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Intent
An Intent describes the operation to be
performed. Intents are used to start an
activity using either of the following
methods – Context.startActivity() or
Activity.startActivityForResult()
Additionally, Intents are used to interact
with services and broadcast receivers.
From an application perspective, the
following diagram shows how Intents
interact with activities, services and
broadcast receivers.
An intent is an Intent object with a message content.
Activities, services and broadcast receivers are started by intents.
An activity is started by Context.startActivity(Intent intent) or
Activity.startActivityForResult(Intent intent, int RequestCode)
A service is started by Context.startService(Intent service)
An application can initiate a broadcast by using an Intent in any of
Context.sendBroadcast(Intent intent), Context.sendOrderedBroadcast(), and
Context.sendStickyBroadcast()
What are Intent Objects?
An Intent object is collection (bundle) of information that needs to be passed to an activity. It typically
contains
*name of the activity that should handle the intent, (e.g. Module2Activity in our earlier example) and
the package in which that component resides e.g. com.learncomputer
*Intent action which describes what operation needs to be done. For activities, the possible intent
actions include the following:
ACTION_CALL – make a phone call
ACTION_EDIT – show data on screen for user editing
ACTION_MAIN – start the main activity
ACTION_SYNC – synchronize device data with server
*Data which is the information required to complete the operation described above. It would typically
be a phone number (for calling), displaying text data (for user editing) etc.
*Category which helps to identify the target activity for intent. Typical categories include
CATEGORY_HOME (return user to home screen), CATEGORY_PREFERENCES (open the preference panel),
CATEGORY_BROWSABLE (invoke browser to open a web page, render an image or load an email
message) etc.
*Extras which provide a means to pass additional info in the form of key-value pairs
Flags determining the various aspects of target activity e.g. whether it should be added to the list of
recent activities.
Types of Intents
Intents can be implicit and explicit.
Explicit intents are identifiable by name and these are typically
used by activities for internal messaging, e.g. to start another
activity.
On the other hand, implicit intents are handled by the Android
system and it in turn chooses the best suited activity to perform
the required action as stated in the intent.
Intent Filters
Intent Filters are used with implicit intents. Activities
which want to handle implicit intents will associate
themselves with Intent Filters. As the name itself
suggests, an Intent Filter essentially signifies what types
of intents an activity would like to receive and which ones
it would like to filter.
Creating Intent Filters
Intent Filters are defined in the manifest file using the
<intent-filter> tag. An Intent Filter has fields which map
one to one with the action, data, and category fields of an
Intent object. For intent to be delivered to an implicit
activity, it must pass all three criteria of action, data and
category
Intent Filter and its associated fields (action,
data and category)
<intent-filter>
<action android:name="com.learncomputer.Module2.SHOW_CURRENT" />
<category android:name="android.intent.category.BROWSABLE" />
<data android:mimeType="audio/mpeg" android:scheme="http" />
</intent-filter>
As per the above Intent Filter rules, the target category must act upon the SHOW_CURRENT action,
must belong to the BROWSABLE category and finally use the data specification (audio/ mpeg) as per the
defined scheme.
In case, intent passes through multiple filters and there are multiple target activities eligible to receive
that intent, it’s developer’s responsibility to decide which activity is to be used.
An intent is an Intent object with a message content.
Activities, services and broadcast receivers are started by intents. ContentProviders are started by
ContentResolvers:
An activity is started by Context.startActivity(Intent intent) or Activity.startActivityForResult(Intent
intent, int RequestCode)
A service is started by Context.startService(Intent service)
An application can initiate a broadcast by using an Intent in any of Context.sendBroadcast(Intent intent),
Context.sendOrderedBroadcast(), and Context.sendStickyBroadcast()
Intent
• Intent object is a messaging object(instance of the
android.content.Intent class) which is used to
signal/activate other components such as activities,
services, and broadcast receivers. E.g., an activity B can
register with intent A’s intent so that the bound activity
B will be notified and activated when the event fires.
• Intent is capable to include multiple components to
perform certain tasks.
• An Intent is sent to the Android OS system and the
Android system will decide which target (for implicit
intent) can do the job best.
• An Intent can also contain data. This data can be used
by the receiving component.
Explicit intent
An explicit intent tells Android system to run
specific component such as ActivityB
Intent intent = new Intent(ActivityA.this,
ActivityB.class);
startActivity(intent);
Implicit intent
Implicit Intents
Implicit intents specify the action which should be performed and optionally data
which provides content for the action.
For example, the following tells the Android system to view a webpage. All installed
web browsers should be registered to the corresponding intent data via an intent
filter.
Intent i = new Intent(Intent.ACTION_VIEW, Uri.parse("http://www.vogella.com"));
startActivity(i);
If an implicit intent is sent to the Android system, it searches for all components which
are registered for the specific action and the fitting data type.
If only one component is found, Android starts this component directly. If several
components are identified by the Android system, the user will get a selection dialog
and can decide which component should be used for the intent
Explicit intent vs. Implicit intent
Android Intents to trigger actions in your own apps (explicit) or some other third party app (implicit where Android
resolves matching intents)
Not specify the name of the target component Intent is called the "hidden-Intent. The IntentFilter Android system will
find the implicit Intent object.
Explicit Intent directly define the target component with the name of the component, this approach is very direct.
However, developers often do not know the other components of the application name, an explicit Intent more used to
deliver the message within the application.
