Bug 1519193 part 1. Add an iterator that implements "shadow-including tree order" traversal. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D16242

--HG--
extra : moz-landing-system : lando

dom/base/ShadowIncludingTreeIterator.h

@@ -0,0 +1,113 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Implementation of
+ * https://dom.spec.whatwg.org/#concept-shadow-including-tree-order in iterator
+ * form.  This can and should be used to avoid recursion on the stack and lots
+ * of function calls during shadow-including tree iteration.
+ */
+
+#ifndef mozilla_dom_ShadowIncludingTreeIterator_h
+#define mozilla_dom_ShadowIncludingTreeIterator_h
+
+#include "nsINode.h"
+#include "nsTArray.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/ShadowRoot.h"
+
+namespace mozilla {
+namespace dom {
+
+class ShadowIncludingTreeIterator {
+ public:
+  /**
+   * Initialize an iterator with aRoot.  After that it can be iterated with a
+   * range-based for loop.  At the moment, that's the only supported form of use
+   * for this iterator.
+   */
+  explicit ShadowIncludingTreeIterator(nsINode& aRoot) : mCurrent(&aRoot) {
+    mRoots.AppendElement(&aRoot);
+  }
+
+#ifdef DEBUG
+  ~ShadowIncludingTreeIterator() {
+    MOZ_ASSERT(
+        !mMutationGuard.Mutated(0),
+        "Don't mutate the DOM while using a ShadowIncludingTreeIterator");
+  }
+#endif  // DEBUG
+
+  // Basic support for range-based for loops.  This will modify the iterator as
+  // it goes.
+  ShadowIncludingTreeIterator& begin() { return *this; }
+
+  ShadowIncludingTreeIterator end() { return ShadowIncludingTreeIterator(); }
+
+  bool operator!=(const ShadowIncludingTreeIterator& aOther) {
+    return mCurrent != aOther.mCurrent;
+  }
+
+  void operator++() { Next(); }
+
+  nsINode* operator*() { return mCurrent; }
+
+ private:
+  // Constructor used only for end() to represent a drained iterator.
+  ShadowIncludingTreeIterator() : mCurrent(nullptr) {}
+
+  void Next() {
+    MOZ_ASSERT(mCurrent, "Don't call Next() after we have no current node");
+
+    // We walk shadow roots immediately after their shadow host.
+    if (Element* element = Element::FromNode(mCurrent)) {
+      if (ShadowRoot* shadowRoot = element->GetShadowRoot()) {
+        mCurrent = shadowRoot;
+        mRoots.AppendElement(shadowRoot);
+        return;
+      }
+    }
+
+    mCurrent = mCurrent->GetNextNode(mRoots.LastElement());
+    while (!mCurrent) {
+      // Nothing left under this root.  Keep trying to pop the stack until we
+      // find a node or run out of stack.
+      nsINode* root = mRoots.PopLastElement();
+      if (mRoots.IsEmpty()) {
+        // No more roots to step out of; we're done.  mCurrent is already set to
+        // null.
+        return;
+      }
+      mCurrent =
+          ShadowRoot::FromNode(root)->Host()->GetNextNode(mRoots.LastElement());
+    }
+  }
+
+  // The current node we're at.
+  nsINode* mCurrent;
+
+  // Stack of roots that we're inside of right now.  An empty stack can only
+  // happen when mCurrent is null (and hence we are done iterating).
+  //
+  // The default array size here is picked based on gut feeling.  We want at
+  // least 1, since we will always add something to it in our constructor.
+  // Having a few more entries probably makes sense, because this is commonly
+  // used in cases when we know we have custom elements, and hence likely have
+  // shadow DOMs.  But the exact value "4" was just picked because it sounded
+  // not too big, not too small.  Feel free to replace it with something else
+  // based on actual data.
+  AutoTArray<nsINode*, 4> mRoots;
+
+#ifdef DEBUG
+  // Make sure no one mutates the DOM while we're walking over it.
+  nsMutationGuard mMutationGuard;
+#endif  // DEBUG
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_ShadowIncludingTreeIterator_h

dom/base/moz.build

@@ -220,6 +220,7 @@ EXPORTS.mozilla.dom += [
     'ScreenLuminance.h',
     'ScreenOrientation.h',
     'Selection.h',
+    'ShadowIncludingTreeIterator.h',
     'ShadowRoot.h',
     'StructuredCloneBlob.h',
     'StructuredCloneHolder.h',