docs: consistency fixit for the quickview boxes in the dev guide

[android-x86/frameworks-base.git] / docs / html / guide / topics / intents / intents-filters.jd

page.title=Intents and Intent Filters
@jd:body

<div id="qv-wrapper">
<div id="qv">

<h2>In this document</h2>
<ol>
<li><a href="#iobjs">Intent Objects</a></li>
<li><a href="#ires">Intent Resolution</a></li>
<li style="margin-left: 2em"><a href="#ifs">Intent filters</a></li>
<li style="margin-left: 2em"><a href="#ccases">Common cases</a></li>
<li style="margin-left: 2em"><a href="#imatch">Using intent matching</a></li>
<li><a href="#npex">Note Pad Example</a></li>
</ol>

<h2>Key classes</h2>
<ol>
<li>{@link android.content.Intent}</li>
<li>{@link android.content.IntentFilter}</li>
<li>{@link android.content.BroadcastReceiver}</li>
<li>{@link android.content.pm.PackageManager}</li>
</ol>

</div>
</div>


<p>
Three of the core components of an application &mdash; activities, services, and 
broadcast receivers &mdash; are activated through messages, called <i>intents</i>.   
Intent messaging is a facility for late run-time binding between components in the same 
or different applications.  The intent itself, an {@link android.content.Intent} 
object, is a passive data structure holding an abstract description of an operation 
to be performed &mdash; or, often in the case of broadcasts, a description of something 
that has happened and is being announced.  There are separate mechanisms for 
delivering intents to each type of component: 
</p>

<ul>
<li>An Intent object is passed to <code>{@link android.content.Context#startActivity 
Context.startActivity()}</code> or <code>{@link android.app.Activity#startActivityForResult 
Activity.startActivityForResult()}</code> to launch an activity or get an existing 
activity to do something new. (It can also be passed to 
<code>{@link android.app.Activity#setResult(int, Intent) Activity.setResult()}</code>
to return information to the activity that called {@code startActivityForResult()}.)</li>

<li><p>An Intent object is passed to <code>{@link android.content.Context#startService 
Context.startService()}</code> to initiate a service or deliver new instructions to an 
ongoing service.  Similarly, an intent can be passed to <code>{@link 
android.content.Context#bindService Context.bindService()}</code> to establish a 
connection between the calling component and a target service.  It can optionally
initiate the service if it's not already running.</p></li>

<li><p>Intent objects passed to any of the broadcast methods (such as <code>{@link 
android.content.Context#sendBroadcast(Intent) Context.sendBroadcast()}</code>, 
<code>{@link android.content.Context#sendOrderedBroadcast(Intent, String) 
Context.sendOrderedBroadcast()}</code>, or <code>{@link 
android.content.Context#sendStickyBroadcast Context.sendStickyBroadcast()}</code>) 
are delivered to all interested broadcast receivers.  Many kinds of broadcasts 
originate in system code.</p></li>
</ul>

<p>
In each case, the Android system finds the appropriate activity, service, or set 
of broadcast receivers to respond to the intent, instantiating them if necessary. 
There is no overlap within these messaging systems:  Broadcast intents are delivered 
only to broadcast receivers, never to activities or services.  An intent passed to 
{@code startActivity()} is delivered only to an activity, never to a service or 
broadcast receiver, and so on.
</p>

<p>
This document begins with a description of Intent objects.  It then describes the 
rules Android uses to map intents to components &mdash; how it resolves which 
component should receive an intent message.  For intents that don't explicitly 
name a target component, this process involves testing the Intent object against 
<i>intent filters</i> associated with potential targets. 
</p>


<h2><a name="iobjs"></a>Intent Objects</h2>

<p>
An {@link android.content.Intent} object is a bundle of information.  It 
contains information of interest to the component that receives the intent 
(such as the action to be taken and the data to act on) plus information 
of interest to the Android system (such as the category of component that 
should handle the intent and instructions on how to launch a target activity).  
Principally, it can contain the following:
</p>

<dl>

<dt><b>Component name</b><a name="cname"></a></dt> 
<dd>The name of the component that should handle the intent.  This field is 
a {@link android.content.ComponentName} object &mdash; a combination of the 
fully qualified class name of the target component (for example "{@code 
com.example.project.app.FreneticActivity}") and the package name set
in the manifest file of the application where the component resides (for 
example, "{@code com.example.project}").  The package part of the component 
name and the package name set in the manifest do not necessarily have to match.

<p>  
The component name is optional.  If it is set, the Intent object is 
delivered to an instance of the designated class.  If it is not set, 
Android uses other information in the Intent object to locate a suitable 
target &mdash; see <a href="#ires">Intent Resolution</a>, later in this 
document.
</p>

<p>
The component name is set by <code>{@link android.content.Intent#setComponent 
setComponent()}</code>,  <code>{@link android.content.Intent#setClass 
setClass()}</code>, or <code>{@link android.content.Intent#setClassName(String, String) 
setClassName()}</code> and read by <code>{@link android.content.Intent#getComponent 
getComponent()}</code>. 
</p>
</dd>

<p><dt><b>Action</b></dt>
<dd>A string naming the action to be performed &mdash; or, in the case of broadcast 
intents, the action that took place and is being reported.  The Intent class defines 
a number of action constants, including these:
</p>

