how an Intent has been used to call an another activity. Android OS uses filters to pinpoint the set of Activities, Services, and Broadcast receivers that can handle the Intent with help of specified set of action, categories, data scheme associated with an Intent. You will use element in the manifest file to list down actions, categories and data types associated with any activity, service, or broadcast receiver.
Following is an example of a part of AndroidManifest.xml file to specify an activity com.example.My Application.CustomActivity which can be invoked by either of the two mentioned actions, one category, and one data −
Once this activity is defined along with above mentioned filters, other activities will be able to invoke this activity using either the android.intent.action.VIEW, or using the com.example.My Application.LAUNCH action provided their category is android.intent.category.DEFAULT.
The element specifies the data type expected by the activity to be called and for above example our custom activity expects the data to start with the "http://"
There may be a situation that an intent can pass through the filters of more than one activity or service, the user may be asked which component to activate. An exception is raised if no target can be found.
There are following test Android checks before invoking an activity −
A filter may list more than one action as shown above but this list cannot be empty; a filter must contain at least one element, otherwise it will block all intents. If more than one actions are mentioned then Android tries to match one of the mentioned actions before invoking the activity.
A filter may list zero, one or more than one categories. if there is no category mentioned then Android always pass this test but if more than one categories are mentioned then for an intent to pass the category test, every category in the Intent object must match a category in the filter.
Each element can specify a URI and a data type (MIME media type). There are separate attributes like scheme, host, port, and path for each part of the URI. An Intent object that contains both a URI and a data type passes the data type part of the test only if its type matches a type listed in the filter.
Wednesday, 17 October 2018
Android - Fragments
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 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.
Android Fragment
The application can embed two fragments in Activity A, when running on a tablet-sized device. However, on a handset-sized screen, there's not enough room for both fragments, so Activity A includes only the fragment for the list of articles, and when the user selects an article, it starts Activity B, which includes the second fragment to read the article.
Fragment Life Cycle
Android fragments have their own life cycle very similar to an android activity. This section briefs different stages of its life cycle.
Fragment
FRAGMENT LIFECYCLE
Here is the list of methods which you can to override in your fragment class −
onAttach()The fragment instance is associated with an activity instance.The fragment and the activity is not fully initialized. Typically you get in this method a reference to the activity which uses the fragment for further initialization work.
onCreate() The system calls this method when creating the fragment. You should initialize essential components of the fragment that you want to retain when the fragment is paused or stopped, then resumed.
onCreateView() The system calls this callback when it's time for the fragment to draw its user interface for the first time. To draw a UI for your fragment, you must return a View component from this method that is the root of your fragment's layout. You can return null if the fragment does not provide a UI.
onActivityCreated()The onActivityCreated() is called after the onCreateView() method when the host activity is created. Activity and fragment instance have been created as well as the view hierarchy of the activity. At this point, view can be accessed with the findViewById() method. example. In this method you can instantiate objects which require a Context object
onStart()The onStart() method is called once the fragment gets visible.
onResume()Fragment becomes active.
onPause() The system calls this method as the first indication that the user is leaving the fragment. This is usually where you should commit any changes that should be persisted beyond the current user session.
onStop()Fragment going to be stopped by calling onStop()
onDestroyView()Fragment view will destroy after call this method
onDestroy()onDestroy() called to do final clean up of the fragment's state but Not guaranteed to be called by the Android platform.
How to use Fragments?
This involves number of simple steps to create Fragments.
First of all decide how many fragments you want to use in an activity. For example let's we want to use two fragments to handle landscape and portrait modes of the device.
Next based on number of fragments, create classes which will extend the Fragment class. The Fragment class has above mentioned callback functions. You can override any of the functions based on your requirements.
Corresponding to each fragment, you will need to create layout files in XML file. These files will have layout for the defined fragments.
Finally modify activity file to define the actual logic of replacing fragments based on your requirement.
Types of Fragments
Basically fragments are divided as three stages as shown below.
Single frame fragments − Single frame fragments are using for hand hold devices like mobiles, here we can show only one fragment as a view.
List fragments − fragments having special list view is called as list fragment
Fragments transaction − Using with fragment transaction. we can move one fragment to another fragment.
