1 files changed, 117 insertions, 0 deletions
diff --git a/js/public/CallNonGenericMethod.h b/js/public/CallNonGenericMethod.h
new file mode 100644
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+++ b/js/public/CallNonGenericMethod.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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 js_CallNonGenericMethod_h
+#define js_CallNonGenericMethod_h
+
+#include "jstypes.h"
+
+#include "js/CallArgs.h"
+
+namespace JS {
+
+// Returns true if |v| is considered an acceptable this-value.
+typedef bool (*IsAcceptableThis)(HandleValue v);
+
+// Implements the guts of a method; guaranteed to be provided an acceptable
+// this-value, as determined by a corresponding IsAcceptableThis method.
+typedef bool (*NativeImpl)(JSContext* cx, const CallArgs& args);
+
+namespace detail {
+
+// DON'T CALL THIS DIRECTLY.  It's for use only by CallNonGenericMethod!
+extern JS_PUBLIC_API(bool)
+CallMethodIfWrapped(JSContext* cx, IsAcceptableThis test, NativeImpl impl, const CallArgs& args);
+
+} // namespace detail
+
+// Methods usually act upon |this| objects only from a single global object and
+// compartment.  Sometimes, however, a method must act upon |this| values from
+// multiple global objects or compartments.  In such cases the |this| value a
+// method might see will be wrapped, such that various access to the object --
+// to its class, its private data, its reserved slots, and so on -- will not
+// work properly without entering that object's compartment.  This method
+// implements a solution to this problem.
+//
+// To implement a method that accepts |this| values from multiple compartments,
+// define two functions.  The first function matches the IsAcceptableThis type
+// and indicates whether the provided value is an acceptable |this| for the
+// method; it must be a pure function only of its argument.
+//
+//   static const JSClass AnswerClass = { ... };
+//
+//   static bool
+//   IsAnswerObject(const Value& v)
+//   {
+//       if (!v.isObject())
+//           return false;
+//       return JS_GetClass(&v.toObject()) == &AnswerClass;
+//   }
+//
+// The second function implements the NativeImpl signature and defines the
+// behavior of the method when it is provided an acceptable |this| value.
+// Aside from some typing niceties -- see the CallArgs interface for details --
+// its interface is the same as that of JSNative.
+//
+//   static bool
+//   answer_getAnswer_impl(JSContext* cx, JS::CallArgs args)
+//   {
+//       args.rval().setInt32(42);
+//       return true;
+//   }
+//
+// The implementation function is guaranteed to be called *only* with a |this|
+// value which is considered acceptable.
+//
+// Now to implement the actual method, write a JSNative that calls the method
+// declared below, passing the appropriate template and runtime arguments.
+//
+//   static bool
+//   answer_getAnswer(JSContext* cx, unsigned argc, JS::Value* vp)
+//   {
+//       JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+//       return JS::CallNonGenericMethod<IsAnswerObject, answer_getAnswer_impl>(cx, args);
+//   }
+//
+// Note that, because they are used as template arguments, the predicate
+// and implementation functions must have external linkage. (This is
+// unfortunate, but GCC wasn't inlining things as one would hope when we
+// passed them as function arguments.)
+//
+// JS::CallNonGenericMethod will test whether |args.thisv()| is acceptable.  If
+// it is, it will call the provided implementation function, which will return
+// a value and indicate success.  If it is not, it will attempt to unwrap
+// |this| and call the implementation function on the unwrapped |this|.  If
+// that succeeds, all well and good.  If it doesn't succeed, a TypeError will
+// be thrown.
+//
+// Note: JS::CallNonGenericMethod will only work correctly if it's called in
+//       tail position in a JSNative.  Do not call it from any other place.
+//
+template<IsAcceptableThis Test, NativeImpl Impl>
+MOZ_ALWAYS_INLINE bool
+CallNonGenericMethod(JSContext* cx, const CallArgs& args)
+{
+    HandleValue thisv = args.thisv();
+    if (Test(thisv))
+        return Impl(cx, args);
+
+    return detail::CallMethodIfWrapped(cx, Test, Impl, args);
+}
+
+MOZ_ALWAYS_INLINE bool
+CallNonGenericMethod(JSContext* cx, IsAcceptableThis Test, NativeImpl Impl, const CallArgs& args)
+{
+    HandleValue thisv = args.thisv();
+    if (Test(thisv))
+        return Impl(cx, args);
+
+    return detail::CallMethodIfWrapped(cx, Test, Impl, args);
+}
+
+} // namespace JS
+
+#endif /* js_CallNonGenericMethod_h */