Log a warning if there was an error while ObjC was retrieving message…

…s from the javascript, instead of throwing an exception while attempting to decode the message as JSON.

This can occur e.g if the webview loaded a new page immediately after dispatching a message to ObjC, in which case WebViewJavascriptBridge may not exist in the JS by the time ObjC reaches in to fetch the message queue.

Fix marcuswestin#159, marcuswestin#183

WebViewJavascriptBridge/WKWebViewJavascriptBridge.m

@@ -85,6 +85,9 @@ - (void) _setupInstance:(WKWebView*)webView {
 
 - (void)WKFlushMessageQueue {
     [_webView evaluateJavaScript:[_base webViewJavascriptFetchQueyCommand] completionHandler:^(NSString* result, NSError* error) {
+        if (error != nil) {
+            NSLog(@"WebViewJavascriptBridge: WARNING: Error when trying to fetch data from WKWebView: %@", error);
+        }
         [_base flushMessageQueue:result];
     }];
 }

WebViewJavascriptBridge/WebViewJavascriptBridgeBase.m

@@ -61,11 +61,12 @@ - (void)sendData:(id)data responseCallback:(WVJBResponseCallback)responseCallbac
 }
 
 - (void)flushMessageQueue:(NSString *)messageQueueString{
-    id messages = [self _deserializeMessageJSON:messageQueueString];
-    if (![messages isKindOfClass:[NSArray class]]) {
-        NSLog(@"WebViewJavascriptBridge: WARNING: Invalid %@ received: %@", [messages class], messages);
+    if (messageQueueString == nil || messageQueueString.length == 0) {
+        NSLog(@"WebViewJavascriptBridge: WARNING: ObjC got nil while fetching the message queue JSON from webview. This can happen if the WebViewJavascriptBridge JS is not currently present in the webview, e.g if the webview just loaded a new page.");
         return;
     }
+
+    id messages = [self _deserializeMessageJSON:messageQueueString];
     for (WVJBMessage* message in messages) {
         if (![message isKindOfClass:[WVJBMessage class]]) {
             NSLog(@"WebViewJavascriptBridge: WARNING: Invalid %@ received: %@", [message class], message);