<table>
<tr>
   <th>Constant</th>
   <th>Target component</th>
   <th>Action</th>
</tr><tr>
   <td>{@code ACTION_CALL}
   <td>activity
   <td>Initiate a phone call.
</tr><tr>
   <td>{@code ACTION_EDIT}
   <td>activity
   <td>Display data for the user to edit.
</tr><tr>
   <td>{@code ACTION_MAIN}
   <td>activity
   <td>Start up as the initial activity of a task, with no data input and no returned output.
</tr><tr>
   <td>{@code ACTION_SYNC}
   <td>activity
   <td>Synchronize data on a server with data on the mobile device.
</tr><tr>
   <td>{@code ACTION_BATTERY_LOW}
   <td>broadcast receiver
   <td>A warning that the battery is low.
</tr><tr>
   <td>{@code ACTION_HEADSET_PLUG}
   <td>broadcast receiver
   <td>A headset has been plugged into the device, or unplugged from it.
</tr><tr>
   <td>{@code ACTION_SCREEN_ON}
   <td>broadcast receiver
   <td>The screen has been turned on.
</tr><tr>
   <td>{@code ACTION_TIMEZONE_CHANGED}
   <td>broadcast receiver
   <td>The setting for the time zone has changed.
</tr>
</table>

<p>
See the {@link android.content.Intent} class description for a list of 
pre-defined constants for generic actions.  Other actions are defined 
elsewhere in the Android API.
You can also define your own action strings for activating the components 
in your application.  Those you invent should include the application 
package as a prefix &mdash; for example: 
"<code>com.example.project.SHOW_COLOR</code>". 
</p>

<p>
The action largely determines how the rest of the intent is structured
&mdash; particularly the <a href="#data">data</a> and 
<a href="#extras">extras</a> fields &mdash; 
much as a method name determines a set of arguments and a return value.
For this reason, it's a good idea to use action names that are
as specific as possible, and to couple them tightly to the other fields of
the intent.  In other words, instead of defining an action in isolation,
define an entire protocol for the Intent objects your components can handle.
</p>

<p>
The action in an Intent object is set by the 
<code>{@link android.content.Intent#setAction setAction()}</code> 
method and read by 
<code>{@link android.content.Intent#getAction getAction()}</code>.
</p>
</dd>

<p><dt><b>Data</b><a name="data"></a></dt>
<dd>The URI of the data to be acted on and the MIME type of that data.  Different 
actions are paired with different kinds of data specifications.  For example, if 
the action field is {@code ACTION_EDIT}, 
the data field would contain the URI of the document to be displayed for editing.  
If the action is {@code ACTION_CALL}, the data field would be a {@code tel:} URI 
with the number to call.  Similarly, if the action is {@code ACTION_VIEW} and the 
data field is an {@code http:} URI, the receiving activity would be called upon 
to download and display whatever data the URI refers to.

<p>
When matching an intent to a component that is capable of handling the data, 
it's often important to know the type of data (its MIME type) in addition to its URI.  
For example, a component able to display image data should not be called
upon to play an audio file.
</p>

<p>
In many cases, the data type can be inferred from the URI &mdash; particularly 
{@code content:} URIs, which indicate that the data is located on the device and
controlled by a content provider (see the  
<a href="{@docRoot}guide/topics/providers/content-providers.html">separate 
discussion on content providers</a>).  But the type can also be explicitly set 
in the Intent object.  
The <code>{@link android.content.Intent#setData setData()}</code> method specifies 
data only as a URI, <code>{@link android.content.Intent#setType setType()}</code> 
specifies it only as a MIME type, and <code>{@link 
android.content.Intent#setDataAndType setDataAndType()}</code> specifies it as both 
a URI and a MIME type.  The URI is read by <code>{@link 
android.content.Intent#getData getData()}</code> and the type by <code>{@link 
android.content.Intent#getType getType()}</code>.
</p>
</dd>

<p><dt><b>Category</b></dt>
<dd>A string containing additional information about the kind of component 
that should handle the intent.  Any number of category descriptions can be 
placed in an Intent object.  As it does for actions, the Intent class defines
several category constants, including these: 

<table>
<tr>
   <th>Constant</th>
   <th>Meaning</th>
</tr><tr>
   <td>{@code CATEGORY_BROWSABLE}
   <td>The target activity can be safely invoked by the browser to display data 
       referenced by a link &mdash; for example, an image or an e-mail message.
</tr><tr>
   <td>{@code CATEGORY_GADGET}
   <td>The activity can be embedded inside of another activity that hosts gadgets.
</tr><tr>
   <td>{@code CATEGORY_HOME}
   <td>The activity displays the home screen, the first screen the user sees when 
       the device is turned on or when the HOME key is pressed.
</tr><tr>
   <td>{@code CATEGORY_LAUNCHER}
   <td>The activity can be the initial activity of a task and is listed in 
       the top-level application launcher.
</tr><tr>
   <td>{@code CATEGORY_PREFERENCE}
   <td>The target activity is a preference panel.
</tr>
</table> 

<p>
See the {@link android.content.Intent} class description for the full list of 
categories.
</p>

<p>
The <code>{@link android.content.Intent#addCategory addCategory()}</code> method 
places a category in an Intent object, <code>{@link android.content.Intent#removeCategory 
removeCategory()}</code> deletes a category previously added, and <code>{@link android.content.Intent#getCategories getCategories()}</code> gets the set of all 
categories currently in the object. 
</p>
</dd>

<p><dt><b>Extras</b><a name="extras"></a></dt>
<dd>Key-value pairs for additional information that should be delivered to the 
component handling the intent.  Just as some actions are paired with particular 
kinds of data URIs, some are paired with particular extras.  For example, an 
{@code ACTION_TIMEZONE_CHANGED} intent has a "{@code time-zone}" extra that 
identifies the new time zone, and {@code ACTION_HEADSET_PLUG} has a 
"{@code state}" extra indicating whether the headset is now plugged in or 
unplugged, as well as a "{@code name}" extra for the type of headset. 
If you were to invent a {@code SHOW_COLOR} action, the color value would
be set in an extra key-value pair.

