Notify the TObjectList when Object changed - delphi

Question

Can the object of (TObjectList) know when some values of (TMyObject) was changed?
Some example:
TMyObject = class
oName: string;
end;
TMyObjectList = class(TObjectList<TMyObject>)
end;
procedure Form1.Button1.Click(Sender: TObject);
var
Obj: TMyObject;
List: TMyObjectList;
Begin
List:= TMyObjectList.Create;
Obj:= TMyObject.Create;
List.Add(Obj);
List[0].oName:= 'Test'; // here a want to know from var (List) when this object (Obj or List[0]) changed his value..
end;
Thanks for any help.

Answer 1

I just added the TObservableList<T> type to Spring4D (feature/observablelist branch). It is mostly modeled after .NET and uses the INotifyPropertyChanged interface to attach its event handler to any objects that support it. This class has been part of DSharp for quite some time and is used in production. It might change a bit in the future and become full part of the library.
Here is a small example how to use it so you get an idea:
program Project60;
{$APPTYPE CONSOLE}
uses
Spring,
Spring.Collections,
SysUtils;
type
TNotifyPropertyChangedBase = class(TInterfaceBase, INotifyPropertyChanged)
private
fOnPropertyChanged: Event<TPropertyChangedEvent>;
function GetOnPropertyChanged: IPropertyChangedEvent;
protected
procedure PropertyChanged(const propertyName: string);
end;
TMyObject = class(TNotifyPropertyChangedBase)
private
fName: string;
procedure SetName(const Value: string);
public
property Name: string read fName write SetName;
end;
TMain = class
procedure ListChanged(Sender: TObject; const item: TMyObject;
action: TCollectionChangedAction);
end;
{ TNotifyPropertyChangedBase }
function TNotifyPropertyChangedBase.GetOnPropertyChanged: IPropertyChangedEvent;
begin
Result := fOnPropertyChanged;
end;
procedure TNotifyPropertyChangedBase.PropertyChanged(
const propertyName: string);
begin
fOnPropertyChanged.Invoke(Self,
TPropertyChangedEventArgs.Create(propertyName) as IPropertyChangedEventArgs);
end;
{ TMyObject }
procedure TMyObject.SetName(const Value: string);
begin
fName := Value;
PropertyChanged('Name');
end;
{ TMain }
procedure TMain.ListChanged(Sender: TObject; const item: TMyObject;
action: TCollectionChangedAction);
begin
case action of
caAdded: Writeln('item added ', item.Name);
caRemoved, caExtracted: Writeln('item removed ', item.Name);
caChanged: Writeln('item changed ', item.Name);
end;
end;
var
main: TMain;
list: IList<TMyObject>;
o: TMyObject;
begin
list := TCollections.CreateObservableList<TMyObject>;
list.OnChanged.Add(main.ListChanged);
o := TMyObject.Create;
o.Name := 'o1';
list.Add(o);
o := TMyObject.Create;
o.Name := 'o2';
list.Add(o);
list[1].Name := 'o3';
Readln;
end.

Answer 2

There is nothing built in that can do what you ask. You will need to implement a notification mechanism yourself. This is the classic scenario for the Observer Pattern.
There are many implementations of this pattern already in existence. One obvious choice would be to use the implementation in Spring4D. Nick Hodges recent book, More Coding in Delphi, includes a chapter on this pattern which I would recommend.

Answer 3

Found the way, how to call method of TObjectList from TMyObject. Using TNotifyEvent in base Object.
Example:
TMyClass = class(TObject)
private
FName: string;
FOnNameEvent: TNotifyEvent;
procedure SetName(value: string);
public
property Name: string read FName write SetName;
property OnNameEvent: TNotifyEvent read FOnNameEvent write FOnNameEvent;
end;
procedure TMyClass.SetName(value: string);
begin
FName := value;
if Assigned(FOnNameEvent) then
FOnNameEvent(Self);
end;
procedure MyNameEvent(Sender: TObject);
var
i: Integer;
begin
for i := 0 to MyListOfMyClassObjects.Count -1 do
if Sender = MyListOfMyClassObjects.Item[i] then
begin
MessageBox(0, PChar(TMyClass(MyListOfMyClassObjects.Item[i]).Name), nil, MB_OK);
break;
end;
end;
procedure MyProc;
var
MyObject: TMyClass;
begin
MyObject := TMyClass.Create;
MyObject.OnNameEvent := MyNameEvent;
MyListOfMyClassObjects.Add(MyObject);
end;