How to modify TComponentProperty to show only particular items on drop down list?

Question

Please consider such scenerio:

I have component called TMenuItemSelector which has two published properties: PopupMenu - allows to pick an instance of TPopupMenu from the form and MenuItem which allows to pick any instance of TMenuItem from the form.

I would like to modify property editor for MenuItem property in a way that when PopupMenu is assigned then only menu items from this PopupMenu are visible in a drop down list.

I know that I need to write my own descendant of TComponentProperty and override GetValues method. The problem is that I do not know how to access the form on which TMenuItemSelector is lying.

Original TComponentProperty is using this method to iterate all available instances:

procedure TComponentProperty.GetValues(Proc: TGetStrProc);
begin
  Designer.GetComponentNames(GetTypeData(GetPropType), Proc);
end;

However, Designer seems to be precompiled so I have no idea how GetComponentNames works.

This is what I have so far, I guess only thing which I am missing is the implementation of GetValues:

unit uMenuItemSelector;

interface

uses
  Classes, Menus, DesignIntf, DesignEditors;

type
  TMenuItemSelector = class(TComponent)
  private
    FPopupMenu: TPopUpMenu;
    FMenuItem: TMenuItem;
    procedure SetPopupMenu(const Value: TPopUpMenu);
    procedure SetMenuItem(const Value: TMenuItem);
  published
    property PopupMenu: TPopUpMenu read FPopupMenu write SetPopupMenu;
    property MenuItem: TMenuItem read FMenuItem write SetMenuItem;
  end;

type
  TMenuItemProp = class(TComponentProperty)
  public
    function GetAttributes: TPropertyAttributes; override;
    procedure GetValues(Proc: TGetStrProc); override;
  end;

procedure Register;

implementation

procedure Register;
begin
  RegisterPropertyEditor(TypeInfo(TMenuItem), TMenuItemSelector, 'MenuItem', TMenuItemProp);
  RegisterComponents('Test', [TMenuItemSelector]);
end;

{ TMenuItemSelector }

procedure TMenuItemSelector.SetMenuItem(const Value: TMenuItem);
begin
  FMenuItem := Value;
end;

procedure TMenuItemSelector.SetPopupMenu(const Value: TPopUpMenu);
begin
  FPopupMenu := Value;
end;

{ TMenuItemProperty }

function TMenuItemProp.GetAttributes: TPropertyAttributes;
begin
  Result := inherited GetAttributes + [paValueList, paSortList];
end;

procedure TMenuItemProp.GetValues(Proc: TGetStrProc);
begin
  //How to filter MenuItems from the form in a way that only
  //MenuItems which belong to TMenuItemSelector.PopupMenu are displayed? \
  //And how to get to that form?
  //inherited;

end;

end.

Anyone could help?

Thanks.

Answer 1

When TMenuItemProp.GetValues() is called, you need to look at the TMenuItemSelector object whose MenuItem property is currently being edited, see if that object has a PopupMenu assigned, and if so then loop through its items as neded, eg:

procedure TMenuItemProp.GetValues(Proc: TGetStrProc); 
var
  Selector: TMenuItemSelector;
  I: Integer;
begin 
  Selector := GetComponent(0) as TMenuItemSelector;
  if Selector.PopupMenu <> nil then
  begin
    with Selector.PopupMenu.Items do
    begin
      for I := 0 to Count-1 do
        Proc(Designer.GetComponentName(Items[I]));
    end;
  end else
    inherited GetValues(Proc);
end; 

BTW, you need to implement TMenuItemSelector and TMenuItemProp in separate packages. With the exception of the RegisterComponents() function, (which is implemented in a runtime package), design-time code is not allowed to be compiled into run-time executables. It is against the EULA, and Embarcadero's design-time pacakges are not allowed to be distributed. You need to implement TMenuItemSelector in a runtime-only package, and then implement TMenuItemProp and Register() in a designtime-only package that Requires the runtime-only package and uses the unit that TMenuItemSelector is declared in, eg:

unit uMenuItemSelector;

interface

uses
  Classes, Menus;

type
  TMenuItemSelector = class(TComponent)
  private
    FPopupMenu: TPopUpMenu;
    FMenuItem: TMenuItem;
    procedure SetPopupMenu(const Value: TPopUpMenu);
    procedure SetMenuItem(const Value: TMenuItem);
  protected
    procedure Notification(AComponent: TComponent; Operation: TOperation); override;
  published
    property PopupMenu: TPopUpMenu read FPopupMenu write SetPopupMenu;
    property MenuItem: TMenuItem read FMenuItem write SetMenuItem;
  end;

implementation

{ TMenuItemSelector }

procedure TMenuItemSelector.Notification(AComponent: TComponent; Operation: TOperation);
begin
  inherited;
  if Operation = opRemove then
  begin
    if AComponent = FPopupMenu then
    begin
      FPopupMenu := nil;
      FMenuItem := nil;
    end
    else if AComponent = FMenuItem then
    begin
      FMenuItem := nil;
    end;
  end;
end;

procedure TMenuItemSelector.SetMenuItem(const Value: TMenuItem);
begin
  if FMenuItem <> Value then
  begin
    if FMenuItem <> nil then FMenuItem.RemoveFreeNotification(Self);
    FMenuItem := Value;
    if FMenuItem <> nil then FMenuItem.FreeNotification(Self);
  end;
end;

procedure TMenuItemSelector.SetPopupMenu(const Value: TPopUpMenu);
begin
  if FPopupMenu <> Value then
  begin
    if FPopupMenu <> nil then FPopupMenu.RemoveFreeNotification(Self);
    FPopupMenu := Value;
    if FPopupMenu <> nil then FPopupMenu.FreeNotification(Self);
    SetMenuItem(nil);
  end;
end;

end.

.

unit uMenuItemSelectorEditor;

interface

uses
  Classes, DesignIntf, DesignEditors;

type
  TMenuItemSelectorMenuItemProp = class(TComponentProperty)
  public
    function GetAttributes: TPropertyAttributes; override;
    procedure GetValues(Proc: TGetStrProc); override;
  end;       

procedure Register;

implementation

uses
  Menus, uMenuItemSelector;

procedure Register;
begin
  RegisterComponents('Test', [TMenuItemSelector]);
  RegisterPropertyEditor(TypeInfo(TMenuItem), TMenuItemSelector, 'MenuItem', TMenuItemSelectorMenuItemProp);
end;

{ TMenuItemSelectorMenuItemProp }

function TMenuItemSelectorMenuItemProp.GetAttributes: TPropertyAttributes;
begin
  Result := inherited GetAttributes + [paValueList, paSortList] - [paMultiSelect];
end;

procedure TMenuItemSelectorMenuItemProp.GetValues(Proc: TGetStrProc);
var
  Selector: TMenuItemSelector;
  I: Integer;
begin
  Selector := GetComponent(0) as TMenuItemSelector;
  if Selector.PopupMenu <> nil then
  begin
    with Selector.PopupMenu.Items do
    begin
      for I := 0 to Count-1 do
        Proc(Designer.GetComponentName(Items[I]));
    end;
  end else
    inherited GetValues(Proc);
end; 

end.