Added method to set 'mutable-content' to 1. Fixes #321, #328.

src/main/java/com/notnoop/apns/PayloadBuilder.java

@@ -34,6 +34,7 @@
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.Map;
+
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.notnoop.apns.internal.Utilities;
 
@@ -262,6 +263,19 @@ public PayloadBuilder instantDeliveryOrSilentNotification() {
         return this;
     }
 
+    /**
+     * In order for the push notification to get picked up by UNNotificationServiceExtension, the
+     * aps dictionary must include 'mutable-content' key with value set to 1.
+     *
+     * @see https://developer.apple.com/reference/usernotifications/unnotificationserviceextension
+     *
+     * @return this
+     */
+    public PayloadBuilder mutableContent() {
+        aps.put("mutable-content", 1);
+        return this;
+    }
+
     /**
      * Set the notification localized key for the alert body
      * message.

src/test/java/com/notnoop/apns/PayloadBuilderTest.java

@@ -570,4 +570,14 @@ public void instantMessageWithAlert() {
         final String actual = builder.toString();
         assertEqualsJson(expected, actual);
     }
+
+    @Test
+    public void mutableConentMessage() {
+        final PayloadBuilder builder = new PayloadBuilder();
+        builder.mutableContent();
+
+        final String expected = "{\"aps\":{\"mutable-content\":1}}";
+        final String actual = builder.toString();
+        assertEqualsJson(expected, actual);
+    }
 }