Tracking filter mutation via SVGElementProxy

third_party/WebKit/Source/core/svg/SVGElementProxy.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SVGElementProxy_h
+#define SVGElementProxy_h
+
+#include "core/fetch/DocumentResource.h"
+#include "platform/heap/Handle.h"
+#include "wtf/text/AtomicString.h"
+#include "wtf/text/WTFString.h"
+
+namespace blink {
+
+class Document;
+class SVGElement;
+class SVGResourceClient;
+class TreeScope;
+
+// A proxy to an SVGElement. Allows access to an element with a certain 'id',
+// and provides its clients with notifications when the proxied element
+// changes.
+//
+// SVGElements can be referenced using CSS, for example like:
+//
+//   filter: url(#baz);
+//
+//    or
+//
+//   filter: url(foo.com/bar.svg#baz);
+//
+// SVGElementProxy provide a mechanism to persistently reference an SVGElement
+// in such cases - regardless of if the proxied element reside in the same
+// document (read: tree scope) or in an external (resource) document. In the
+// latter case, SVGElementProxy will transparently take care of the loading of
+// the document (if requested, see SVGElementProxy::resolve.)
+//
+// The proxy uses an 'id' (c.f getElementById) to determine which element to
+// proxy. The id is looked up in the TreeScope of the element for a
+// same-document reference and in the TreeScope of the Document for a external
+// document reference.
+//
+// For same document references, the proxy will track changes that could affect
+// the 'id' lookup, to handle elements being added, removed or having their
+// 'id' mutated. (This does not apply for the external document case because
+// it's assumed they will not mutate after load, due to scripts not being run
+// et.c.)
+//
+// SVGElementProxys are created by CSSURIValue, and have SVGResourceClients as
+// a means to deliver change notifications. Clients that are interested in
+// change notifications hence need to register a SVGResourceClient with the
+// proxy. Most commonly this registration would take place when the
+// ComputedStyle changes.
+//
+// Registration of a client does not establish a link to the proxied element
+// right away. That is instead deferred until element(...) is called on the
+// SVGElementProxy. At this point, the proxy will be registered with either the
+// element (on success) or a set of "pending proxies" managed by
+// SVGDocumentExtensions. (For external references lookups will not be
+// registered, per the same assumption as mentioned above.)
+//
+// As content is mutated, clients will get notified via the proxied element
+// which in turn will notify all registered proxies, which will notify all
+// registered clients.
+//
+// <event> -> SVG...Element -> SVGElementProxy(0..N) -> SVGResourceClient(0..N)
+//
+class SVGElementProxy : public GarbageCollectedFinalized<SVGElementProxy>,
+                        private DocumentResourceClient {
+  USING_GARBAGE_COLLECTED_MIXIN(SVGElementProxy);
+  USING_PRE_FINALIZER(SVGElementProxy, detachDocumentClient);
+
+ public:
+  // Create a proxy to an element in the same document. (See also
+  // SVGURLReferenceResolver and the definition of 'local url'.)
+  static SVGElementProxy* create(const AtomicString& id) {
+    return new SVGElementProxy(id);
+  }
+
+  // Create a proxy to an element in a different document (indicated by |url|.)
+  static SVGElementProxy* create(const String& url, const AtomicString& id) {
+    return new SVGElementProxy(url, id);
+  }
+  virtual ~SVGElementProxy();
+
+  void addClient(SVGResourceClient*);
+  void removeClient(SVGResourceClient*);
+
+  // Resolve a potentially external document reference.
+  void resolve(Document&);
+
+  // Returns the proxied element, or null if the proxy is invalid.
+  SVGElement* element(TreeScope&);

[esprehn, 2016/10/25 01:18:42]

getElement or findElement?

[fs, 2016/10/25 15:00:53]

Done.

+
+  // Notify the proxy that the structure of the subtree rooted at the proxied
+  // element has mutated. This should generally only be called via a proxy
+  // tracker.
+  void contentChanged();
+
+  // Notify the proxy that the reference to the proxied element may now be
+  // pointing somewhere else (the id of an element changed, the resource
+  // document of the proxy finished loading, the element was removed from the
+  // document tree etc.)  This should generally only be called via a proxy
+  // tracker.
+  void proxiedElementChanged();
+
+  const AtomicString& id() const { return m_id; }
+
+  // The current "generation" of this proxy. This is essentially a sequence
+  // number (or counter) incremented with each change to the proxied element
+  // (or its descendants, although that depends on the type of element being
+  // proxied.)
+  unsigned generation() const { return m_generation; }

[esprehn, 2016/10/25 01:18:42]

I'd prefer a dirty bit specific to the subtree or something, my experience is
the generation model is very hard to debug

[fs, 2016/10/25 15:00:53]

This mechanism is only here for the case of the target element changing. With a
different mechanism (IdTargetObserver?) I think it'd be possible to eliminate
this. I don't see where we'd be served by a dirty bit, since there's no single
point of validation where we'd be resetting it.

+
+  DECLARE_VIRTUAL_TRACE();
+
+ private:
+  explicit SVGElementProxy(const AtomicString&);
+  SVGElementProxy(const String&, const AtomicString&);
+
+  void detachDocumentClient();
+  // ResourceClient implementation. Used when referencing an external resource
+  // document.
+  void notifyFinished(Resource*) override;
+  String debugName() const override { return "SVGElementProxy"; }
+
+  void incrementGeneration() { m_generation++; }
+  TreeScope* treeScopeForLookup(TreeScope&) const;
+
+  HeapHashSet<Member<SVGResourceClient>> m_clients;
+  Member<DocumentResource> m_document;
+  AtomicString m_id;
+  // URL for resolving references to external resource documents. Contains an
+  // absolute URL to the resource to load. Cleared when a load has been
+  // initiated. Ignored if m_isLocal is true.
+  String m_url;
+  unsigned m_generation;
+  bool m_isLocal;
+};
+
+// Collection of SVGElementProxys. This is hosted by elements that can be
+// subject to proxies (currently only SVGFilterElement), and is mainly a helper
+// for dealing with the many-to-one structure of SVGElementProxy.
+class SVGElementProxySet : public GarbageCollected<SVGElementProxySet> {
+ public:
+  void add(SVGElementProxy&);
+  bool isEmpty() const;
+
+  // Notify proxy clients that the (content of the) proxied element has
+  // changed.
+  void notifyContentChanged();
+
+  // Notify proxy clients that the reference to the proxied element is no
+  // longer valid. This also clears references to the proxied element.
+  void invalidateProxies();
+
+  DECLARE_TRACE();
+
+ private:
+  using ProxySet = HeapHashSet<WeakMember<SVGElementProxy>>;
+  ProxySet m_elementProxies;
+};
+
+}  // namespace blink
+
+#endif  // SVGElementProxy_h