For example, in an application, an Activity to start a service. The implicit Intent on the contrary, it does not define the
need to activate the target component, component name, it is more widely used to pass messages between different
applications.
In the the explicit Intent message, decided the only elements of the target component is the component name,
Therefore, if you Intent has been clearly defined in the name of the target component, then you completely do not
have to define the contents of the other Intent.
For implicit Intent is different, because there is no clear target component name, must help by Android system
application to find components that most closely matches the intent of the request with Intent. Specific selection
method: Android Intent of the contents of the request and called IntentFilter filter comparison, the IntentFilter system
contains all the possible components to be selected. IntentFilter matches a component implicit Intent requested
content, the Android on the choice of the components as Cain Intent target component.
Android How do I know the application to handle certain types of Intent to request it? This requires the application
statement in the Android-the Manifest.xml in the components contained in the filter (that can match what Intent
request). A statement Intent-Filter components not only respond to his name specified explicitly Intent request, and
unable to respond implicit Intent request. And a statement of the the IntentFilter components either in response to the
explicit Intent request, you can also respond to the implicit Intent request.
Understanding Android Intent Filters (Receiving
Implicit Intents)
Intents sends signals to the Android system telling it that some action needs to be
performed by another component (activity, services, broadcast receivers) in the same
app or a different app.
The system starts resolving which component in which app is responsible to handle
this event that just got triggered.
Implicit intent that opens a webpage URL in the browser.
String url = "http://google.com";
Intent intent = new Intent(Intent.ACTION_VIEW);
intent.setData(Uri.parse(url));
startActivity(intent);
ACTION_VIEW intent action and data to google.com. Once startActivity() is triggered
Android will start looking for all the apps that can handle this and most probably find
just one which is the browser (chrome) that can receive it. It’ll launch chrome and
open google.com in it.
If we wanted to advertise that our app can also handle such an implicit intent where
we take an HTTP URL and do some thing. In such a case Android will give two choices
to the user – single browser installed (chrome) and our app – from which he can make
a selection.
Register a component of our app that can
receive an implicit intent
Note: An explicit intent has its target specified (while creation) does not need registration.
To register a component of our app that can receive an implicit intent for a specific action and data, we
can declare an <intent-filter> for that particular component (activity, service or a broadcast receiver) in
our manifest file.
<activity
android:name="com.pycitup.pyc.WebViewActivity"
android:label="@string/title_activity_web_view" >
<intent-filter>
<action android:name="android.intent.action.VIEW" />
<category android:name="android.intent.category.DEFAULT" />
<data android:scheme="http" />
</intent-filter>
</activity>
Make sure that both the sending and receiving activities have requested permission for the type of
action (ACTION_VIEW) to be performed. Need the INTERNET permission in our manifest:
<uses-permission android:name="android.permission.INTERNET" />
When the intent code is fired, the user will be given multiple choices among which one of them will be
our own app that’ll trigger WebViewActivity. Inside the Activity you could then do things like show up
the webpage in a WebView
Subelement of intent filters
Each activity in the manifest can contain multiple intent filters blocks. Each intent filter
specifies the type of intents it’ll accept based on the <action>, <category> and <data>
subelements. So as soon as an implicit intent passes through (or conforms to) one of
the intent filters it is delivered to that activity component or at least considered in the
list of initial dialog with choices.
<action> – Declares the intent action that you want to be accepted by the intent filter,
specified by the android:name attribute. Every <intent-filter> element must contain
one or more <action> elements, else no Intent objects will get through the filter. Also
note that the value must be the string value of the intent action, not the class constant
(for example "android.intent.action.VIEW" and not Intent.ACTION_VIEW).
<category> – Specifies the accepted intent category in the android:name attribute.
Should contain the string value rather than the class constant
("android.intent.category.DEFAULT" instead of Intent.CATEGORY_DEFAULT).
<data> – Declares the type of data accepted using various attributes that you can find
here. Examples are there in the code samples above which makes this element simple
to understand.
Action which is the most reasonable action to perform and Data that contains the
basic information that drives our action were pretty easy to comprehend but Category
is something that might need a bit of an explanation. Using the category attribute, we
give/mention additional information about the action to execute (that’ll be matched
for intent resolution).
CATEGORY
The CATEGORY_DEFAULT category is applied to all implicit
intents (automatically) passed to startActivity() and
startActivityForResult(). So if you want your activity to
receive implicit intents, it must include a category for
"android.intent.category.DEFAULT" in its intent filters.
CATEGORY_LAUNCHER – Show up in the Launcher
(Google Experience Launcher or Google Now Launcher or
Nova or some other that you installed and use) as a toplevel application. In other words the activity is the initial
activity of a task (will launch when app starts) and is listed
in the system’s application launcher (app drawer).
An implicit intent tells Android system to run an activity(ActivityB in this case) that can do these things
that are specified in the intent filter in a manifest
Intent intent = new Intent();
intent.addAction("thisAction");
intent.addCategory("thisCategory");
startActivity(intent);
Android manifest file:
<activity android:name="ActivityB">
<intent-filter>
<action android:name="thisAction"/>
<category android:name="thisCategory"/>
</intent-filter>
</activity>
Android system will locate a best matched component such as ActivityB based on action and category
requirement. Implicit intent also leaves a security vulnerability hole.
Demo
https://sites.google.com/site/mobilesecuritylab
ware/home/android-components/activity-intent