fix: use mutable content and content available when set.

Content available is for background notifications hence the notification
shouldn't have an alert or sound.
Ref:
https://developer.apple.com/documentation/usernotifications/generating-a-remote-notification

notify/notification_fcm.go

@@ -71,29 +71,81 @@ func GetAndroidNotification(req *PushNotification) []*messaging.Message {
 		if req.Image != "" {
 			req.Notification.ImageURL = req.Image
 		}
+		if req.MutableContent {
+			req.APNS = &messaging.APNSConfig{
+				Payload: &messaging.APNSPayload{
+					Aps: &messaging.Aps{
+						MutableContent: req.MutableContent,
+					},
+				},
+			}
+		}
 	}
 
-	// Check if the notification has a sound
-	if req.Sound != nil {
-		sound, ok := req.Sound.(string)
-
-		if req.APNS == nil && ok {
+	// content-available is for background notifications and a badge, alert
+	// and sound keys should not be present.
+	// See: https://developer.apple.com/documentation/usernotifications/generating-a-remote-notification
+	if req.Badge == nil && req.Title == "" && req.Message == "" && req.ContentAvailable {
+		if req.Sound != nil {
+			_, ok := req.Sound.(string)
+			if !ok {
+				req.APNS = &messaging.APNSConfig{
+					Payload: &messaging.APNSPayload{
+						Aps: &messaging.Aps{
+							ContentAvailable: req.ContentAvailable,
+						},
+					},
+				}
+			}
+		} else {
 			req.APNS = &messaging.APNSConfig{
 				Payload: &messaging.APNSPayload{
 					Aps: &messaging.Aps{
-						Sound: sound,
+						ContentAvailable: req.ContentAvailable,
 					},
 				},
 			}
 		}
+	}
+
+	// Check if the notification has a sound
+	if req.Sound != nil {
+		sound, ok := req.Sound.(string)
+		if ok {
+			switch {
+			case req.APNS == nil:
+				req.APNS = &messaging.APNSConfig{
+					Payload: &messaging.APNSPayload{
+						Aps: &messaging.Aps{
+							Sound: sound,
+						},
+					},
+				}
+			case req.APNS.Payload == nil:
+				req.APNS.Payload = &messaging.APNSPayload{
+					Aps: &messaging.Aps{
+						Sound: sound,
+					},
+				}
 
-		if req.Android == nil && ok {
-			req.Android = &messaging.AndroidConfig{
-				Notification: &messaging.AndroidNotification{
+			case req.APNS.Payload.Aps == nil:
+				req.APNS.Payload.Aps = &messaging.Aps{
 					Sound: sound,
-				},
+				}
+			default:
+				req.APNS.Payload.Aps.Sound = sound
+
+			}
+
+			if req.Android == nil {
+				req.Android = &messaging.AndroidConfig{
+					Notification: &messaging.AndroidNotification{
+						Sound: sound,
+					},
+				}
 			}
 		}
+
 	}
 
 	// Check if the notification is a topic

notify/notification_fcm_test.go

@@ -149,13 +149,13 @@ func TestFCMMessage(t *testing.T) {
 func TestAndroidNotificationStructure(t *testing.T) {
 	test := "test"
 	req := &PushNotification{
-		Tokens:           []string{"a", "b"},
-		Message:          "Welcome",
-		To:               test,
-		Priority:         HIGH,
-		ContentAvailable: true,
-		Title:            test,
-		Sound:            test,
+		Tokens:         []string{"a", "b"},
+		Message:        "Welcome",
+		To:             test,
+		Priority:       HIGH,
+		MutableContent: true,
+		Title:          test,
+		Sound:          test,
 		Data: D{
 			"a": "1",
 			"b": 2,
@@ -177,6 +177,9 @@ func TestAndroidNotificationStructure(t *testing.T) {
 	assert.Equal(t, "1", messages[0].Data["a"])
 	assert.Equal(t, "2", messages[0].Data["b"])
 	assert.Equal(t, "{\"c\":\"3\",\"d\":4}", messages[0].Data["json"])
+	assert.NotNil(t, messages[0].APNS)
+	assert.Equal(t, req.Sound, messages[0].APNS.Payload.Aps.Sound)
+	assert.Equal(t, req.MutableContent, messages[0].APNS.Payload.Aps.MutableContent)
 
 	// test empty body
 	req = &PushNotification{
@@ -190,3 +193,27 @@ func TestAndroidNotificationStructure(t *testing.T) {
 
 	assert.Equal(t, "", messages[0].Notification.Body)
 }
+
+func TestAndroidBackgroundNotificationStructure(t *testing.T) {
+	req := &PushNotification{
+		Tokens:           []string{"a", "b"},
+		Priority:         HIGH,
+		ContentAvailable: true,
+		Data: D{
+			"a": "1",
+			"b": 2,
+			"json": map[string]interface{}{
+				"c": "3",
+				"d": 4,
+			},
+		},
+	}
+
+	messages := GetAndroidNotification(req)
+
+	assert.Equal(t, "1", messages[0].Data["a"])
+	assert.Equal(t, "2", messages[0].Data["b"])
+	assert.Equal(t, "{\"c\":\"3\",\"d\":4}", messages[0].Data["json"])
+	assert.NotNil(t, messages[0].APNS)
+	assert.Equal(t, req.ContentAvailable, messages[0].APNS.Payload.Aps.ContentAvailable)
+}