The Inverse: <intent-filter>

When we use an explicit Intent with startActivity(), it is fairly clear how Android determines what activity to display: it is the one whose class is listed in the Intent.

When we use an implicit Intent — one with an action string and maybe a Uri, but no class — somehow Android needs to find out what activity (or activities) can handle that.

This is where the <intent-filter> comes in.

In our manifest, inside of an <activity> element, you can optionally have one or more <intent-filter> elements. These describe certain implicit Intent structures that our activity can handle. If some code tries starting an implicit Intent with a matching structure, our activity will be considered a candidate. So, in the case of our ACTION_PICK and ACTION_VIEW requests in the preceding sections, we are hoping that there are 1+ activities with an <intent-filter> that matches our Intent.

Most apps will have an activity with one particular <intent-filter>:

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

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

This would match an Intent like:

Intent(Intent.ACTION_MAIN).addCategory(Intent.CATEGORY_LAUNCHER)

A “category” is simply an identifier of related Intent structures and is one facet of an implicit Intent, along with things like the action string and “data” Uri.

A launcher can ask Android “what activities support this Intent?”. Android will return a list of matches, and the launcher can use this to provide options for the user. When the user clicks on a specific activity, then the launcher has the details to create an explicit Intent, which it can use to start the activity. Since this is the standard behavior of launchers, and since most Android apps want to have 1+ activities appear in the launcher, most apps will have 1+ activities with the MAIN/LAUNCHER <intent-filter>.

Most apps will not need <intent-filter> on an <activity> beyond this one, though. However, they might use other apps that have activities with other <intent-filter> options, such as the ACTION_PICK and ACTION_VIEW scenarios.

We will see an example of this sort of structure in an upcoming chapter, where we will examine an app that supports ACTION_SEND, the standard Intent action for “share” options in apps.