Content Provider
A content provider component supplies data from one application to others on request. Such requests are handled by the methods of the ContentResolver class. A content provider can use different ways to store its data and the data can be stored in a database, in files, or even over a network.
content provider
CONTENTPROVIDER
sometimes it is required to share data across applications. This is where content providers become very useful.
Content providers let you centralize content in one place and have many different applications access it as needed. A content provider behaves very much like a database where you can query it, edit its content, as well as add or delete content using insert(), update(), delete(), and query() methods. In most cases this data is stored in an SQlite database.
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 My Application extends ContentProvider {
}
Content URIs
To query a content provider, you specify the query string in the form of a URI which has following format −
:////
Here is the detail of various parts of the URI −
Sr.No Part & Description
1
prefix
This is always set to content://
2
authority
This specifies the name of the content provider, for example contacts, browser etc. For third-party content providers, this could be the fully qualified name, such as com.tutorialspoint.statusprovider
3
data_type
This indicates the type of data that this particular provider provides. For example, if you are getting all the contacts from the Contacts content provider, then the data path would be people and URI would look like thiscontent://contacts/people
4
id
This specifies the specific record requested. For example, if you are looking for contact number 5 in the Contacts content provider then URI would look like this content://contacts/people/5.
Create Content Provider
This involves number of simple steps to create your own content provider.
First of all you need to create a Content Provider class that extends the ContentProviderbaseclass.
Second, you need to define your content provider URI address which will be used to access the content.
Next you will need to create your own database to keep the content. Usually, Android uses SQLite database and framework needs to override onCreate() method which will use SQLite Open Helper method to create or open the provider's database. When your application is launched, the onCreate() handler of each of its Content Providers is called on the main application thread.
Next you will have to implement Content Provider queries to perform different database specific operations.
Finally register your Content Provider in your activity file using tag.
Here is the list of methods which you need to override in Content Provider class to have your Content Provider working −
content provider
CONTENTPROVIDER
onCreate() This method is called when the provider is started.
query() This method receives a request from a client. The result is returned as a Cursor object.
insert()This method inserts a new record into the content provider.
delete() This method deletes an existing record from the content provider.
update() This method updates an existing record from the content provider.
getType() This method returns the MIME type of the data at the given URI.
Example
This example will explain you how to create your own ContentProvider. So let's follow the following steps to similar to what we followed while creating Hello World Example−
Step Description
1 You will use Android StudioIDE to create an Android application and name it as My Application under a package com.example.MyApplication, with blank Activity.
2 Modify main activity file MainActivity.java to add two new methods onClickAddName() and onClickRetrieveStudents().
3 Create a new java file called StudentsProvider.java under the package com.example.MyApplication to define your actual provider and associated methods.
4 Register your content provider in your AndroidManifest.xml file using STUDENTS_PROJECTION_MAP;
static final int STUDENTS = 1;
static final int STUDENT_ID = 2;
static final UriMatcher uriMatcher;
static{
uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
uriMatcher.addURI(PROVIDER_NAME, "students", STUDENTS);
uriMatcher.addURI(PROVIDER_NAME, "students/#", STUDENT_ID);
}
/**
* Database specific constant declarations
*/
private SQLiteDatabase db;
static final String DATABASE_NAME = "College";
static final String STUDENTS_TABLE_NAME = "students";
static final int DATABASE_VERSION = 1;
static final String CREATE_DB_TABLE =
" CREATE TABLE " + STUDENTS_TABLE_NAME +
" (_id INTEGER PRIMARY KEY AUTOINCREMENT, " +
" name TEXT NOT NULL, " +
" grade TEXT NOT NULL);";
/**
* Helper class that actually creates and manages
* the provider's underlying data repository.
*/
private static class DatabaseHelper extends SQLiteOpenHelper {
DatabaseHelper(Context context){
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(CREATE_DB_TABLE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS " + STUDENTS_TABLE_NAME);
onCreate(db);
}
}
@Override
public boolean onCreate() {
Context context = getContext();
DatabaseHelper dbHelper = new DatabaseHelper(context);
/**
* Create a write able database which will trigger its
* creation if it doesn't already exist.
