Fix Cold-Start Notification Opened Handler

• Previously, if a user tapped a notification while the app was closed, the Flutter OneSignal SDK wasn't correctly calling the notification opened handler
• Fixed by adding a handleNotificationAction block for the first OneSignal init() call, which will save a reference to the OSNotificationOpenedResult object until the Flutter channel is up.

Author: Nightsd01

ios/Classes/OneSignalPlugin.h

@@ -29,6 +29,8 @@
 #import <OneSignal/OneSignal.h>
 
 @interface OneSignalPlugin : NSObject<FlutterPlugin, OSSubscriptionObserver, OSPermissionObserver, OSEmailSubscriptionObserver>
-@property (strong, nonatomic) FlutterMethodChannel *channel;
+
+// Do NOT initialize instances of this class.
+// You must only reference the shared instance.
 + (instancetype)sharedInstance;
 @end

ios/Classes/OneSignalPlugin.m

@@ -30,12 +30,32 @@
 #import "OneSignalTagsController.h"
 
 @interface OneSignalPlugin ()
-@property (nonatomic) BOOL waitingForUserConsent;
+
+@property (strong, nonatomic) FlutterMethodChannel *channel;
+
+/*
+    Will be true if the SDK is waiting for the
+    user's consent before initializing.
+    This is important because if the plugin
+    doesn't know the SDK is waiting for consent,
+    it will add the observers (ie. subscription)
+*/
+@property (atomic) BOOL waitingForUserConsent;
+
+/*
+    holds reference to any notifications received before the
+    flutter runtime channel has been opened
+    Thus, if a user taps a notification while the app is
+    terminated, the SDK will still notify the app once the
+    channel is open
+*/
+@property (strong, nonatomic) OSNotificationOpenedResult *coldStartOpenResult;
+
 @end
 
-@implementation OneSignalPlugin
 
-BOOL waitingForUserConsent;
+
+@implementation OneSignalPlugin
 
 + (instancetype)sharedInstance
 {
@@ -50,7 +70,10 @@ + (instancetype)sharedInstance
 
 #pragma mark FlutterPlugin
 + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
-    [OneSignal initWithLaunchOptions:nil appId:nil];
+    
+    [OneSignal initWithLaunchOptions:nil appId:nil handleNotificationAction:^(OSNotificationOpenedResult *result) {
+        
+    }];
 
     OneSignalPlugin.sharedInstance.channel = [FlutterMethodChannel
                                      methodChannelWithName:@"OneSignal"
@@ -111,7 +134,7 @@ - (void)initOneSignal:(FlutterMethodCall *)call withResult:(FlutterResult)result
     // until consent has been provided.
 
     if (OneSignal.requiresUserPrivacyConsent) {
-        waitingForUserConsent = true;
+        self.waitingForUserConsent = true;
     } else {
         [self addObservers];
     }
@@ -135,7 +158,7 @@ -(void)changeConsentStatus:(FlutterMethodCall *)call withResult:(FlutterResult)r
 
     [OneSignal consentGranted:granted];
 
-    if (waitingForUserConsent && granted) {
+    if (self.waitingForUserConsent && granted) {
         [self addObservers];
     }