1 files changed, 145 insertions, 0 deletions
diff --git a/layout/generic/nsQueryFrame.h b/layout/generic/nsQueryFrame.h
new file mode 100644
index 0000000000..326b29266c
--- /dev/null
+++ b/layout/generic/nsQueryFrame.h
@@ -0,0 +1,145 @@
+/* -*- 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/. */
+
+#ifndef nsQueryFrame_h
+#define nsQueryFrame_h
+
+#include <type_traits>
+
+#include "nscore.h"
+#include "mozilla/Assertions.h"
+
+// NOTE: the long lines in this file are intentional to make compiler error
+// messages more readable.
+
+#define NS_DECL_QUERYFRAME_TARGET(classname)      \
+  static const nsQueryFrame::FrameIID kFrameIID = \
+      nsQueryFrame::classname##_id;               \
+  typedef classname Has_NS_DECL_QUERYFRAME_TARGET;
+
+#define NS_DECL_QUERYFRAME void* QueryFrame(FrameIID id) const override;
+
+#define NS_QUERYFRAME_HEAD(class)               \
+  void* class ::QueryFrame(FrameIID id) const { \
+    switch (id) {
+#define NS_QUERYFRAME_ENTRY(class)                                    \
+  case class ::kFrameIID: {                                           \
+    static_assert(                                                    \
+        std::is_same_v<class, class ::Has_NS_DECL_QUERYFRAME_TARGET>, \
+        #class " must declare itself as a queryframe target");        \
+    return const_cast<class*>(static_cast<const class*>(this));       \
+  }
+
+#define NS_QUERYFRAME_ENTRY_CONDITIONAL(class, condition)               \
+  case class ::kFrameIID:                                               \
+    if (condition) {                                                    \
+      static_assert(                                                    \
+          std::is_same_v<class, class ::Has_NS_DECL_QUERYFRAME_TARGET>, \
+          #class " must declare itself as a queryframe target");        \
+      return const_cast<class*>(static_cast<const class*>(this));       \
+    }                                                                   \
+    break;
+
+#define NS_QUERYFRAME_TAIL_INHERITING(class) \
+  default:                                   \
+    break;                                   \
+    }                                        \
+    return class ::QueryFrame(id);           \
+    }
+
+#define NS_QUERYFRAME_TAIL_INHERITANCE_ROOT                          \
+  default:                                                           \
+    break;                                                           \
+    }                                                                \
+    MOZ_ASSERT(id != GetFrameId(),                                   \
+               "A frame failed to QueryFrame to its *own type*. "    \
+               "It may be missing NS_DECL_QUERYFRAME, or a "         \
+               "NS_QUERYFRAME_ENTRY() line with its own type name"); \
+    return nullptr;                                                  \
+    }
+
+class nsQueryFrame {
+ public:
+  enum FrameIID {
+#define FRAME_ID(classname, ...) classname##_id,
+#define ABSTRACT_FRAME_ID(classname) classname##_id,
+#include "mozilla/FrameIdList.h"
+#undef FRAME_ID
+#undef ABSTRACT_FRAME_ID
+  };
+
+  // A strict subset of FrameIID above for frame classes that we instantiate.
+  enum class ClassID : uint8_t {
+#define FRAME_ID(classname, ...) classname##_id,
+#define ABSTRACT_FRAME_ID(classname)
+#include "mozilla/FrameIdList.h"
+#undef FRAME_ID
+#undef ABSTRACT_FRAME_ID
+  };
+
+  virtual void* QueryFrame(FrameIID id) const = 0;
+};
+
+class nsIFrame;
+
+template <class Source>
+class do_QueryFrameHelper {
+ public:
+  explicit do_QueryFrameHelper(Source* s) : mRawPtr(s) {}
+
+  // The return and argument types here are arbitrarily selected so no
+  // corresponding member function exists.
+  typedef void (do_QueryFrameHelper::*MatchNullptr)(double, float);
+  // Implicit constructor for nullptr, trick borrowed from already_AddRefed.
+  MOZ_IMPLICIT do_QueryFrameHelper(MatchNullptr aRawPtr) : mRawPtr(nullptr) {}
+
+  template <class Dest>
+  operator Dest*() {
+    static_assert(std::is_same_v<std::remove_const_t<Dest>,
+                                 typename Dest::Has_NS_DECL_QUERYFRAME_TARGET>,
+                  "Dest must declare itself as a queryframe target");
+    if (!mRawPtr) {
+      return nullptr;
+    }
+    if (Dest* f = FastQueryFrame<Source, Dest>::QueryFrame(mRawPtr)) {
+      MOZ_ASSERT(
+          f == reinterpret_cast<Dest*>(mRawPtr->QueryFrame(Dest::kFrameIID)),
+          "fast and slow paths should give the same result");
+      return f;
+    }
+    return reinterpret_cast<Dest*>(mRawPtr->QueryFrame(Dest::kFrameIID));
+  }
+
+ private:
+  // For non-nsIFrame types there is no fast-path.
+  template <class Src, class Dst, typename = void, typename = void>
+  struct FastQueryFrame {
+    static Dst* QueryFrame(Src* aFrame) { return nullptr; }
+  };
+
+  // Specialization for any nsIFrame type to any nsIFrame type -- if the source
+  // instance's mClass matches kFrameIID of the destination type then
+  // downcasting is safe.
+  template <class Src, class Dst>
+  struct FastQueryFrame<
+      Src, Dst, std::enable_if_t<std::is_base_of<nsIFrame, Src>::value>,
+      std::enable_if_t<std::is_base_of<nsIFrame, Dst>::value>> {
+    static Dst* QueryFrame(Src* aFrame) {
+      return nsQueryFrame::FrameIID(aFrame->mClass) == Dst::kFrameIID
+                 ? reinterpret_cast<Dst*>(aFrame)
+                 : nullptr;
+    }
+  };
+
+  Source* mRawPtr;
+};
+
+template <class T>
+inline do_QueryFrameHelper<T> do_QueryFrame(T* s) {
+  return do_QueryFrameHelper<T>(s);
+}
+
+#endif  // nsQueryFrame_h