*/
db = dbHelper.getWritableDatabase();
return (db == null)? false:true;
}
@Override
public Uri insert(Uri uri, ContentValues values) {
/**
* Add a new student record
*/
long rowID = db.insert( STUDENTS_TABLE_NAME, "", values);
/**
* If record is added successfully
*/
if (rowID > 0) {
Uri _uri = ContentUris.withAppendedId(CONTENT_URI, rowID);
getContext().getContentResolver().notifyChange(_uri, null);
return _uri;
}
throw new SQLException("Failed to add a record into " + uri);
}
@Override
public Cursor query(Uri uri, String[] projection,
String selection,String[] selectionArgs, String sortOrder) {
SQLiteQueryBuilder qb = new SQLiteQueryBuilder();
qb.setTables(STUDENTS_TABLE_NAME);
switch (uriMatcher.match(uri)) {
case STUDENTS:
qb.setProjectionMap(STUDENTS_PROJECTION_MAP);
break;
case STUDENT_ID:
qb.appendWhere( _ID + "=" + uri.getPathSegments().get(1));
break;
default:
}
if (sortOrder == null || sortOrder == ""){
/**
* By default sort on student names
*/
sortOrder = NAME;
}
Cursor c = qb.query(db, projection, selection,
selectionArgs,null, null, sortOrder);
/**
* register to watch a content URI for changes
*/
c.setNotificationUri(getContext().getContentResolver(), uri);
return c;
}
@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
int count = 0;
switch (uriMatcher.match(uri)){
case STUDENTS:
count = db.delete(STUDENTS_TABLE_NAME, selection, selectionArgs);
break;
case STUDENT_ID:
String id = uri.getPathSegments().get(1);
count = db.delete( STUDENTS_TABLE_NAME, _ID + " = " + id +
(!TextUtils.isEmpty(selection) ? "
AND (" + selection + ')' : ""), selectionArgs);
break;
default:
throw new IllegalArgumentException("Unknown URI " + uri);
}
getContext().getContentResolver().notifyChange(uri, null);
return count;
}
@Override
public int update(Uri uri, ContentValues values,
String selection, String[] selectionArgs) {
int count = 0;
switch (uriMatcher.match(uri)) {
case STUDENTS:
count = db.update(STUDENTS_TABLE_NAME, values, selection, selectionArgs);
break;
case STUDENT_ID:
count = db.update(STUDENTS_TABLE_NAME, values,
_ID + " = " + uri.getPathSegments().get(1) +
(!TextUtils.isEmpty(selection) ? "
AND (" +selection + ')' : ""), selectionArgs);
break;
default:
throw new IllegalArgumentException("Unknown URI " + uri );
}
getContext().getContentResolver().notifyChange(uri, null);
return count;
}
@Override
public String getType(Uri uri) {
switch (uriMatcher.match(uri)){
/**
* Get all student records
*/
case STUDENTS:
return "vnd.android.cursor.dir/vnd.example.students";
/**
* Get a particular student
*/
case STUDENT_ID:
return "vnd.android.cursor.item/vnd.example.students";
default:
throw new IllegalArgumentException("Unsupported URI: " + uri);
}
}
}
Following will the modified content of AndroidManifest.xml file. Here we have added
Following will be the content of res/layout/activity_main.xml file−
Make sure you have following content of res/values/strings.xml file:
My Application
;
Let's try to run our modified My Application application we just created. I assume you had created your AVD while doing environment set-up. To run the app from Android Studio IDE, open one of your project's activity files and click Run Android StudioRun Icon icon from the tool bar. Android Studio installs the app on your AVD and starts it and if everything is fine with your set-up and application, it will display following Emulator window, be patience because it may take sometime based on your computer speed −
Android Content Provider Demo
Now let's enter student Name and Grade and finally click on Add Name button, this will add student record in the database and will flash a message at the bottom showing ContentProvider URI along with record number added in the database. This operation makes use of our insert() method. Let's repeat this process to add few more students in the database of our content provider.
Add Records using ContentProvider
Once you are done with adding records in the database, now its time to ask ContentProvider to give us those records back, so let's click Retrieve Students button which will fetch and display all the records one by one which is as per our the implementation of our query() method.
You can write activities against update and delete operations by providing callback functions in MainActivity.java file and then modify user interface to have buttons for update and deleted operations in the same way as we have done for add and read operations.
