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29 5월 2013

Handling Phone Call Requests the Right Way for Users

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Posted by Dirk Dougherty, Android Developer Relations

One of the things users like most about Android is the flexibility to choose which apps should handle common tasks on their devices — from opening a web page or sending an SMS to playing a music file, taking a picture, or making phone calls. This flexibility is provided by Intents.

Intents give you a powerful way to integrate your apps deeply into the system — users can even choose to let your apps replace functionality provided by system apps. In those cases, it’s essential to make sure that anything your app can’t or doesn’t handle can still be handled properly by the default system app.

Proper implementation and testing are especially important for apps that provide telephony services. Make sure that your app doesn't interfere with emergency calling by listening for the wrong intent — CALL_PRIVILEGED. Follow the best practices below to handle outgoing calls the right way, using the NEW_OUTGOING_CALL intent.

Listening for outgoing call requests

Apps that provide phone calling services (such as VOIP or number management) can set up Intent filters to handle outgoing call requests, such as those made from the Dialer or other installed apps. This provides a seamless integration for the user, who can transition directly to the calling service without having to redial or launch another app.

When the user initiates a call, the system notifies interested apps by sending an ordered broadcast of the NEW_OUTGOING_CALL Intent, attaching the original phone number, URI, and other information as extras. This gives apps such as Google Voice and others a chance to modify, reroute, or cancel the call before it’s passed to the system’s default phone app.

If you want your phone calling app to be able to handle outgoing call requests, implement a broadcast receiver that receives the NEW_OUTGOING_CALL Intent, processes the number, and initiates a call as needed. Make sure to declare an intent filter for NEW_OUTGOING_CALL in the receiver, to let the system know that your app is interested in the broadcast. You’ll also need to request the PROCESS_OUTGOING_CALLS permission in order to receive the Intent.

Note that the system broadcasts NEW_OUTGOING_CALL only for numbers that are not associated with core dialing capabilities such as emergency numbers. This means that NEW_OUTGOING_CALL can not interfere with access to emergency services the way your use of CALL_PRIVILEGED might.

Here’s an example broadcast receiver declared in an app’s manifest file:

    <uses-permission android:name="android.permission.PROCESS_OUTGOING_CALLS" />  
        <receiver android:name=MyOutgoingCallHandler">
                <action android:name="android.intent.action.NEW_OUTGOING_CALL" />
                <category android:name="android.intent.category.DEFAULT" />

The implementation of the corresponding broadcast receiver would look something like this:

public class MyOutgoingCallHandler extends BroadcastReceiver {
  public void onReceive(Context context, Intent intent) {
    // Extract phone number reformatted by previous receivers
    String phoneNumber = getResultData();
    if (phoneNumber == null) {
      // No reformatted number, use the original
      phoneNumber = intent.getStringExtra(Intent.EXTRA_PHONE_NUMBER);
    // My app will bring up the call, so cancel the broadcast
    // Start my app to bring up the call

Because the NEW_OUTGOING_CALL broadcast is ordered, your app can choose whether to consume the call request itself or simply process the number and pass the result data on to other apps that may be interested. In this example, the broadcast receiver brings up a phone call on it’s own service and sets the result data to null. This prevents the call request from reaching the default phone app.

An anti-pattern

Rather than listening for NEW_OUTGOING_CALL Intents, some apps have mistakenly set up intent filters for CALL_PRIVILEGED Intents as a way to handle outgoing calls. This is not a recommended approach, because the system may send a CALL_PRIVILEGED Intent for any number, including emergency numbers. Since non-system apps can’t reformat emergency numbers or place emergency calls, attempting to handle CALL_PRIVILEGED could inadvertently interfere with access to emergency numbers.

CALL_PRIVILEGED should only be used by apps that have the necessary signatureOrSystem-level permission — it is not designed for use by any third-party apps.

Check your apps for proper use of NEW_OUTGOING_CALL

If your app provides phone calling services and already uses intent filters to handle outgoing call requests, take a few minutes to make sure it is listening for the proper Intent: NEW_OUTGOING_CALL.

If your app includes intent filters that listen for CALL_PRIVILEGED Intents, make sure to remove those filters and related code from the app (in favor of NEW_OUTGOING_CALL) and publish the updated app as soon as possible.