I just considered something. I think this solution is focusing too
much on event handlers but event handlers are not the only type of
callback where this problem occurs. I can't give examples right now
but I'm sure there are many other types of callbacks where similar
problem occurs right now which is also solved with embedding
interface. So perhaps we should find a way to generalize this so that
it can work for a general callback type system?
Perhaps using a template?
Greetings,
On 8/3/06, Michael D. Adams <[EMAIL PROTECTED]> wrote:
> On 7/29/06, Jorrit Tyberghein <[EMAIL PROTECTED]> wrote:
> > > *token = new csWeakEventHandler(listener);
> >
> > This is invalid code. You can't use a weak ref like that. Keep in mind
> > that if the only thing that holds a reference to something is a weak
> > reference then the object will be removed. You'll have to rework the
> > code a bit so that after allocation you have a real reference for a
> > while and then you release that reference as soon as you register to
> > the event registry.
> >
> > I do understand the basic idea of your solution though and it seems
> > like a possibility.
>
> Below is a redraft of the code. I changed two things. But I'm not
> sure whether I am using csHandlerID correctly for the second change.
>
> First, I fixed my use of csRef as Jorrit noted should be done.
>
> The second change is to make the csWeakEventListener completely
> invisible to the client code. Now the csHandlerID that is handed back
> (by RegisterListener via RegisterWeakListener) is used to lookup the
> iEventHandler (really a csWeakEventListener) that should be passed on
> to RemoveListener by RemoveWeakListener. (As a side benefit, this
> eliminates the need for an output parameter in RemoveWeakListener to
> hand back the "csRef<csWeakEventListener>*".)
>
> As far as I can tell, there are two kinds of csHandlerID, generic and
> instance. The csHandlerID returned by RegisterListener is an instance
> csHandlerID. What I'm not 100% sure on is that "instance" in this
> context means "per object instance". If it doesn't, then this code
> probably wont work and I should go back to handing back a
> "csRef<csWeakEventListener>*". So can anyone confirm that
> understanding of "instance csHandlerID"?
>
> Michael D. Adams
> [EMAIL PROTECTED]
>
> // The indirection class. Hidden from client code
> class csWeakEventHandler :
> scfImplementation1<csWeakEventHandler, iEventHandler>
> {
> private:
> csWeakRef<iEventHandler> parent;
> public:
> csWeakEventHandler (iEventHandler *parent) :
> scfImplementationType (this), parent(parent) { }
> bool HandleEvent (iEvent& e) { parent->HandleEvent(e); }
> /* Similar code to HandleEvent for all the other methods of iEventHandler */
> };
>
> // Helper function used by client code
> // May be unnecessary. The client code can just call the code on its own.
> csHandlerID RegisterWeakListener (iEventQueue *q, iEventHandler * listener)
> {
> csRef<csWeakEventHandler> handler;
> handler.AttachNew (new csWeakEventHandler (listener));
> return q->RegisterListener (handler);
> }
>
> // Helper function used by client code.
> // May be unnecessary. The client code can just call the code on its own.
> void RemoveWeakListener (iEventQueue *q, iObjectRegistry *r, csHandlerId id)
> {
> q->RemoveListener(csEventHandlerRegistry::Handler(r, id));
> }
>
> // Example client code
> class csFoo : scfImplementation2<csFoo, iComponent, iEventHandler>
> {
> private:
> csHandlerID id;
> public:
> csFoo() : id(CS_HANDLER_INVALID) { /* ... */ }
> ~csFoo();
> bool Initialize (iObjectRegistry *object_reg);
> bool HandleEvent (iEvent& e) { /* ... */ }
> };
>
> bool csFoo::Initialize (iObjectRegistry *object_reg)
> {
> /* ... */
> csRef<iEventQueue> q (CS_QUERY_REGISTRY (object_reg, iEventQueue));
> if (q != 0)
> {
> id = RegisterWeakListener(q, this);
> }
> /* ... */
> return true;
> }
>
> csFoo::~csFoo ()
> {
> /* ... */
> if (id != CS_HANDLER_INVALID)
> {
> RemoveWeakListener(q, object_reg, id);
> }
> /* ... */
> }
>
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