This way you can use existing Content Provider like Address Book or you can use Content Provider concept in developing nice database oriented applications where you can perform all sort of database operations like read, write, update and delete as explained above in the example.
Broadcast Receivers
Android -
Broadcast Receivers simply respond to broadcast messages from other applications or from the system itself. These messages are sometime called events or intents. 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.
There are following two important steps to make BroadcastReceiver works for the system broadcasted intents −
Creating the Broadcast Receiver.
Registering Broadcast Receiver
There is one additional steps in case you are going to implement your custom intents then you will have to create and broadcast those intents.
Creating the Broadcast Receiver
A broadcast receiver is implemented as a subclass of BroadcastReceiver class and overriding the onReceive() method where each message is received as a Intent object parameter.
public class MyReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
Toast.makeText(context, "Intent Detected.", Toast.LENGTH_LONG).show();
}
}
Registering Broadcast Receiver
An application listens for specific broadcast intents by registering a broadcast receiver in AndroidManifest.xml file. Consider we are going to register MyReceiver for system generated event ACTION_BOOT_COMPLETED which is fired by the system once the Android system has completed the boot process.
broadcast
BROADCAST-RECEIVER
Now whenever your Android device gets booted, it will be intercepted by BroadcastReceiver MyReceiver and implemented logic inside onReceive() will be executed.
There are several system generated events defined as final static fields in the Intent class. The following table lists a few important system events.
Sr.No Event Constant & Description
1
android.intent.action.BATTERY_CHANGED
Sticky broadcast containing the charging state, level, and other information about the battery.
2
android.intent.action.BATTERY_LOW
Indicates low battery condition on the device.
3
android.intent.action.BATTERY_OKAY
Indicates the battery is now okay after being low.
4
android.intent.action.BOOT_COMPLETED
This is broadcast once, after the system has finished booting.
5
android.intent.action.BUG_REPORT
Show activity for reporting a bug.
6
android.intent.action.CALL
Perform a call to someone specified by the data.
7
android.intent.action.CALL_BUTTON
The user pressed the "call" button to go to the dialer or other appropriate UI for placing a call.
8
android.intent.action.DATE_CHANGED
The date has changed.
9
android.intent.action.REBOOT
Have the device reboot.
Broadcasting Custom Intents
If you want your application itself should generate and send custom intents then you will have to create and send those intents by using the sendBroadcast() method inside your activity class. If you use the sendStickyBroadcast(Intent) method, the Intent is sticky, meaning the Intent you are sending stays around after the broadcast is complete.
public void broadcastIntent(View view) {
Intent intent = new Intent();
intent.setAction("com.tutorialspoint.CUSTOM_INTENT");
sendBroadcast(intent);
}
This intent com.tutorialspoint.CUSTOM_INTENT can also be registered in similar way as we have regsitered system generated intent.
Example
This example will explain you how to create BroadcastReceiver to intercept custom intent. Once you are familiar with custom intent, then you can program your application to intercept system generated intents. So let's follow the following steps to modify the Android application we created in Hello World Example chapter −
Step Description
1 You will use Android studio to create an Android application and name it as My Application under a package com.example.tutorialspoint7.myapplication as explained in the Hello World Example chapter.
2 Modify main activity file MainActivity.java to add broadcastIntent() method.
3 Create a new java file called MyReceiver.java under the package com.example.tutorialspoint7.myapplication to define a BroadcastReceiver.
4 An application can handle one or more custom and system intents without any restrictions. Every intent you want to intercept must be registered in your AndroidManifest.xml file using
Following will be the content of res/layout/activity_main.xml file to include a button to broadcast our custom intent −
Let's try to run our modified Hello World! application we just modified. I assume you had created your AVD while doing environment set-up. To run the app from Android studio, open one of your project's activity files and click Run Eclipse Run Icon icon from the tool bar. Android Studio installs the app on your AVD and starts it and if everything is fine with your set-up and application, it will display following Emulator window −
Android Broadcast Demo
Now to broadcast our custom intent, let's click on Broadcast Intent button, this will broadcast our custom intent "com.tutorialspoint.CUSTOM_INTENT" which will be intercepted by our registered BroadcastReceiver i.e. MyReceiver and as per our implemented logic a toast will appear on the bottom of the the simulator as follows −
Android Broadcast Intent
You can try implementing other BroadcastReceiver to intercept system generated intents like system boot up, date changed, low battery etc.br />
Android - Services
A service is a component that runs in the background to perform long-running operations without needing to interact with the user and it works even if application is destroyed. A service can essentially take two states −
Sr.No. State & Description
1
Started
A service is started when an application component, such as an activity, starts it by calling startService(). Once started, a service can run in the background indefinitely, even if the component that started it is destroyed.
