weak_reference.h

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/*! \file
  \brief  weak reference and related classes

  \par Copyright Information

  Copyright (c) 2016 by The DiSTI Corporation.<br>
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  <br>
  All rights reserved.<br>

  This Software contains proprietary trade secrets of DiSTI and may not be
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Derivative Works based on this or other copyrighted DiSTI Software.

  NO WARRANTY. THE SOFTWARE IS PROVIDED "AS-IS," WITHOUT WARRANTY OF ANY KIND,
AND ANY USE OF THIS SOFTWARE PRODUCT IS AT YOUR OWN RISK. TO THE MAXIMUM EXTENT
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AND CONDITIONS, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
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  LIMITATION OF LIABILITY. TO THE MAXIMUM EXTENT PERMITTED BY APPLICABLE LAW,
IN NO EVENT SHALL DISTI OR ITS SUPPLIERS BE LIABLE FOR ANY SPECIAL, INCIDENTAL,
INDIRECT, OR CONSEQUENTIAL DAMAGES WHATSOEVER (INCLUDING, WITHOUT LIMITATION,
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*/
#ifndef INCLUDED_WEAK_REFERENCE_H
#define INCLUDED_WEAK_REFERENCE_H

#include "scoped_ptr.h"
#include "disti_assert.h"

namespace disti
{
    class WeakReferenceable;

    /** Interface for holding a weak reference to an object.  This class should generally not be used directly.  Use WeakRef instead */
    class WeakReference
    {
    public:
        /** Get a pointer to the object.  Returns NULL if the object was deleted */
        virtual WeakReferenceable* GetReferent() const = 0;
        
        /** Notify this weak reference that the object destroyed */
        virtual void NotifyReferentDestroyed() = 0;

        virtual ~WeakReference() {}
    };

    /** Interface for getting a weak reference to an object.  To implement this class, inherit from WeakReferenceableMixin (see DisplayObject as an example) */
    class WeakReferenceable
    {
    public:
        /** Add a weak reference to this object, that gets notified when this object is destroyed.  Does not take ownership of weakRef.  
          * \param weakRef the weak reference to add.
          */
        virtual void AddWeakReference( WeakReference* weakRef ) = 0;

        /** Notify this object that its weak reference is being destroyed */
        virtual void NotifyWeakReferenceDestroyed( WeakReference* weakReference ) = 0;

        virtual ~WeakReferenceable() {}
    };

    /** helper class to simplify working with a weak reference 
      * Ex Usage:
      *   DisplayObject* obj = GetSomeDisplayObject();
      *   WeakRef< DisplayObject > weakRef = obj;
      *   ...
      *   ... some code that may or may not have deleted obj
      *   ...
      *   if( !weakRef.IsNull() )
      *   {
      *       weakRef.Get()->Location( 0, 0, 0 );
      *   }
      */
    template< class T >
    class WeakRef
    {
    private:
        /** Implementation for a weak reference */
        class WeakReferenceImpl : virtual public WeakReference
        {
        public:
            /** constructor
              * \param referent a pointer to the WeakReferenceable object
              */
            WeakReferenceImpl( WeakReferenceable* referent )
                : _referent( referent )
            {
                if( referent )
                {
                    referent->AddWeakReference( this );
                }
            }

            /** destructor.  Notifies the WeakReferenceable that this WeakReference is being deleted */
            ~WeakReferenceImpl()
            {
                if( _referent )
                {
                    _referent->NotifyWeakReferenceDestroyed( this );
                }
            }

            /** \see WeakReference */
            WeakReferenceable* GetReferent() const DISTI_METHOD_OVERRIDE { return _referent; }

            /** notifies the WeakReference that the WeakReferenceable object is being destroyed so it can set its referent to NULL */
            void NotifyReferentDestroyed()
            {
                _referent = NULL;
            }

        private:
            /** pointer to the WeakReferenceable object */
            WeakReferenceable* _referent;
        };

    public:
        /** default ctor -- initializes weak reference to NULL
          */
        WeakRef()
        {
            GLS_ASSERT( IsNull() );
        }

        /** construct from a WeakReferenceable pointer
          * \param[in] weakReferenceable pointer to an WeakReferenceable interface ( can be NULL )
          */
        WeakRef( WeakReferenceable* weakReferenceable )
        {
            SetWeakReference( weakReferenceable );
        }

        /** construct from an WeakRef< T >
          * \param[in] w WeakRef< T > ( can be NULL )
          */
        WeakRef( const WeakRef& w )
        {
            SetWeakReference( w.GetWeakReferenceable() );
        }

        /** construct from an WeakRef< U >
          * \param[in] w WeakRef< U > ( can be NULL )
          */
        template <class U>
        WeakRef( const WeakRef<U>& w )
        {
            SetWeakReference( w.GetWeakReferenceable() );
        }

        /** assign this weak ref to reference the given WeakReferenceable*
          * \param weakReferenceable WeakReferenceable* in question (can be NULL)
          * \return a ref to this
          */
        WeakRef& operator=( WeakReferenceable* weakReferenceable )
        {
            SetWeakReference( weakReferenceable );
            return *this ;
        }

        /** assign this pointer to point to the given WeakRef< T >
          * \param w WeakRef< T > in question (can be NULL)
          * \return a ref to this
          */
        WeakRef& operator=( const WeakRef& w )
        {
            SetWeakReference( w.GetWeakReferenceable() );
            return *this;
        }

        /** assign this pointer to point to the given WeakRef< U >
          * \param w WeakRef< U > in question (can be NULL)
          * \return a ref to this
          */
        template <class U>
        WeakRef& operator=( const WeakRef<U>& w )
        {
            SetWeakReference( w.GetWeakReferenceable() );
            return *this;
        }

        /** determine if weak reference is NULL
          * \return true if weak ref is NULL else false
          */
        bool IsNull() const
        {
            return ( _weakReference.IsNull() || NULL == _weakReference->GetReferent() );
        }

        /** Get a T*
          * \return if referent exists and supports interface T then return a pointer to T, else return NULL
          */
        T* Get() const
        {
            T* ret = NULL;

            // if there is still a weak ref, see if it has a referent that supports interface T
            if( !IsNull() )
            {
                ret = dynamic_cast<T*>( _weakReference->GetReferent() );
            }

            return ret;
        }

        /** get the WeakReferenceable* contained in this weak ref
          * \return the WeakReferenceable pointer contained in this weak ref ( can be NULL )
          */
        WeakReferenceable* GetWeakReferenceable() const
        {
            return !_weakReference.IsNull() ? _weakReference->GetReferent() : NULL;
        }

    private:
        void SetWeakReference( WeakReferenceable* weakReferenceable )
        {
            WeakReference* weakRef = NULL;
            if( weakReferenceable )
            {
                // Important - this weak reference must be allocated in the same unit that will delete it, which is
                // why its only allocated here, right before we store it in the object that will delete it.
                weakRef = new WeakReferenceImpl( weakReferenceable );
                GLS_ASSERT( NULL != weakRef->GetReferent() );
            }

            _weakReference.Reset( weakRef );
        }

        ScopedPtr<WeakReference> _weakReference; /**< weak reference else NULL */
    };

} // namespace disti

#endif