nsWrapperCache.h
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* ***** BEGIN LICENSE BLOCK *****
* Version: MPL 1.1/GPL 2.0/LGPL 2.1
*
* The contents of this file are subject to the Mozilla Public License Version
* 1.1 (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
* http://www.mozilla.org/MPL/
*
* Software distributed under the License is distributed on an "AS IS" basis,
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* for the specific language governing rights and limitations under the
* License.
*
* The Original Code is Gecko DOM code.
*
* The Initial Developer of the Original Code is
* Mozilla Foundation.
* Portions created by the Initial Developer are Copyright (C) 2008
* the Initial Developer. All Rights Reserved.
*
* Contributor(s):
* Peter Van der Beken <peterv@propagandism.org>
*
* Alternatively, the contents of this file may be used under the terms of
* either the GNU General Public License Version 2 or later (the "GPL"), or
* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
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#ifndef nsWrapperCache_h___
#define nsWrapperCache_h___
#include "nsCycleCollectionParticipant.h"
struct JSObject;
struct JSContext;
class nsContentUtils;
class XPCWrappedNativeScope;
typedef PRUptrdiff PtrBits;
#define NS_WRAPPERCACHE_IID \
{ 0x6f3179a1, 0x36f7, 0x4a5c, \
{ 0x8c, 0xf1, 0xad, 0xc8, 0x7c, 0xde, 0x3e, 0x87 } }
/**
* Class to store the wrapper for an object. This can only be used with objects
* that only have one non-security wrapper at a time (for an XPCWrappedNative
* this is usually ensured by setting an explicit parent in the PreCreate hook
* for the class).
*
* An instance of nsWrapperCache can be gotten from an object that implements
* a wrapper cache by calling QueryInterface on it. Note that this breaks XPCOM
* rules a bit (this object doesn't derive from nsISupports).
*
* The cache can store objects other than wrappers. We allow wrappers to use a
* separate JSObject to store their state (mostly expandos). If the wrapper is
* collected and we want to preserve this state we actually store the state
* object in the cache.
*
* The cache can store 3 types of objects:
*
* If WRAPPER_IS_PROXY is not set (IsProxy() returns false):
* - a slim wrapper or the JSObject of an XPCWrappedNative wrapper
*
* If WRAPPER_IS_PROXY is set (IsProxy() returns true):
* - a proxy wrapper
* - an expando object
*
* If a proxy wrapper is GCed and it has an expando object we'll store the
* expando object in the cache. If we create a new proxy wrapper and the cache
* contains an expando object we'll store the expando object in the new wrapper
* and store the new wrapper in the cache. Unlinking from the cycle collector
* clears anything stored in the cache.
*
* A number of the methods are implemented in nsWrapperCacheInlines.h because we
* have to include some JS headers that don't play nicely with the rest of the
* codebase. Include nsWrapperCacheInlines.h if you need to call those methods.
*/
class nsWrapperCache
{
friend class nsContentUtils;
public:
NS_DECLARE_STATIC_IID_ACCESSOR(NS_WRAPPERCACHE_IID)
nsWrapperCache() : mWrapperPtrBits(0)
{
}
~nsWrapperCache()
{
NS_ASSERTION(!PreservingWrapper(),
"Destroying cache with a preserved wrapper!");
RemoveExpandoObject();
}
/**
* Get the cached wrapper.
*
* This getter clears the gray bit before handing out the JSObject which means
* that the object is guaranteed to be kept alive past the next CC.
*/
JSObject* GetWrapper() const;
/**
* Get the cached wrapper.
*
* This getter does not change the color of the JSObject meaning that the
* object returned is not guaranteed to be kept alive past the next CC.
*
* This should only be called if you are certain that the return value won't
* be passed into a JS API function and that it won't be stored without being
* rooted (or otherwise signaling the stored value to the CC).
*/
JSObject* GetWrapperPreserveColor() const;
/**
* Get the expando object, used for storing expando properties, if there is
* one available. If the cache holds a DOM proxy binding that proxy's expando
* object will be returned.
*
* This getter does not change the color of the JSObject meaning that the
* object returned is not guaranteed to be kept alive past the next CC.