2
Bound
A service is bound when an application component binds to it by calling bindService(). A bound service offers a client-server interface that allows components to interact with the service, send requests, get results, and even do so across processes with interprocess communication (IPC).
A service has life cycle callback methods that you can implement to monitor changes in the service's state and you can perform work at the appropriate stage. The following diagram on the left shows the life cycle when the service is created with startService() and the diagram on the right shows the life cycle when the service is created with bindService(): (image courtesy : android.com )
Android Service lifecycle
To create an service, you create a Java class that extends the Service base class or one of its existing subclasses. The Service base class defines various callback methods and the most important are given below. 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
onStartCommand()
The system calls this method when another component, such as an activity, requests that the service be started, by calling startService(). If you implement this method, it is your responsibility to stop the service when its work is done, by calling stopSelf() or stopService() methods.
2
onBind()
The system calls this method when another component wants to bind with the service by calling bindService(). If you implement this method, you must provide an interface that clients use to communicate with the service, by returning an IBinder object. You must always implement this method, but if you don't want to allow binding, then you should return null.
3
onUnbind()
The system calls this method when all clients have disconnected from a particular interface published by the service.
4
onRebind()
The system calls this method when new clients have connected to the service, after it had previously been notified that all had disconnected in its onUnbind(Intent).
5
onCreate()
The system calls this method when the service is first created using onStartCommand() or onBind(). This call is required to perform one-time set-up.
6
onDestroy()
The system calls this method when the service is no longer used and is being destroyed. Your service should implement this to clean up any resources such as threads, registered listeners, receivers, etc.
The following skeleton service demonstrates each of the life cycle methods −
package com.tutorialspoint;
import android.app.Service;
import android.os.IBinder;
import android.content.Intent;
import android.os.Bundle;
public class HelloService extends Service {
/** indicates how to behave if the service is killed */
int mStartMode;
/** interface for clients that bind */
IBinder mBinder;
/** indicates whether onRebind should be used */
boolean mAllowRebind;
/** Called when the service is being created. */
@Override
public void onCreate() {
}
/** The service is starting, due to a call to startService() */
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
return mStartMode;
}
/** A client is binding to the service with bindService() */
@Override
public IBinder onBind(Intent intent) {
return mBinder;
}
/** Called when all clients have unbound with unbindService() */
@Override
public boolean onUnbind(Intent intent) {
return mAllowRebind;
}
/** Called when a client is binding to the service with bindService()*/
@Override
public void onRebind(Intent intent) {
}
/** Called when The service is no longer used and is being destroyed */
@Override
public void onDestroy() {
}
}
Example
This example will take you through simple steps to show how to create your own Android Service. Follow the following steps to modify the Android application we created in Hello World Example chapter −
Step Description
1 You will use Android StudioIDE to create an Android application and name it as My Application under a package com.example.tutorialspoint7.myapplication as explained in the Hello World Example chapter.
2 Modify main activity file MainActivity.java to add startService() and stopService() methods.
3 Create a new java file MyService.java under the package com.example.My Application. This file will have implementation of Android service related methods.
4 Define your service in AndroidManifest.xml file using
Following will be the content of res/layout/activity_main.xml file to include two buttons −
Let's try to run our modified Hello World! application we just modified. I assume you had created your AVD while doing environment setup. To run the app from Android studio, open one of your project's activity files and click Run Android StudioRun Icon icon from the tool bar. Android Studio installs the app on your AVD and starts it and if everything is fine with your set-up and application, it will display following Emulator window −
Android Service Demo
Now to start your service, let's click on Start Service button, this will start the service and as per our programming in onStartCommand() method, a message Service Started will appear on the bottom of the the simulator as follows −
Android Service Start
To stop the service, you can click the Stop Service button.