<p>
The Intent object has a series of {@code put...()} methods for inserting various 
types of extra data and a similar  set of {@code get...()} methods for reading 
the data.  These methods parallel those for {@link android.os.Bundle} objects.  
In fact, the extras can be installed and read as a Bundle using the <code>{@link 
android.content.Intent#putExtras putExtras()}</code> and <code>{@link 
android.content.Intent#getExtras getExtras()}</code> methods.
</p>
</dd>

<p><dt><b>Flags</b></dt>
<dd>Flags of various sorts.  Many instruct the Android system how to launch an 
activity (for example, which task the activity should belong to) and how to treat 
it after it's launched (for example, whether it belongs in the list of recent 
activities).  All these flags are defined in the Intent class.   
</dd>

</dl>

<p>
The Android system and the applications that come with the platform employ 
Intent objects both to send out system-originated broadcasts and to activate 
system-defined components.  To see how to structure an intent to activate a 
system component, consult the 
<a href="{@docRoot}guide/appendix/g-app-intents.html">list of intents</a> 
in the reference.  
</p>


<h2><a name="ires"></a>Intent Resolution</h2>

<p>
Intents can be divided into two groups:
</p>

<ul>
<li><i>Explicit intents</i> designate the target component by its 
name (the <a href="#cname">component name field</a>, mentioned earlier, 
has a value set).  Since component names would generally not be known to 
developers of other applications, explicit intents are typically used 
for application-internal messages &mdash; such as an activity starting 
a subordinate service or launching a sister activity.</li>

<li><p><i>Implicit intents</i> do not name a target (the field for 
the component name is blank).  Implicit intents are often used to 
activate components in other applications.</p></li>   
</ul>

<p>
Android delivers an explicit intent to an instance of the designated 
target class.  Nothing in the Intent object other than the component 
name matters for determining which component should get the intent.  
</p>

<p>
A different strategy is needed for implicit intents.  In the absence of a 
designated target, the Android system must find the best component (or 
components) to handle the intent &mdash; a single activity or service to 
perform the requested action or the set of broadcast receivers to respond 
to the broadcast announcement.  It does so by comparing the contents of 
the Intent object to <i>intent filters</i>, structures associated with 
components that can potentially receive intents.  Filters advertise the 
capabilities of a component and delimit the intents it can handle.  They 
open the component to the possibility of receiving implicit intents of 
the advertised type.  If a component does not have any intent filters, 
it can receive only explicit intents.  A component with filters can 
receive both explicit and implicit intents.
</p>

<p>
Only three aspects of an Intent object are consulted when the object
is tested against an intent filter:
</p>

<p style="margin-left: 2em">action
<br/>data (both URI and data type)
<br/>category</p>

<p>
The extras and flags play no part in resolving which component receives
an intent.
</p>


<h3><a name="ifs"></a>Intent filters</h3>

<p>
To inform the system which implicit intents they can handle, activities, 
services, and broadcast receivers can have one or more intent filters.  
Each filter describes a capability of the component, a set of intents that 
the component is willing to receive.  It, in effect, filters in 
intents of a desired type, while filtering out unwanted 
intents &mdash; but only unwanted implicit intents (those that don't name
a target class).  An explicit intent is always delivered to its target, 
no matter what it contains; the filter is not consulted.  But an implicit 
intent is delivered to a component only if it can pass through one of the 
component's filters.
</p>

<p>
A component has separate filters for each job it can do, each face it can 
present to the user.  For example, the NoteEditor activity of the sample
Note Pad application has two filters &mdash; one for starting up with a
specific note that the user can view or edit, and another for starting 
with a new, blank note that the user can fill in and save.  (All of Note 
Pad's filters are described in the <a href="#npex">Note Pad Example</a>
section, later.)
</p>

<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Filters and security</h2>
<p>An intent filter cannot be relied on for security.  While it opens a
component to receiving only certain kinds of implicit intents, it does 
nothing to prevent explicit intents from targeting the component.  Even 
though a filter restricts the intents a component will be asked to handle
to certain actions and data sources, someone could always put 
together an explicit intent with a different action and data source, and 
name the component as the target.
</p>
</div>
</div> 

<p>
An intent filter is an instance of the {@link android.content.IntentFilter} class.  
However, since the Android system must know about the capabilities of a component 
before it can launch that component, intent filters are generally not set up in 
Java code, but in the application's manifest file (AndroidManifest.xml) as 
<code><a href="{@docRoot}guide/topics/manifest/intent-filter-element.html">&lt;intent-filter&gt;</a></code> 
elements.  (The one exception would be filters for 
broadcast receivers that are registered dynamically by calling <code>{@link android.content.Context#registerReceiver(BroadcastReceiver, IntentFilter, String, 
Handler) Context.registerReceiver()}</code>; they are directly created as 
IntentFilter objects.)
</p>

