Issue 1282 - Show error message when user tries to block whitelisted elements

background.js

Index: background.js
===================================================================
--- a/background.js
+++ b/background.js
@@ -519,6 +519,55 @@
       else
         sendResponse(false);
       break;
+    case "can-block":
+      var documentHost = extractHostFromFrame(sender.frame);
+
+      if (isFrameWhitelisted(sender.page, sender.frame, "DOCUMENT"))
+      {
+        sendResponse({canBlock: false, reason: ext.i18n.getMessage("cannot_block_because_domain_whitelisted", documentHost)});
+        break;
+      }
+
+      var sitekey = getKey(sender.page, sender.frame);
+      var exception = null;
+
+      for (var i = 0; !exception && i < msg.urls.length; i++)
+      {
+        var requestHost = extractHostFromURL(msg.url);
+        var thirdParty = isThirdParty(requestHost, documentHost);
+
+        var filter = defaultMatcher.matchesAny(msg.urls[i], msg.mediatype, documentHost, thirdParty, sitekey);
+        if (filter instanceof WhitelistFilter)
+          exception = filter;
+      }
+
+      for (var i = 0; !exception && i < msg.elemHideFilters.length; i++)
+        exception = ElemHide.getException(Filter.fromText(msg.elemHideFilters[i]), documentHost);
+
+      if (exception)
+      {
+        var reason = ext.i18n.getMessage("cannot_block_because_exception_rule", exception.text);
+
+        for (var i = 0; i < exception.subscriptions.length; i++)
+        {
+          var subscription = exception.subscriptions[i];
+          if (!subscription.disabled)
+          {
+            if (subscription.url == Prefs.subscriptions_exceptionsurl)
+              reason = ext.i18n.getMessage("cannot_block_because_acceptable_ads");
+            else if (subscription.title)
+              reason = ext.i18n.getMessage("cannot_block_because_exception_rule_with_origin", [exception.text, '"' + subscription.title + '"']);
+
+            break;
+          }
+        }
+
+        sendResponse({canBlock: false, reason: reason});
+        break;
+      }
+
+      sendResponse({canBlock: true});
+      break;
     case "get-domain-enabled-state":
       // Returns whether this domain is in the exclusion list.
       // The browser action popup asks us this.