Android - Activities
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 )
Android Activity lifecycle
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.
Example
This example will take you through simple steps to show Android application activity life cycle. Follow the following steps to modify the Android application we created in Hello World Example chapter −
Step Description
1 You will use Android studio to create an Android application and name it as HelloWorld under a package com.example.helloworld as explained in the Hello World Example chapter.
2 Modify main activity file MainActivity.java as explained below. Keep rest of the files unchanged.
3 Run the application to launch Android emulator and verify the result of the changes done in the application.
Following is the content of the modified main activity file src/com.example.helloworld/MainActivity.java. This file includes each of the fundamental life cycle methods. The Log.d() method has been used to generate log messages −
package com.example.helloworld;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
public class MainActivity extends Activity {
String msg = "Android : ";
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.d(msg, "The onCreate() event");
}
/** Called when the activity is about to become visible. */
@Override
protected void onStart() {
super.onStart();
Log.d(msg, "The onStart() event");
}
/** Called when the activity has become visible. */
@Override
protected void onResume() {
super.onResume();
Log.d(msg, "The onResume() event");
}
/** Called when another activity is taking focus. */
@Override
protected void onPause() {
super.onPause();
Log.d(msg, "The onPause() event");
}
/** Called when the activity is no longer visible. */
@Override
protected void onStop() {
super.onStop();
Log.d(msg, "The onStop() event");
}
/** Called just before the activity is destroyed. */
@Override
public void onDestroy() {
super.onDestroy();
Log.d(msg, "The onDestroy() event");
}
}
An activity class loads all the UI component using the XML file available in res/layout folder of the project. Following statement loads UI components from res/layout/activity_main.xml file:
setContentView(R.layout.activity_main);
An application can have one or more activities without any restrictions. Every activity you define for your application must be declared in your AndroidManifest.xml file and the main activity for your app must be declared in the manifest with an that includes the MAIN action and LAUNCHER category as follows:
If either the MAIN action or LAUNCHER category are not declared for one of your activities, then your app icon will not appear in the Home screen's list of apps.
Let's try to run our modified Hello World! application we just modified. I assume you had created your AVD while doing environment setup. To run the app from Android studio, open one of your project's activity files and click Run Eclipse Run Icon icon from the toolbar. Android studio installs the app on your AVD and starts it and if everything is fine with your setup and application, it will display Emulator window and you should see following log messages in LogCat window in Android studio −
08-23 10:32:07.682 4480-4480/com.example.helloworld D/Android :: The onCreate() event
08-23 10:32:07.683 4480-4480/com.example.helloworld D/Android :: The onStart() event
08-23 10:32:07.685 4480-4480/com.example.helloworld D/Android :: The onResume() event
Android LotCat Window
Let us try to click lock screen button on the Android emulator and it will generate following events messages in LogCat window in android studio:
08-23 10:32:53.230 4480-4480/com.example.helloworld D/Android :: The onPause() event
08-23 10:32:53.294 4480-4480/com.example.helloworld D/Android :: The onStop() event
Let us again try to unlock your screen on the Android emulator and it will generate following events messages in LogCat window in Android studio:
08-23 10:34:41.390 4480-4480/com.example.helloworld D/Android :: The onStart() event
08-23 10:34:41.392 4480-4480/com.example.helloworld D/Android :: The onResume() event
Next, let us again try to click Back button Android Back Button on the Android emulator and it will generate following events messages in LogCat window in Android studio and this completes the Activity Life Cycle for an Android Application.
Android Resources Organizing & Accessing
There are many more items which you use to build a good Android application. Apart from coding for the application, you take care of various other resources like static content that your code uses, such as bitmaps, colors, layout definitions, user interface strings, animation instructions, and more. These resources are always maintained separately in various sub-directories under res/ directory of the project.
This tutorial will explain you how you can organize your application resources, specify alternative resources and access them in your applications.
Organize resource in Android Studio
MyProject/
app/
manifest/
AndroidManifest.xml
java/
MyActivity.java
res/
drawable/
icon.png
layout/
activity_main.xml
info.xml
values/
strings.xml
Sr.No. Directory & Resource Type
1
anim/
XML files that define property animations. They are saved in res/anim/ folder and accessed from the R.anim class.