*
* This should only be called if you are certain that the return value won't
* be passed into a JS API function and that it won't be stored without being
* rooted (or otherwise signaling the stored value to the CC).
*/
JSObject* GetExpandoObjectPreserveColor() const;
void SetWrapper(JSObject* aWrapper);
/**
* Clear the wrapper, but keep the expando object alive if the wrapper has
* one. This should be called from the finalizer for the wrapper.
*/
void ClearWrapper();
/**
* Clear the wrapper if it's a proxy, doesn't keep the expando object alive.
* This should be called when unlinking the cache.
*/
void ClearWrapperIfProxy();
bool PreservingWrapper()
{
return (mWrapperPtrBits & WRAPPER_BIT_PRESERVED) != 0;
}
void SetIsProxy()
{
NS_ASSERTION(!mWrapperPtrBits,
"This flag should be set before creating any wrappers.");
mWrapperPtrBits = WRAPPER_IS_PROXY;
}
void ClearIsProxy()
{
NS_ASSERTION(!mWrapperPtrBits || mWrapperPtrBits == WRAPPER_IS_PROXY,
"This flag should be cleared before creating any wrappers.");
mWrapperPtrBits = 0;
}
bool IsProxy() const
{
return (mWrapperPtrBits & WRAPPER_IS_PROXY) != 0;
}
/**
* Wrap the object corresponding to this wrapper cache. If non-null is
* returned, the object has already been stored in the wrapper cache and the
* value set in triedToWrap is meaningless. If null is returned then
* triedToWrap indicates whether an error occurred, if it's false then the
* object doesn't actually support creating a wrapper through its WrapObject
* hook.
*/
virtual JSObject* WrapObject(JSContext *cx, XPCWrappedNativeScope *scope,
bool *triedToWrap)
{
*triedToWrap = false;
return nsnull;
}
private:
// Only meant to be called by nsContentUtils.
void SetPreservingWrapper(bool aPreserve)
{
if(aPreserve) {
mWrapperPtrBits |= WRAPPER_BIT_PRESERVED;
}
else {
mWrapperPtrBits &= ~WRAPPER_BIT_PRESERVED;
}
}
JSObject *GetJSObjectFromBits() const
{
return reinterpret_cast<JSObject*>(mWrapperPtrBits & ~kWrapperBitMask);
}
void SetWrapperBits(void *aWrapper)
{
mWrapperPtrBits = reinterpret_cast<PtrBits>(aWrapper) |
(mWrapperPtrBits & WRAPPER_IS_PROXY);
}
void RemoveExpandoObject();
static JSObject *GetExpandoFromSlot(JSObject *obj);
/**
* If this bit is set then we're preserving the wrapper, which in effect ties
* the lifetime of the JS object stored in the cache to the lifetime of the
* native object. We rely on the cycle collector to break the cycle that this
* causes between the native object and the JS object, so it is important that
* any native object that supports preserving of its wrapper
* traces/traverses/unlinks the cached JS object (see
* NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER,
* NS_IMPL_CYCLE_COLLECTION_TRAVERSE_SCRIPT_OBJECTS and
* NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER).
*/
enum { WRAPPER_BIT_PRESERVED = 1 << 0 };
/**
* If this bit is set then the wrapper for the native object is a proxy. Note
* that that doesn't necessarily mean that the JS object stored in the cache
* is a JS proxy, as we sometimes store objects other than the wrapper in the
* cache.
*/
enum { WRAPPER_IS_PROXY = 1 << 1 };
enum { kWrapperBitMask = (WRAPPER_BIT_PRESERVED | WRAPPER_IS_PROXY) };
PtrBits mWrapperPtrBits;
};
NS_DEFINE_STATIC_IID_ACCESSOR(nsWrapperCache, NS_WRAPPERCACHE_IID)
#define NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY \
if ( aIID.Equals(NS_GET_IID(nsWrapperCache)) ) { \
*aInstancePtr = static_cast<nsWrapperCache*>(this); \
return NS_OK; \
}
#endif /* nsWrapperCache_h___ */
