[fpc-devel] TCollection.FOwner

[Juha Manninen]

I am proposing to add a private
  FOwner: TPersistent;
to TCollection and remove it from derived classes which already have it.
At least TParams has it but it is not used.
The next step would be to assign a proper owner for derived
TCollection instances, case by case.
Lazarus has code to go up in hierarchy by TPersistent.GetOwner calls. See:
 function GetLookupRootForComponent(APersistent: TPersistent): TPersistent;
So, it already works perfectly if only the Owners are set.

My fundamental goal is to fix issue
  https://bugs.freepascal.org/view.php?id=25068
There is no link from a TCollection to its "owner" object which is
called "LookupRoot" in Lazarus sources. The Collection is not visible
in Object Inspector and changing a CollectionItem cannot mark the
top-level Form/DataUnit as modified.

Mattias has made a hack where the "LookupRoot" of certain TCollection
derivatives can be queried.
It was implemented for TFieldDefs with an explicit Dataset propery.
In r56855 changed it to query TDefCollection which actually defines
the Dataset and thus includes derived IndexDefs, too.
Yes, now FieldDefs and IndexDefs both show in OI and changing them
sets Modified flag correctly.
But, it is still a hack and does not scale. We need a general solution
for all TCollection derivatives.
What more, the "Owner" of TDefCollection is not used which is poor
design. It could be used instead of "Dataset".

TParams has FOwner which is correctly returned by overridden method GetOwner.
However it is never set which can be considered a bug.
I tried to assign owner DataSource.DataSet for it in the only place of
creation I found, but it has no effect.

  Function TCustomSQLStatement.CreateParams: TSQLDBParams;
  var
    DS: TDataSet;
  begin
    if Assigned(DataSource) then
      DS := DataSource.DataSet
    else
      DS := Nil;
    Result:=TSQLDBParams.Create(DS);
  end;

TFPWebActions is another TCollection derivative without an owner.
I am sure there are plenty more but I did not study them.
I can provide a patch to add FOwner to TCollection and to return it by
GetOwner but I don't know much about the derived classes in FPC libs.
Help is needed there.

One note: the TCollection.Owner explained here is not inherently tied
to memory management like TComponent.Owner is.

Juha