2
color/
XML files that define a state list of colors. They are saved in res/color/ and accessed from the R.color class.
3
drawable/
Image files like .png, .jpg, .gif or XML files that are compiled into bitmaps, state lists, shapes, animation drawable. They are saved in res/drawable/ and accessed from the R.drawable class.
4
layout/
XML files that define a user interface layout. They are saved in res/layout/ and accessed from the R.layout class.
5
menu/
XML files that define application menus, such as an Options Menu, Context Menu, or Sub Menu. They are saved in res/menu/ and accessed from the R.menu class.
6
raw/
Arbitrary files to save in their raw form. You need to call Resources.openRawResource() with the resource ID, which is R.raw.filename to open such raw files.
7
values/
XML files that contain simple values, such as strings, integers, and colors. For example, here are some filename conventions for resources you can create in this directory −
arrays.xml for resource arrays, and accessed from the R.array class.
integers.xml for resource integers, and accessed from the R.integer class.
bools.xml for resource boolean, and accessed from the R.bool class.
colors.xml for color values, and accessed from the R.color class.
dimens.xml for dimension values, and accessed from the R.dimen class.
strings.xml for string values, and accessed from the R.string class.
styles.xml for styles, and accessed from the R.style class.
8
xml/
Arbitrary XML files that can be read at runtime by calling Resources.getXML(). You can save various configuration files here which will be used at run time.
Alternative Resources
Your application should provide alternative resources to support specific device configurations. For example, you should include alternative drawable resources ( i.e.images ) for different screen resolution and alternative string resources for different languages. At runtime, Android detects the current device configuration and loads the appropriate resources for your application.
To specify configuration-specific alternatives for a set of resources, follow the following steps −
Create a new directory in res/ named in the form -. Here resources_name will be any of the resources mentioned in the above table, like layout, drawable etc. The qualifier will specify an individual configuration for which these resources are to be used. You can check official documentation for a complete list of qualifiers for different type of resources.
Save the respective alternative resources in this new directory. The resource files must be named exactly the same as the default resource files as shown in the below example, but these files will have content specific to the alternative. For example though image file name will be same but for high resolution screen, its resolution will be high.
Below is an example which specifies images for a default screen and alternative images for high resolution screen.
MyProject/
app/
manifest/
AndroidManifest.xml
java/
MyActivity.java
res/
drawable/
icon.png
background.png
drawable-hdpi/
icon.png
background.png
layout/
activity_main.xml
info.xml
values/
strings.xml
Below is another example which specifies layout for a default language and alternative layout for Arabic language.
MyProject/
app/
manifest/
AndroidManifest.xml
java/
MyActivity.java
res/
drawable/
icon.png
background.png
drawable-hdpi/
icon.png
background.png
layout/
activity_main.xml
info.xml
layout-ar/
main.xml
values/
strings.xml
Accessing Resources
During your application development you will need to access defined resources either in your code, or in your layout XML files. Following section explains how to access your resources in both the scenarios −
Accessing Resources in Code
When your Android application is compiled, a R class gets generated, which contains resource IDs for all the resources available in your res/ directory. You can use R class to access that resource using sub-directory and resource name or directly resource ID.
Example
To access res/drawable/myimage.png and set an ImageView you will use following code −
ImageView imageView = (ImageView) findViewById(R.id.myimageview);
imageView.setImageResource(R.drawable.myimage);
Here first line of the code make use of R.id.myimageview to get ImageView defined with id myimageview in a Layout file. Second line of code makes use of R.drawable.myimage to get an image with name myimage available in drawable sub-directory under /res.
Example
Consider next example where res/values/strings.xml has following definition −
Hello, World!
Now you can set the text on a TextView object with ID msg using a resource ID as follows −
TextView msgTextView = (TextView) findViewById(R.id.msg);
msgTextView.setText(R.string.hello);
Example
Consider a layout res/layout/activity_main.xml with the following definition −
This application code will load this layout for an Activity, in the onCreate() method as follows −
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
Accessing Resources in XML
Consider the following resource XML res/values/strings.xml file that includes a color resource and a string resource −
#f00
Hello!
Now you can use these resources in the following layout file to set the text color and text string as follows −
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