Implement Cold Start Notification Open

• Adds ability for the SDK to notify Flutter clients if the user opened the app by tapping a notification/tapping a button on a notification.

Author: Nightsd01

ios/Classes/OneSignalPlugin.m

@@ -72,7 +72,9 @@ + (instancetype)sharedInstance
 + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
 
     [OneSignal initWithLaunchOptions:nil appId:nil handleNotificationAction:^(OSNotificationOpenedResult *result) {
-        
+        @synchronized (OneSignalPlugin.sharedInstance.coldStartOpenResult) {
+            OneSignalPlugin.sharedInstance.coldStartOpenResult = result;
+        }
     }];
 
     OneSignalPlugin.sharedInstance.channel = [FlutterMethodChannel
@@ -139,6 +141,12 @@ - (void)initOneSignal:(FlutterMethodCall *)call withResult:(FlutterResult)result
         [self addObservers];
     }
 
+    @synchronized(self.coldStartOpenResult) {
+        if (self.coldStartOpenResult) {
+            [self handleNotificationOpened:self.coldStartOpenResult];
+        }
+    }
+    
     result(@[]);
 }