Add notes about calling register

Author: Jawnnypoo

gcm/README.md

@@ -15,7 +15,15 @@ dependencies {
     implementation "com.google.android.gms:play-services-gcm:latest.version.here"
 }
 ```
-You will then need to register some things in your manifest, specifically:
+You will then need to register some things in your manifest, firstly, the GCM sender ID:
+```xml
+<meta-data
+    android:name="com.parse.push.gcm_sender_id"
+    android:value="id:YOUR_SENDER_ID_HERE" />
+```
+The sender ID should be all numbers. Make sure you are keeping the `id:` in the front
+
+Next:
 ```xml
 <receiver
     android:name="com.google.android.gms.gcm.GcmReceiver"
@@ -60,7 +68,20 @@ After these services are registered in the Manifest, you then need to register y
     </intent-filter>
 </receiver>
 ```
-And from there, you should be good to go.
+After all this, you will need to register GCM in your `Application.onCreate()` like so:
+```java
+@Override
+public void onCreate() {
+    super.onCreate();
+    Parse.Configuration configuration = new Parse.Configuration.Builder(this)
+            //...
+            .build();
+    Parse.initialize(configuration);
+    ParseGCM.register(this);
+}
+```
+
+After this, you are all set.
 
 ## Custom Notifications
 If you need to customize the notification that is sent out from a push, you can do so easily by extending `ParsePushBroadcastReceiver` with your own class and registering it instead in the Manifest.

gcm/src/main/java/com/parse/gcm/ParseGCM.java

@@ -12,6 +12,11 @@
 import android.os.Bundle;
 import android.support.annotation.Nullable;
 
+import com.firebase.jobdispatcher.Constraint;
+import com.firebase.jobdispatcher.FirebaseJobDispatcher;
+import com.firebase.jobdispatcher.GooglePlayDriver;
+import com.firebase.jobdispatcher.Job;
+import com.firebase.jobdispatcher.RetryStrategy;
 import com.parse.ManifestInfo;
 import com.parse.PLog;
 
@@ -24,6 +29,31 @@ public class ParseGCM {
 
     static final String TAG = "ParseGCM";
 
+    /**
+     * Register your app to start receiving GCM pushes
+     *
+     * @param context context
+     */
+    public static void register(Context context) {
+        //kicks off the background job
+        PLog.v(TAG, "Scheduling job to register Parse GCM");
+        FirebaseJobDispatcher dispatcher = new FirebaseJobDispatcher(new GooglePlayDriver(context.getApplicationContext()));
+        Job job = dispatcher.newJobBuilder()
+                .setRecurring(false)
+                .setReplaceCurrent(true)
+                // retry with exponential backoff
+                .setRetryStrategy(RetryStrategy.DEFAULT_EXPONENTIAL)
+                .setConstraints(
+                        // only run on a network
+                        Constraint.ON_ANY_NETWORK
+                )
+                .setService(ParseGCMJobService.class) // the JobService that will be called
+                .setTag("initialize")        // uniquely identifies the job
+                .build();
+
+        dispatcher.mustSchedule(job);
+    }
+
     @Nullable
     static String gcmSenderFromManifest(Context context) {
         // Look for an element like this as a child of the <application> element:
@@ -70,7 +100,7 @@ private static String actualSenderIDFromExtra(Object senderIDExtra) {
             return null;
         }
 
-        String senderID = (String)senderIDExtra;
+        String senderID = (String) senderIDExtra;
         if (!senderID.startsWith("id:")) {
             return null;
         }

gcm/src/main/java/com/parse/gcm/ParseGCMInstanceIDListenerService.java

@@ -16,20 +16,6 @@ public class ParseGCMInstanceIDListenerService extends InstanceIDListenerService
     public void onTokenRefresh() {
         super.onTokenRefresh();
 
-        FirebaseJobDispatcher dispatcher = new FirebaseJobDispatcher(new GooglePlayDriver(getApplicationContext()));
-        Job job = dispatcher.newJobBuilder()
-                .setRecurring(false)
-                .setReplaceCurrent(true)
-                // retry with exponential backoff
-                .setRetryStrategy(RetryStrategy.DEFAULT_EXPONENTIAL)
-                .setConstraints(
-                        // only run on a network
-                        Constraint.ON_ANY_NETWORK
-                )
-                .setService(ParseGCMJobService.class) // the JobService that will be called
-                .setTag("initialize")        // uniquely identifies the job
-                .build();
-
-        dispatcher.mustSchedule(job);
+        ParseGCM.register(getApplicationContext());
     }
 }