For instance, if you build a social app that can share messages or photos with the user's friends, it's in your best interest to support the ACTION_SEND intent so users can initiate a "share" action from another app and launch your app to perform the action.
To allow other apps to start your activity, you need to add an <intent-filter> element in your manifest file for the corresponding <activity> element.
When your app is installed on a device, the system identifies your intent filters and adds the information to an internal catalog of intents supported by all installed apps. When an app calls startActivity() or startActivityForResult(), with an implicit intent, the system finds which activity (or activities) can respond to the intent.
Add an Intent Filter
In order to properly define which intents your activity can handle, each intent filter you add should be as specific as possible in terms of the type of action and data the activity accepts.
The system may send a given Intent to an activity if that activity has an intent filter fulfills the following criteria of the Intent object:
A string naming the action to perform. Usually one of the platform-defined values such as ACTION_SEND or ACTION_VIEW.
Specify this in your intent filter with the <action> element. The value you specify in this element must be the full string name for the action, instead of the API constant (see the examples below).
A description of the data associated with the intent.
Specify this in your intent filter with the <data> element. Using one or more attributes in this element, you can specify just the MIME type, just a URI prefix, just a URI scheme, or a combination of these and others that indicate the data type accepted.
Note: If you don't need to declare specifics about the data Uri (such as when your activity handles to other kind of "extra" data, instead of a URI), you should specify only the android:mimeType attribute to declare the type of data your activity handles, such as text/plain or image/jpeg.
Provides an additional way to characterize the activity handling the intent, usually related to the user gesture or location from which it's started. There are several different categories supported by the system, but most are rarely used. However, all implicit intents are defined with CATEGORY_DEFAULT by default.
Specify this in your intent filter with the <category> element.
In your intent filter, you can declare which criteria your activity accepts by declaring each of them with corresponding XML elements nested in the <intent-filter> element.
For example, here's an activity with an intent filter that handles the ACTION_SEND intent when the data type is either text or an image:
Each incoming intent specifies only one action and one data type, but it's OK to declare multiple instances of the <action>, <category>, and <data> elements in each <intent-filter>.
If any two pairs of action and data are mutually exclusive in their behaviors, you should create separate intent filters to specify which actions are acceptable when paired with which data types.