<p>
A filter has fields that parallel the action, data, and category fields of an 
Intent object.  An implicit intent is tested against the filter in all three areas.  
To be delivered to the component that owns the filter, it must pass all three tests.  
If it fails even one of them, the Android system won't deliver it to the 
component &mdash; at least not on the basis of that filter.  However, since a 
component can have multiple intent filters, an intent that does not pass 
through one of a component's filters might make it through on another.
</p>

<p>
Each of the three tests is described in detail below:
</p>

<dl>

<dt><b>Action test</b></dt>
<dd>An 
<code><a href="{@docRoot}guide/topics/manifest/intent-filter-element.html">&lt;intent-filter&gt;</a></code>
element in the manifest file lists actions as 
<code><a href="{@docRoot}guide/topics/manifest/action-element.html">&lt;action&gt;</a></code> 
subelements.  For example:

<pre>&lt;intent-filter . . . &gt;
    &lt;action android:name="com.example.project.SHOW_CURRENT" /&gt;
    &lt;action android:name="com.example.project.SHOW_RECENT" /&gt;
    &lt;action android:name="com.example.project.SHOW_PENDING" /&gt;
    . . .
&lt;/intent-filter&gt;</pre>

<p>
As the example shows, while an Intent object names just a single action, 
a filter may list more than one.  The list cannot be empty; a filter must 
contain at least one {@code &lt;action&gt;} element, or it
will block all intents.
</p>

<p>
To pass this test, the action specified in the Intent object must match 
one of the actions listed in the filter.  If the object or the filter 
does not specify an action, the results are as follows: 
</p>

<ul>
<li>If the filter fails to list any actions, there is nothing for an 
intent to match, so all intents fail the test.  No intents can get 
through the filter.</li>

<li><p>On the other hand, an Intent object that doesn't specify an 
action automatically passes the test &mdash; as long as the filter 
contains at least one action.</p></li>
</ul
</dd>

<dt><b>Category test</b></dt>
<dd>An 
<code><a href="{@docRoot}guide/topics/manifest/intent-filter-element.html">&lt;intent-filter&gt;</a></code> 
element also lists categories as subelements.  For example:

<pre>&lt;intent-filter . . . &gt;
    &lt;category android:name="android.intent.category.DEFAULT" /&gt;
    &lt;category android:name="android.intent.category.BROWSABLE" /&gt;
    . . .
&lt;/intent-filter&gt;</pre>

<p>
Note that the constants described earlier for actions and categories are not
used in the manifest file.  The full string values are used instead.  For 
instance, the "{@code android.intent.category.BROWSABLE}" string in the example 
above corresponds to the {@code CATEGORY_BROWSABLE} constant mentioned earlier 
in this document.  Similarly, the string "{@code android.intent.action.EDIT}" 
corresponds to the {@code ACTION_EDIT} constant.
</p>

<p>
For an intent to pass the category test, every category in the Intent object 
must match a category in the filter.  The filter can list additional categories, 
but it cannot omit any that are in the intent.
</p>

<p>
In principle, therefore, an Intent object with no categories should always pass 
this test, regardless of what's in the filter.  That's mostly true.  However, 
with one exception, Android treats all implicit intents passed to {@link 
android.content.Context#startActivity startActivity()} as if they contained 
at least one category:  "{@code android.intent.category.DEFAULT}" (the
{@code CATEGORY_DEFAULT} constant).  
Therefore, activities that are willing to receive implicit intents must 
include "{@code android.intent.category.DEFAULT}" in their intent filters. 
(Filters with "{@code android.intent.action.MAIN}" and 
"{@code android.intent.category.LAUNCHER}" settings are the exception.  
They mark activities that begin new tasks and that are represented on the 
launcher screen.  They can include "{@code android.intent.category.DEFAULT}" 
in the list of categories, but don't need to.)  See <a href="#imatch">Using
intent matching</a>, later, for more on these filters.)
</p>
<dd>

<dt><b>Data test</b></dt>
<dd>Like the action and categories, the data specification for an intent filter 
is contained in a subelement.  And, as in those cases, the subelement can appear 
multiple times, or not at all.  For example:

<pre>&lt;intent-filter . . . &gt;
    &lt;data android:mimeType="video/mpeg" android:scheme="http" . . . /&gt; 
    &lt;data android:mimeType="audio/mpeg" android:scheme="http" . . . /&gt;
    . . .
&lt;/intent-filter&gt;</pre>

<p>
Each 
<code><a href="{@docRoot}guide/topics/manifest/data-element.html">&lt;data&gt;</a></code> 
element can specify a URI and a data type (MIME media type).  There are separate 
attributes &mdash; {@code scheme}, {@code host}, {@code port}, 
and {@code path} &mdash; for each part of the URI:  
</p>

<p style="margin-left: 2em">{@code scheme://host:port/path}</p>

<p>
For example, in the following URI,
</p>

<p style="margin-left: 2em">{@code content://com.example.project:200/folder/subfolder/etc}</p>

<p> the scheme is "{@code content}", the host is "{@code com.example.project}", 
the port is "{@code 200}", and the path is "{@code folder/subfolder/etc}".  
The host and port together constitute the URI <i>authority</i>; if a host is 
not specified, the port is ignored.
</p>

