nsWrapperCache.h

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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 * 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
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 *
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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
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 * 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>
 *
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 * 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;
class nsContentUtils;

typedef PRUptrdiff PtrBits;

#define NS_WRAPPERCACHE_IID \
{ 0x3a51ca81, 0xddab, 0x422c, \
  { 0x95, 0x3a, 0x13, 0x06, 0x28, 0x0e, 0xee, 0x14 } }

/**
 * Class to store the XPCWrappedNative for an object. This can only be used
 * with objects that only have one XPCWrappedNative at a time (usually ensured
 * by setting an explicit parent in the PreCreate hook for the class). This
 * object can be gotten by calling QueryInterface, note that this breaks XPCOM
 * rules a bit (this object doesn't derive from nsISupports).
 */
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!");
  }

  /**
   * 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.
   *
   * Implemented in xpcpublic.h because we have to include some JS headers that
   * don't play nicely with the rest of the codebase. Include xpcpublic.h if you
   * need to call this method.
   */
  inline JSObject* GetWrapper() const;

  /**
   * 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
  {
    return reinterpret_cast<JSObject*>(mWrapperPtrBits & ~kWrapperBitMask);
  }

  void SetWrapper(JSObject* aWrapper)
  {
    NS_ASSERTION(!PreservingWrapper(), "Clearing a preserved wrapper!");
    mWrapperPtrBits = reinterpret_cast<PtrBits>(aWrapper) |
                      (mWrapperPtrBits & WRAPPER_IS_PROXY);
  }

  void ClearWrapper()
  {
    NS_ASSERTION(!PreservingWrapper(), "Clearing a preserved wrapper!");
    mWrapperPtrBits = 0;
  }

  PRBool PreservingWrapper()
  {
    return (mWrapperPtrBits & WRAPPER_BIT_PRESERVED) != 0;
  }

  void SetIsProxy()
  {
    mWrapperPtrBits |= WRAPPER_IS_PROXY;
  }

  PRBool IsProxy()
  {
    return (mWrapperPtrBits & WRAPPER_IS_PROXY) != 0;
  }

private:
  // Only meant to be called by nsContentUtils.
  void SetPreservingWrapper(PRBool aPreserve)
  {
    if(aPreserve) {
      mWrapperPtrBits |= WRAPPER_BIT_PRESERVED;
    }
    else {
      mWrapperPtrBits &= ~WRAPPER_BIT_PRESERVED;
    }
  }

  enum { WRAPPER_BIT_PRESERVED = 1 << 0 };
  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___ */