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118 lines
4.4 KiB
C++
118 lines
4.4 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
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* vim: set ts=8 sts=4 et sw=4 tw=99:
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef js_CallNonGenericMethod_h
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#define js_CallNonGenericMethod_h
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#include "jstypes.h"
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#include "js/CallArgs.h"
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namespace JS {
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// Returns true if |v| is considered an acceptable this-value.
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typedef bool (*IsAcceptableThis)(HandleValue v);
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// Implements the guts of a method; guaranteed to be provided an acceptable
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// this-value, as determined by a corresponding IsAcceptableThis method.
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typedef bool (*NativeImpl)(JSContext* cx, const CallArgs& args);
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namespace detail {
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// DON'T CALL THIS DIRECTLY. It's for use only by CallNonGenericMethod!
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extern JS_PUBLIC_API(bool)
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CallMethodIfWrapped(JSContext* cx, IsAcceptableThis test, NativeImpl impl, const CallArgs& args);
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} // namespace detail
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// Methods usually act upon |this| objects only from a single global object and
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// compartment. Sometimes, however, a method must act upon |this| values from
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// multiple global objects or compartments. In such cases the |this| value a
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// method might see will be wrapped, such that various access to the object --
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// to its class, its private data, its reserved slots, and so on -- will not
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// work properly without entering that object's compartment. This method
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// implements a solution to this problem.
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//
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// To implement a method that accepts |this| values from multiple compartments,
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// define two functions. The first function matches the IsAcceptableThis type
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// and indicates whether the provided value is an acceptable |this| for the
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// method; it must be a pure function only of its argument.
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//
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// static const JSClass AnswerClass = { ... };
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//
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// static bool
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// IsAnswerObject(const Value& v)
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// {
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// if (!v.isObject())
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// return false;
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// return JS_GetClass(&v.toObject()) == &AnswerClass;
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// }
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//
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// The second function implements the NativeImpl signature and defines the
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// behavior of the method when it is provided an acceptable |this| value.
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// Aside from some typing niceties -- see the CallArgs interface for details --
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// its interface is the same as that of JSNative.
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//
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// static bool
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// answer_getAnswer_impl(JSContext* cx, JS::CallArgs args)
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// {
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// args.rval().setInt32(42);
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// return true;
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// }
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//
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// The implementation function is guaranteed to be called *only* with a |this|
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// value which is considered acceptable.
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//
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// Now to implement the actual method, write a JSNative that calls the method
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// declared below, passing the appropriate template and runtime arguments.
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//
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// static bool
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// answer_getAnswer(JSContext* cx, unsigned argc, JS::Value* vp)
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// {
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// JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
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// return JS::CallNonGenericMethod<IsAnswerObject, answer_getAnswer_impl>(cx, args);
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// }
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//
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// Note that, because they are used as template arguments, the predicate
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// and implementation functions must have external linkage. (This is
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// unfortunate, but GCC wasn't inlining things as one would hope when we
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// passed them as function arguments.)
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//
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// JS::CallNonGenericMethod will test whether |args.thisv()| is acceptable. If
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// it is, it will call the provided implementation function, which will return
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// a value and indicate success. If it is not, it will attempt to unwrap
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// |this| and call the implementation function on the unwrapped |this|. If
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// that succeeds, all well and good. If it doesn't succeed, a TypeError will
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// be thrown.
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//
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// Note: JS::CallNonGenericMethod will only work correctly if it's called in
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// tail position in a JSNative. Do not call it from any other place.
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//
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template<IsAcceptableThis Test, NativeImpl Impl>
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MOZ_ALWAYS_INLINE bool
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CallNonGenericMethod(JSContext* cx, const CallArgs& args)
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{
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HandleValue thisv = args.thisv();
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if (Test(thisv))
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return Impl(cx, args);
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return detail::CallMethodIfWrapped(cx, Test, Impl, args);
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}
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MOZ_ALWAYS_INLINE bool
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CallNonGenericMethod(JSContext* cx, IsAcceptableThis Test, NativeImpl Impl, const CallArgs& args)
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{
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HandleValue thisv = args.thisv();
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if (Test(thisv))
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return Impl(cx, args);
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return detail::CallMethodIfWrapped(cx, Test, Impl, args);
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}
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} // namespace JS
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#endif /* js_CallNonGenericMethod_h */
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