<p>
Each of these attributes is optional, but they are not independent of each other:  
For an authority to be meaningful, a scheme must also be specified.  
For a path to be meaningful, both a scheme and an authority must be specified.  
</p>

<p>
When the URI in an Intent object is compared to a URI specification in a filter, 
it's compared only to the parts of the URI actually mentioned in the filter.  
For example, if a filter specifies only a scheme, all URIs with that scheme match 
the filter.  If a filter specifies a scheme and an authority but no path, all URIs 
with the same scheme and authority match, regardless of their paths.  If a filter 
specifies a scheme, an authority, and a path, only URIs with the same scheme, 
authority, and path match.  However, a path specification in the filter can 
contain wildcards to require only a partial match of the path.
</p>

<p>
The {@code type} attribute of a {@code &lt;data&gt;} element specifies the MIME type 
of the data.  It's more common in filters than a URI.  Both the Intent object and 
the filter can use a "*" wildcard for the subtype field &mdash; for example, 
"{@code text/*}" or "{@code audio/*}" &mdash; indicating any subtype matches.
</p>

<p>
The data test compares both the URI and the data type in the Intent object to a URI 
and data type specified in the filter.  The rules are as follows:
</p>

<ol type="a">
<li>An Intent object that contains neither a URI nor a data type passes the 
test only if the filter likewise does not specify any URIs or data types.</li>

<li><p>An Intent object that contains a URI but no data type (and a type cannot 
be inferred from the URI) passes the test only if its URI matches a URI in the 
filter and the filter likewise does not specify a type.  This will be the case 
only for URIs like {@code mailto:} and {@code tel:} that do not refer to actual data.</p></li>

<li><p>An Intent object that contains a data type but not a URI passes the test 
only if the filter lists the same data type and similarly does not specify a URI.</p></li>

<li><p>An Intent object that contains both a URI and a data type (or a data type 
can be inferred from the URI) passes the data type part of the test only if its 
type matches a type listed in the filter.  It passes the URI part of the test 
either if its URI matches a URI in the filter or if it has a {@code content:} 
or {@code file:} URI and the filter does not specify a URI.  In other words, 
a component is presumed to support {@code content:} and {@code file:} data if
its filter lists only a data type.</p></li>
</ol>
</dl>

<p>
If 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.
</p>


<h3><a name="ccases"></a>Common cases</h3>

<p>
The last rule shown above for the data test, rule (d), reflects the expectation 
that components are able to get local data from a file or content provider.
Therefore, their filters can list just a data type and do not need to explicitly
name the {@code content:} and {@code file:} schemes.
This is a typical case.  A {@code &lt;data&gt;} element like the following, 
for example, tells Android that the component can get image data from a content 
provider and display it:
</p>

<pre>&lt;data android:mimeType="image/*" /&gt;</pre>

<p>
Since most available data is dispensed by content providers, filters that 
specify a data type but not a URI are perhaps the most common.
</p>

<p>
Another common configuration is filters with a scheme and a data type.  For
example, a {@code &lt;data&gt;} element like the following tells Android that
the component can get video data from the network and display it:
</p>

<pre>&lt;data android:scheme="http" android:type="video/*" /&gt;</pre>

<p>
Consider, for example, what the browser application does when
the user follows a link on a web page.   It first tries to display the data 
(as it could if the link was to an HTML page).  If it can't display the data, 
it puts together an implicit intent with the scheme and data type and tries
to start an activity that can do the job.  If there are no takers, it asks the
download manager to download the data.  That puts it under the control
of a content provider, so a potentially larger pool of activities
(those with filters that just name a data type) can respond.
</p>

<p>
Most applications also have a way to start fresh, without a reference
to any particular data.  Activities that can initiate applications 
have filters with "{@code android.intent.action.MAIN}" specified as
the action.  If they are to be represented in the application launcher,
they also specify the "{@code android.intent.category.LAUNCHER}" 
category:
</p>

<pre>&lt;intent-filter . . . &gt;
    &lt;action android:name="code android.intent.action.MAIN" /&gt;
    &lt;category android:name="code android.intent.category.LAUNCHER" /&gt;
&lt;/intent-filter&gt;</pre>


<h3><a name="imatch"></a>Using intent matching</h3>

<p>
Intents are matched against intent filters not only to discover a target
component to activate, but also to discover something about the set of
components on the device.  For example, the Android system populates the 
application launcher, the top-level screen that shows the applications 
that are available for the user to launch, by finding all the  activities
 with intent filters that specify the "{@code android.intent.action.MAIN}" 
action and "{@code android.intent.category.LAUNCHER}" category 
(as illustrated in the previous section).  It then displays the icons and 
labels of those activities in the launcher.  Similarly, it discovers the 
home screen by looking for the activity with 
"{@code android.intent.category.HOME}" in its filter.
</p>

<p>
Your application can use intent matching is a similar way.
The {@link android.content.pm.PackageManager} has a set of {@code query...()}
methods that return all components that can accept a particular intent, and 
a similar series of {@code resolve...()} methods that determine the best
component to respond to an intent.  For example, 
{@link android.content.pm.PackageManager#queryIntentActivities 
queryIntentActivities()} returns a list of all activities that can perform
the intent passed as an argument, and {@link 
android.content.pm.PackageManager#queryIntentServices 
queryIntentServices()} returns a similar list of services.  
Neither method activates the components; they just list the ones that
can respond.  There's a similar method, 
{@link android.content.pm.PackageManager#queryBroadcastReceivers 
queryBroadcastReceivers()}, for broadcast receivers.
</p>

