Question

The documentation says you can specify a custom category.

• When, why and how would you do it?
• What would be the use of it?

Answer 1

The way I understand it, categories are public directives to the android operating system(and other apps) that represent different categories that your app should be a part of.

Example

• When the launcher icon is tapped on the home screen, the home application looks through every installed app’s manifest for the HOME category — and if so it displays it in the app drawer.

However, there’s more. You can specify categories in your applications manifest that lets the system know that you application can handle the intent category. For example, by putting a ALTERNATIVE category, other apps in the system know that your app can handle that category without specifically knowing the action name! In the following example, custom intent categories are passed through this intent, which is filtered and the corresponding object gets edited(taken from the Notes example app):

<intent-filter android:label="@string/resolve_title">
 <action android:name="com.android.notepad.action.EDIT_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="vnd.android.cursor.item/vnd.google.note" />
</intent-filter>

By registering this intent filter in an <activity /> tag, you can edit a “note”. The intent data would contain the note, and the intent would get routed to the activity that this filter is registered in.

In Conclusion:

There isn’t really a reason you’d use a custom category. They are for Android, and thus don’t really make sense in application use. But, if you choose to use them, they can be used in the methods described above. “They provide some specific semantic rules, and if those rules are useful to you then feel free to use them”(Hackbod).