Bug 93002

As per http://lists.webkit.org/pipermail/webkit-gtk/2012-August/001186.html, the following bug (incomplete functionality) exists with the WebKitDOMTreeWalker API: webkit_dom_document_create_tree_walker is not possible to use, because: 1) It requires a non-NULL WebKitDOMNodeFilter pointer As per the usage of document.createTreeWalker in Javascript, this is a bug because you are allowed to pass "null" for the Filter parameter in Javascript. In WebKitDOMDocument.cpp, in the function body for webkit_dom_document_create_tree_walker, the line g_return_val_if_fail(filter, 0) should be removed, because WebCore::Document::createTreeWalker() will accept a NULL filter. The Document::createTreeWalker() function will convert the null raw pointer to a smart pointer, and pass it to a WebCore::Traversal instance. WebCore::Traversal will then check for a NULL filter in Traversal::acceptNode. So there is no reason for the GTK layer to check for a NULL with g_return_val_if_fail(filter, 0). 2) The Webkit GTK API does not expose any means to create a WebKitDOMNodeFilter object. (The header file WebKitDOMNodeFilter.h does not expose a constructor.) In combination with (1), this second bug makes WebKitDOMTreeWalker impossible to use at all, since the Webkit GTK API exposes no way to instantiate a WebKitDOMNodeFilter instance. In WebKitDOMNodeFilter.h there needs to be a constructor function exposed, which wraps a WebCore::NodeFilter object. It's also unclear how the WebKit GTK API handles user-provided Node Filters (which are handled using the virtual function NodeFilterCondition::acceptNode() in WebCore.) Presumably, a derived class of webCore::NodeFilterCondition would need to be created which just defers to a wrapped C function provided at the Webkit-GTK layer.