<h2 id="npex">Note Pad Example</h2>

<p>
The Note Pad sample application enables users to browse through a list 
of notes, view details about individual items in the list, edit the items, 
and add a new item to the list.  This section looks at the intent filters
declared in its manifest file.  (If you're working offline in the SDK, you 
can find all the source files for this sample application, including its 
manifest file, at {@code &lt;sdk&gt;/samples/NotePad/index.html}.  
If you're viewing the documentation online, the source files are in the 
<a href="{@docRoot}resources/samples/index.html">Tutorials and Sample Code</a> 
section <a href="{@docRoot}resources/samples/NotePad/index.html">here</a>.)
</p>

<p>
In its manifest file, the Note Pad application declares three activities,
each with at least one intent filter.  It also declares a content provider 
that manages the note data.  Here is the manifest file in its entirety:
</p>

<pre>&lt;manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.example.android.notepad"&gt;
    &lt;application android:icon="@drawable/app_notes"
                 android:label="@string/app_name" &gt;

        &lt;provider android:name="NotePadProvider"
                  android:authorities="com.google.provider.NotePad" /&gt;

        &lt;activity android:name="NotesList" android:label="@string/title_notes_list"&gt;
            &lt;intent-filter&gt;
                &lt;action android:name="android.intent.action.MAIN" /&gt;
                &lt;category android:name="android.intent.category.LAUNCHER" /&gt;
            &lt;/intent-filter&gt;
            &lt;intent-filter&gt;
                &lt;action android:name="android.intent.action.VIEW" /&gt;
                &lt;action android:name="android.intent.action.EDIT" /&gt;
                &lt;action android:name="android.intent.action.PICK" /&gt;
                &lt;category android:name="android.intent.category.DEFAULT" /&gt;
                &lt;data android:mimeType="vnd.android.cursor.dir/vnd.google.note" /&gt;
            &lt;/intent-filter&gt;
            &lt;intent-filter&gt;
                &lt;action android:name="android.intent.action.GET_CONTENT" /&gt;
                &lt;category android:name="android.intent.category.DEFAULT" /&gt;
                &lt;data android:mimeType="vnd.android.cursor.item/vnd.google.note" /&gt;
            &lt;/intent-filter&gt;
        &lt;/activity&gt;
        
        &lt;activity android:name="NoteEditor"
                  android:theme="@android:style/Theme.Light"
                  android:label="@string/title_note" &gt;
            &lt;intent-filter android:label="@string/resolve_edit"&gt;
                &lt;action android:name="android.intent.action.VIEW" /&gt;
                &lt;action android:name="android.intent.action.EDIT" /&gt;
                &lt;action android:name="com.android.notepad.action.EDIT_NOTE" /&gt;
                &lt;category android:name="android.intent.category.DEFAULT" /&gt;
                &lt;data android:mimeType="vnd.android.cursor.item/vnd.google.note" /&gt;
            &lt;/intent-filter&gt;
            &lt;intent-filter&gt;
                &lt;action android:name="android.intent.action.INSERT" /&gt;
                &lt;category android:name="android.intent.category.DEFAULT" /&gt;
                &lt;data android:mimeType="vnd.android.cursor.dir/vnd.google.note" /&gt;
            &lt;/intent-filter&gt;
        &lt;/activity&gt;
        
        &lt;activity android:name="TitleEditor" 
                  android:label="@string/title_edit_title"
                  android:theme="@android:style/Theme.Dialog"&gt;
            &lt;intent-filter android:label="@string/resolve_title"&gt;
                &lt;action android:name="com.android.notepad.action.EDIT_TITLE" /&gt;
                &lt;category android:name="android.intent.category.DEFAULT" /&gt;
                &lt;category android:name="android.intent.category.ALTERNATIVE" /&gt;
                &lt;category android:name="android.intent.category.SELECTED_ALTERNATIVE" /&gt;
                &lt;data android:mimeType="vnd.android.cursor.item/vnd.google.note" /&gt;
            &lt;/intent-filter&gt;
        &lt;/activity&gt;
        
    &lt;/application&gt;
&lt;/manifest&gt;</pre>

<p>
The first activity, NotesList, is 
distinguished from the other activities by the fact that it operates 
on a directory of notes (the note list) rather than on a single note.  
It would generally serve as the initial user interface into the 
application.  It can do three things as described by its three intent 
filters:
</p>

<ol>
<li><pre>&lt;intent-filter&gt;
    &lt;action android:name="android.intent.action.MAIN" /&gt;
    &lt;category android:name="android.intent.category.LAUNCHER" /&gt;
&lt;/intent-filter&gt;</pre>

<p>
This filter declares the main entry point into the Note Pad application.  
The standard {@code MAIN} action is an entry point that does not require 
any other information in the Intent (no data specification, for example), 
and the {@code LAUNCHER} category says that this entry point should be 
listed in the application launcher.
</p></li>

