[fpc-devel] Recent added support for Observer pattern in FCL

[michael.vancanneyt at wisa.be]

On Mon, 27 Aug 2012, Luiz Americo Pereira Camara wrote:

> Back in 2010, in the thread about observer support in fcl, i asked if the 
> actions would be customizable so i could, e.g., notify/observe if a child was 
> added or removed in a tree structure. The answer was yes.
>
> Looking at the recent added support, it limit the actions (called operations) 
> to 5 types
>
> TFPObservedOperation = (ooChanged,ooFree,ooAddItem,ooDeleteItem,ooCustom);
>
> I ask to change to a mechanism that could allow customizable operations (more 
> than one)

That is why you have ooCustom.

>
>
> I'm asking that because i have implemented a similar mechanism that support 
> operations like Load, ChildAdd, ChildDelete, ChildChange. I believe that more 
> developers are in the same situation,i.e., using observer pattern to an 
> action not handled by those 5 types. It would be helpful to use the native 
> implementation otherwise i will have to keep(re-implement) the observer 
> support over TPersistent.

That's why you have ooCustom and Data at your disposal.

I imagined 3 scenarios for this:

The first:

Procedure TmyObject.SomethingChanged;

Var
   Rec : TMyOperationInfo; // Contains custom information about your operation.

begin
   Rec.XYZ:=MoreInfo; // Set up as needed.
   FPONotifyObservers(Self,ooCustom, at Rec);
end;

Or, you can do

Procedure TmyObject.SomethingChanged;

Var
   Obj : TMyOperationInfo; // Contains custom information about your operation.

begin
   Obj:=TMyOperationInfo.Create(MyParams);
   try
     FPONotifyObservers(Self,ooCustom,Pointer(Obj));
   finally
     Obj.free;
   end;
end;


Procedure TmyObject.SomeThingChanged(APropName : String);

begin
   FPONotifyObservers(Self,ooCustom,GetPropInfo(Self,APropName));
end;


But I think the additional Create/Free may be heavy. That is why I decided
on a data pointer, not an object. It allows to use records as well as
objects.

The last scenario was for property change notifications:

Procedure TmyObject.SomeThingChanged(Const APropName : String);

begin
   FPONotifyObservers(Self,ooCustom,GetPropInfo(Self,APropName));
end;

This could be called from a property setter.

There are many ways to implement an observer pattern. It is always a balance
between lightweight and extensibility. I believe the implemented mechanism
is both: it does not require additional objects (and thus heap operations)
but does allow using them.

> BTW:
> The names of the TFPObservedOperation are not consistent:
> ooChanged is in preterit while others not
>
> ooChanged does not has Item suffix

It does not need one. The sender has changed, not an item in a list.
The 'Item'-s are  for when sender is a list, and an item is added to the list.
(TList, TStringList, TCollection)

That's a matter of documentation.

Michael.