Clear iOS platform instances synchronously

ios/Classes/AblyFlutter.h

@@ -25,14 +25,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 -(ARTPaginatedResult *) getPaginatedResult:(NSNumber *const) handle;
 
-/**
- This method must be called from the main dispatch queue. It may only be called
- once.
-
- @param completionHandler Will be called on the main dispatch queue when all
- platform objects have been closed down cleanly.
- */
--(void)disposeWithCompletionHandler:(dispatch_block_t)completionHandler;
+-(void)dispose;
 
 @end
 

ios/Classes/AblyFlutter.m

@@ -126,21 +126,6 @@ -(ARTPaginatedResult *) getPaginatedResult:(NSNumber *const) handle {
     return [_paginatedResults objectForKey:handle];
 }
 
--(void)disposeWithCompletionHandler:(const dispatch_block_t)completionHandler {
-    if (!completionHandler) {
-        [NSException raise:NSInvalidArgumentException format:@"completionHandler cannot be nil."];
-    }
-
-    dispatch_assert_queue(dispatch_get_main_queue());
-
-    // This is contrived for now but the point is that we can introduce a clean,
-    // asynchronous close via a background queue here if required.
-    dispatch_async(dispatch_get_main_queue(), ^{
-        [self dispose];
-        completionHandler();
-    });
-}
-
 -(void)dispose {
     for (ARTRealtime *const r in _realtimeInstances.allValues) {
         [r close];

ios/Classes/AblyFlutterPlugin.m

@@ -22,7 +22,7 @@
  static FlutterHandle declarations.
  */
 @interface AblyFlutterPlugin ()
--(void)registerWithCompletionHandler:(FlutterResult)completionHandler;
+-(void)reset;
 @end
 
 NS_ASSUME_NONNULL_END
@@ -36,7 +36,8 @@ -(void)registerWithCompletionHandler:(FlutterResult)completionHandler;
 };
 
 static const FlutterHandler _resetAblyClients = ^void(AblyFlutterPlugin *const plugin, FlutterMethodCall *const call, const FlutterResult result) {
-    [plugin registerWithCompletionHandler:result];
+    [plugin reset];
+    result(nil);
 };
 
 static const FlutterHandler _createRestWithOptions = ^void(AblyFlutterPlugin *const plugin, FlutterMethodCall *const call, const FlutterResult result) {
@@ -652,14 +653,9 @@ -(void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
     }
 }
 
--(void)registerWithCompletionHandler:(const FlutterResult)completionHandler {
-    if (!completionHandler) {
-        [NSException raise:NSInvalidArgumentException format:@"completionHandler cannot be nil."];
-    }
-    [_ably disposeWithCompletionHandler:^{
-        [self->_streamsChannel reset];
-        completionHandler(nil);
-    }];
+-(void)reset {
+    [_ably dispose];
+    [self->_streamsChannel reset];
 }
 
 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {