Using intent matching

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

"android.intent.action.MAIN" action and "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 "android.intent.category.HOME" in its filter.

<category android:name="android.intent.category.LAUNCHER" /> </intent-filter> <intent-filter>

<action android:name="android.intent.action.VIEW" /> <action android:name="android.intent.action.EDIT" /> <action android:name="android.intent.action.PICK" /> <category android:name="android.intent.category.DEFAULT" /> <data android:mimeType="" /> </intent-filter> <intent-filter>

<action android:name="android.intent.action.GET_CONTENT" /> <category android:name="android.intent.category.DEFAULT" /> <data android:mimeType="" /> </intent-filter> </activity>

<activity android:name="NoteEditor"

android:theme="@android:style/Theme.Light" android:label="@string/title note" > <intent-filter android:label="@string/resolve edit">

<action android:name="android.intent.action.VIEW" /> <action android:name="android.intent.action.EDIT" /> <action android:name=" NOTE" /> <category android:name="android.intent.category.DEFAULT" /> <data android:mimeType="" /> </intent-filter> <intent-filter>

<action android:name="android.intent.action.INSERT" />

<category android:name="android.intent.category.DEFAULT" />

<data android:mimeType="" />

</intent-filter> </activity>

<activity android:name="TitleEditor"

android:label="@string/title edit title" android:theme="@android:style/Theme.Dialog"> <intent-filter android:label="@string/resolve title">

<action android:name=" TITLE" /> <category android:name="android.intent.category.DEFAULT" /> <category android:name="android.intent.category.ALTERNATIVE" /> <category android:name="android.intent.category.SELECTED_ALTERNATIVE" />

<data android:mimeType="" /> </intent-filter> </activity> </application> </manifest>

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:


<action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter>

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


<action android:name="android.intent.action.VIEW" /> <action android:name="android.intent.action.EDIT" /> <action android:name="android.intent.action.PICK" /> <category android:name="android.intent.category.DEFAULT" /> <data android:mimeType="" /> </intent-filter>

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 VIEW and EDIT actions), or to pick a particular note from the directory (via the PICK action).

The mimeType attribute of the <data> 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 ( from a content provider that holds Note Pad data ( The Intent object that launches the activity would include a content: URI specifying the exact data of this type that the activity should open.

Note also the DEFAULT category supplied in this filter. It's there because the Context.startActivity() and Activity.startActivityForResult() methods treat all intents as if they contained the DEFAULT category — with just two exceptions:

o Intents that explicitly name the target activity o Intents consisting of the MAIN action and LAUNCHER category

Therefore, the DEFAULT category is required for all filters — except for those with the MAIN action and LAUNCHER category. (Intent filters are not consulted for explicit intents.)

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