<li><pre>&lt;intent-filter&gt;
    &lt;action android:name="android.intent.action.VIEW" /&gt;
    &lt;action android:name="android.intent.action.EDIT" /&gt;
    &lt;action android:name="android.intent.action.PICK" /&gt;
    &lt;category android:name="android.intent.category.DEFAULT" /&gt;
    &lt;data android:mimeType="vnd.android.cursor.dir/vnd.google.note" /&gt;
&lt;/intent-filter&gt;</pre>

<p>
This filter declares the things that the activity can do on a directory 
of notes.  It can allow the user to view or edit the directory (via
the {@code VIEW} and {@code EDIT} actions), or to pick a particular note 
from the directory (via the {@code PICK} action).
</p> 

<p>
The {@code mimeType} attribute of the 
<code><a href="{@docRoot}guide/topics/manifest/data-element.html">&lt;data&gt;</a></code> 
element specifies the kind of data that these actions operate on.  It 
indicates that the activity can get a Cursor over zero or more items 
({@code vnd.android.cursor.dir}) from a content provider that holds
Note Pad data ({@code vnd.google.note}).  The Intent object that launches
the activity would include a {@code content:} URI specifying the exact 
data of this type that the activity should open.
</p>

<p>
Note also the {@code DEFAULT} category supplied in this filter.  It's
there because the <code>{@link android.content.Context#startActivity  
Context.startActivity()}</code> and 
<code>{@link android.app.Activity#startActivityForResult  
Activity.startActivityForResult()}</code> methods treat all intents 
as if they contained the {@code DEFAULT} category &mdash; with just
two exceptions:
</p>

<ul>
<li>Intents that explicitly name the target activity</li>
<li>Intents consisting of the {@code MAIN} action and {@code LAUNCHER} 
category</li>
</ul>

<p>
Therefore, the {@code DEFAULT} category is <em>required</em> for all 
filters &mdash; except for those with the {@code MAIN} action and 
{@code LAUNCHER} category.  (Intent filters are not consulted for
explicit intents.)  
</p></li>

<li><pre>&lt;intent-filter&gt;
    &lt;action android:name="android.intent.action.GET_CONTENT" /&gt;
    &lt;category android:name="android.intent.category.DEFAULT" /&gt;
    &lt;data android:mimeType="vnd.android.cursor.item/vnd.google.note" /&gt;
&lt;/intent-filter&gt;</pre>

<p>
This filter describes the activity's ability to return a note selected by
the user without requiring any specification of the directory the user should 
choose from.  The {@code GET_CONTENT} action is similar to the {@code PICK} 
action.  In both cases, the activity returns the URI for a note selected by 
the user.  (In each case, it's returned to the activity that called
<code>{@link android.app.Activity#startActivityForResult 
startActivityForResult()}</code> to start the NoteList activity.)  Here, 
however, the caller specifies the type of data desired instead of the 
directory of data the user will be picking from.
</p>

<p>
The data type, <code>vnd.android.cursor.item/vnd.google.note</code>, 
indicates the type of data the activity can return &mdash; a URI for 
a single note.  From the returned URI, the caller can get a Cursor for 
exactly one item ({@code vnd.android.cursor.item}) from the content 
provider that holds Note Pad data ({@code vnd.google.note}).
</p>

<p>
In other words, for the {@code PICK} action in the previous filter, 
the data type indicates the type of data the activity could display to the 
user.  For the {@code GET_CONTENT} filter, it indicates the type of data 
the activity can return to the caller.
</p></li>
</ol>

<p>
Given these capabilities, the following intents will resolve to the
NotesList activity:
</p>

<dl style="margin-left: 2em">
<dt>action: <code>android.intent.action.MAIN</code></dt>
<dd>Launches the activity with no data specified.</dd>

<dt>action: <code>android.intent.action.MAIN</code>
<br/>category: <code>android.intent.category.LAUNCHER</code></dt>
<dd> Launches the activity with no data selected specified. 
This is the actual intent used by the Launcher to populate its top-level 
list.  All activities with filters that match this action and category
are added to the list.</dd>

<dt>action: <code>android.intent.action.VIEW</code>
<br/>data: <code>content://com.google.provider.NotePad/notes</code></dt>
<dd>Asks the activity to display a list of all the notes under
<code>content://com.google.provider.NotePad/notes</code>.  The user can then 
browse through the list and get information about the items in it.</dd>

<dt>action: <code>android.intent.action.PICK</code>
<br/>data: <code>content://com.google.provider.NotePad/notes</code></dt>
<dd>Asks the activity to display a list of the notes under
<code>content://com.google.provider.NotePad/notes</code>.
The user can then pick a note from the list, and the activity will return 
the URI for that item back to the activity that started the NoteList activity.</dd>

<dt>action: <code>android.intent.action.GET_CONTENT</code>
<br/>data type: <code>vnd.android.cursor.item/vnd.google.note</code></dt>
<dd>Asks the activity to supply a single item of Note Pad data.</dd>
</dl>

<p>
The second activity, NoteEditor, shows 
users a single note entry and allows them to edit it.  It can do two things 
as described by its two intent filters:

<ol>
<li><pre>&lt;intent-filter android:label="@string/resolve_edit"&gt;
    &lt;action android:name="android.intent.action.VIEW" /&gt;
    &lt;action android:name="android.intent.action.EDIT" /&gt;
    &lt;action android:name="com.android.notepad.action.EDIT_NOTE" /&gt;
    &lt;category android:name="android.intent.category.DEFAULT" /&gt;
    &lt;data android:mimeType="vnd.android.cursor.item/vnd.google.note" /&gt;
&lt;/intent-filter&gt;</pre>

