delphi component property: TObjectList<TPicture> - delphi

Question

I'm trying to create a VCL component, that lets you insert multiple TImages of different sizes as properties.
I was told to best use a TObjectList ( Delphi component with a variable amount of TPictures ), but now I'm struggling to make the single TPictures assignable in the Property editor.
What i have at the moment: (it compiles)
unit ImageMultiStates;
interface
uses
Vcl.Graphics, Vcl.StdCtrls, System.SysUtils, System.Classes, Vcl.Controls, Vcl.ExtCtrls, Forms, Generics.Collections;
type
TImageMultiStates = class(TImage)
private
FPictures: TObjectList<TPicture>;
procedure SetPicture(Which: Integer; APicture: TPicture);
function GetPicture(Which: Integer): TPicture;
public
Count: integer;
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
procedure Activate(Which: Integer);
published
// property Pictures: TObjectList<TPicture> read GetPicture write SetPicture;
// property Pictures[Index: Integer]: TObjectList<TPicture> read GetPicture write SetPicture;
property Pictures: TObjectList<TPicture> read FPictures write FPictures;
end;
procedure Register;
implementation
constructor TImageMultiStates.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
FPictures := TObjectList<TPicture>.Create;
end;
destructor TImageMultiStates.Destroy;
begin
FPictures.Free;
inherited Destroy;
end;
procedure TImageMultiStates.SetPicture(Which: Integer; APicture: TPicture);
begin
FPictures[Which] := APicture;
if Which=0 then
Picture.Assign(APicture);
end;
function TImageMultiStates.GetPicture(Which: Integer): TPicture;
begin
Result := FPictures[Which];
end;
procedure TImageMultiStates.Activate(Which: Integer);
begin
Picture.Assign(FPictures[Which]);
end;
procedure Register;
begin
RegisterComponents('Standard', [TImageMultiStates]);
end;
end.
What doesn't work is the final result in the PropertyEditor. It shows one single item named "Pictures", with the value "(TObjectList)". Clicking it doesn't do anything, i don't get a proper editor. Other ideas for the line in question have been commented out, they bring other errors:
The first one throws the compiler error "E2008 Incompatible Types", The second one throws "Published property 'Pictures' can not be of type ARRAY".

Answer 1

The IDE has no idea how to edit a TObjectList at design-time, and the DFM streaming system has no idea how to stream a TObjectList. You would have to implement a custom property editor and custom streaming logic. While that is certainly possible, it is a LOT of work.
What you are attempting to do is better handled by using System.Classes.TCollection instead. Both the IDE and the DFM streaming system have built-in support for handling TCollection editing and streaming automatically for you.
Try something more like this:
unit ImageMultiStates;
interface
uses
System.Classes, Vcl.Controls, Vcl.ExtCtrls, Vcl.Graphics;
type
TImagePictureItem = class(TCollectionItem)
private
FPicture: TPicture;
procedure PictureChanged(Sender: TObject);
procedure SetPicture(Value: TPicture);
public
constructor Create(Collection: TCollection); override;
destructor Destroy; override;
published
property Picture: TPicture read FPicture write SetPicture;
end;
TImagePictureEvent = procedure(Sender: TObject; Index: Integer) of object;
TImagePictures = class(TOwnedCollection)
private
FOnPictureChange: TImagePictureEvent;
function GetPicture(Index: Integer): TImagePictureItem;
procedure SetPicture(Index: Integer; Value: TImagePictureItem);
protected
procedure Update(Item: TCollectionItem); override;
public
constructor Create(Owner: TComponent); reintroduce;
property Pictures[Index: Integer]: TImagePictureItem read GetPicture write SetPicture; default;
property OnPictureChange: TImagePictureEvent read FOnPictureChange write FOnPictureChange;
end;
TImageMultiStates = class(TImage)
private
FActivePicture: Integer;
FPictures: TImagePictures;
function GetPicture(Index: Integer): TPicture;
procedure PictureChanged(Sender: TObject; Index: Integer);
procedure SetActivePicture(Index: Integer);
procedure SetPicture(Index: Integer; Value: TPicture);
procedure SetPictures(Value: TImagePictures);
protected
procedure Loaded; override;
public
constructor Create(Owner: TComponent); override;
function Count: integer;
property Pictures[Index: Integer]: TPicture read GetPicture write SetPicture;
published
property ActivePicture: Integer read FActivePicture write SetActivePicture default -1;
property Picture stored False;
property Pictures: TImagePictures read FPictures write SetPictures;
end;
procedure Register;
implementation
{ TImagePictureItem }
constructor TImagePictureItem.Create(Collection: TCollection);
begin
inherited Create(Collection);
FPicture := TPicture.Create;
FPicture.OnChange := PictureChanged;
end;
destructor TImagePictureItem.Destroy;
begin
FPicture.Free;
inherited;
end;
procedure TImagePictureItem.PictureChanged(Sender: TObject);
begin
Changed(False);
end;
procedure TImagePictureItem.SetPicture(Value: TPicture);
begin
FPicture.Assign(Value);
end;
{ TImagePictures }
constructor TImagePictures.Create(Owner: TComponent);
begin
inherited Create(Owner, TImagePictureItem);
end;
function TImagePictures.GetPicture(Index: Integer): TImagePictureItem;
begin
Result := TImagePictureItem(inherited GetItem(Index));
end;
procedure TImagePictures.SetPicture(Index: Integer; Value: TImagePictureItem);
begin
inherited SetItem(Index, Value);
end;
procedure TImagePictures.Update(Item: TCollectionItem);
begin
if Assigned(FOnPictureChange) then
begin
if Item <> nil then
FOnPictureChange(Self, Item.Index)
else
FOnPictureChange(Self, -1);
end;
end;
{ TImageMultiStates }
constructor TImageMultiStates.Create(Owner: TComponent);
begin
inherited Create(Owner);
FPictures := TImagePictures.Create(Self);
FPictures.OnPictureChange := PictureChanged;
FActivePicture := -1;
end;
procedure TImageMultiStates.Loaded;
begin
inherited;
PictureChanged(nil, FActivePicture);
end;
function TImageMultiStates.Count: Integer;
begin
Result := FPictures.Count;
end;
procedure TImageMultiStates.PictureChanged(Sender: TObject; Index: Integer);
begin
if (FActivePicture <> -1) and ((Index = -1) or (Index = FActivePicture)) then
Picture.Assign(GetPicture(FActivePicture));
end;
function TImageMultiStates.GetPicture(Index: Integer): TPicture;
begin
Result := FPictures[Index].Picture;
end;
procedure TImageMultiStates.SetPicture(Index: Integer; Value: TPicture);
begin
FPictures[Index].Picture.Assign(Value);
end;
procedure TImageMultiStates.SetActivatePicture(Value: Integer);
begin
if FActivePicture <> Value then
begin
if ComponentState * [csLoading, csReading] = [] then
Picture.Assign(GetPicture(Value));
FActivePicture := Value;
end;
end;
procedure Register;
begin
RegisterComponents('Standard', [TImageMultiStates]);
// the inherited TImage.Picture property is published, and you cannot
// decrease the visibility of an existing property. However, if you move
// this procedure into a separate design-time package, you can then use
// DesignIntf.UnlistPublishedProperty() to hide the inherited
// Picture property at design-time, at least:
//
// UnlistPublishedProperty(TImageMultiStates, 'Picture');
//
// Thus, users are forced to use the TImageMultiStates.Pictures and
// TImageMultiStates.ActivePicture at design-time. The inherited
// Picture property will still be accessible in code at runtime, though...
end;
end.