Improved window.postMessage implementation

Summary:
Adds a queue to postMessage so that messages sent close together are not lost.

Setting location="a";location="b" results in only "b" reaching shouldStartLoadWithRequest. Making the second update asynchronous with setTimeout does not fix the issue unless a delay is added.

With this update, postMessage queues "b" until it gets a "message:received" event that confirms "a" has already been processed.

The included test sends two messages from a webview and checks that both are received. It fails against the preexisting code with the first message being dropped.
Closes https://github.com/facebook/react-native/pull/11304

Differential Revision: D5481385

Pulled By: hramos

fbshipit-source-id: 9b6af195eeff8f20c820e2fcdac997c90763e840

React/Views/RCTWebView.m

@@ -250,6 +250,11 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
     [event addEntriesFromDictionary: @{
       @"data": data,
     }];
+
+    NSString *source = @"document.dispatchEvent(new MessageEvent('message:received'));";
+
+    [_webView stringByEvaluatingJavaScriptFromString:source];
+
     _onMessage(event);
   }
 
@@ -301,10 +306,28 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
     }
     #endif
     NSString *source = [NSString stringWithFormat:
-      @"window.originalPostMessage = window.postMessage;"
-      "window.postMessage = function(data) {"
-        "window.location = '%@://%@?' + encodeURIComponent(String(data));"
-      "};", RCTJSNavigationScheme, kPostMessageHost
+      @"(function() {"
+        "window.originalPostMessage = window.postMessage;"
+
+        "var messageQueue = [];"
+        "var messagePending = false;"
+
+        "function processQueue() {"
+          "if (!messageQueue.length || messagePending) return;"
+          "messagePending = true;"
+          "window.location = '%@://%@?' + encodeURIComponent(messageQueue.shift());"
+        "}"
+
+        "window.postMessage = function(data) {"
+          "messageQueue.push(String(data));"
+          "processQueue();"
+        "};"
+
+        "document.addEventListener('message:received', function(e) {"
+          "messagePending = false;"
+          "processQueue();"
+        "});"
+      "})();", RCTJSNavigationScheme, kPostMessageHost
     ];
     [webView stringByEvaluatingJavaScriptFromString:source];
   }