<p>
The first, primary, purpose of this activity is to enable the user to 
interact with a single note &mdash to either {@code VIEW} the note or 
{@code EDIT} it.  (The {@code EDIT_NOTE} category is a synonym for 
{@code EDIT}.)  The intent would contain the URI for data matching the 
MIME type <code>vnd.android.cursor.item/vnd.google.note</code> &mdash;
that is, the URI for a single, specific note.  It would typically be a 
URI that was returned by the {@code PICK} or {@code GET_CONTENT} 
actions of the NoteList activity.
</p>

<p>
As before, this filter lists the {@code DEFAULT} category so that the 
activity can be launched by intents that don't explicitly specify the
NoteEditor class.
</p></li>

<li><pre>&lt;intent-filter&gt;
    &lt;action android:name="android.intent.action.INSERT" /&gt;
    &lt;category android:name="android.intent.category.DEFAULT" /&gt;
    &lt;data android:mimeType="vnd.android.cursor.dir/vnd.google.note" /&gt;
&lt;/intent-filter&gt;</pre>

<p>
The secondary purpose of this activity is to enable the user to create a new 
note, which it will {@code INSERT} into an existing directory of notes.  The 
intent would contain the URI for data matching the MIME type
<code>vnd.android.cursor.dir/vnd.google.note</code> &mdash; that 
is, the URI for the directory where the note should be placed.
</p></li>  
</ol>

<p>
Given these capabilities, the following intents will resolve to the
NoteEditor activity:
</p>

<dl style:"margin-left: 2em">
<dt>action: <code>android.intent.action.VIEW</code>
<br/>data: <code>content://com.google.provider.NotePad/notes/<var>ID</var></code></dt>
<dd>Asks the activity to display the content of the note identified 
by {@code <var>ID</var>}. (For details on how {@code content:} URIs
specify individual members of a group, see 
<a href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a>.)

<dt>action: <code>android.intent.action.EDIT</code>
<br/>data: <code>content://com.google.provider.NotePad/notes/<var>ID</var></code></dt>
<dd>Asks the activity to display the content of the note identified 
by {@code <var>ID</var>}, and to let the user edit it.  If the user 
saves the changes, the activity updates the data for the note in the 
content provider.</dd>

<dt>action: <code>android.intent.action.INSERT</code>
<br/>data: <code>content://com.google.provider.NotePad/notes</code></dt>
<dd>Asks the activity to create a new, empty note in the notes list at
<code>content://com.google.provider.NotePad/notes</code>
and allow the user to edit it.  If the user saves the note, its URI
is returned to the caller.
</dd>
</dl>

<p>The last activity, TitleEditor, 
enables the user to edit the title of a note.  This could be implemented 
by directly invoking the activity (by explicitly setting its component 
name in the Intent), without using an intent filter.  But here we take 
the opportunity to show how to publish alternative operations on existing 
data:
</p>

<pre>&lt;intent-filter android:label="@string/resolve_title"&gt;
    &lt;action android:name="com.android.notepad.action.EDIT_TITLE" /&gt;
    &lt;category android:name="android.intent.category.DEFAULT" /&gt;
    &lt;category android:name="android.intent.category.ALTERNATIVE" /&gt;
    &lt;category android:name="android.intent.category.SELECTED_ALTERNATIVE" /&gt;
    &lt;data android:mimeType="vnd.android.cursor.item/vnd.google.note" /&gt;
&lt;/intent-filter&gt;</pre>

<p>
The single intent filter for this activity uses a custom action called
"<code>com.android.notepad.action.EDIT_TITLE</code>".  It must be invoked on
a specific note (data type <code>vnd.android.cursor.item/vnd.google.note</code>),
like the previous {@code VIEW} and {@code EDIT} actions.  However, here the 
activity displays the title contained in the note data, not the content of 
the note itself.
</p>

<p>
In addition to supporting the usual {@code DEFAULT} category, the title 
editor also supports two other standard categories: 
<code>{@link android.content.Intent#CATEGORY_ALTERNATIVE ALTERNATIVE}</code>
and <code>{@link android.content.Intent#CATEGORY_SELECTED_ALTERNATIVE 
SELECTED_ALTERNATIVE}</code>.
These categories identify activities that can be presented to users in 
a menu of options (much as the {@code LAUNCHER} category identifies 
activities that should be presented to user in the application launcher).  
Note that the filter also supplies an explicit label (via 
<code>android:label="@string/resolve_title"</code>) to better control
what users see when presented with this activity as an alternative
action to the data they are currently viewing.  (For more information 
on these categories and building options menus, see the 
<code>{@link android.content.pm.PackageManager#queryIntentActivityOptions 
PackageManager.queryIntentActivityOptions()}</code> and 
<code>{@link android.view.Menu#addIntentOptions Menu.addIntentOptions()}</code>
methods.)
</p>

<p>
Given these capabilities, the following intent will resolve to the
TitleEditor activity:
</p>

<dl style="margin-left: 2em">
<dt>action: <code>com.android.notepad.action.EDIT_TITLE</code>
<br/>data: <code>content://com.google.provider.NotePad/notes/<var>ID</var></code></dt>
<dd>Asks the activity to display the title associated with note <var>ID</var>, and 
allow the user to edit the title.</dd>
</dl>