How to simulate an OnDestroy event on a TFrame in Delphi?

How can I simulate an OnDestroy event for a TFrame in Delphi?

I naively added a constructor and destructor to my frame, thinking that is what TForm does:

TframeEditCustomer = class(TFrame)
...
public
   constructor Create(AOwner: TComponent); override;
   destructor Destroy; override;
   ...
end;

constructor TframeEditCustomer.Create(AOwner: TComponent)
begin
    inherited Create(AOwner);
    
    //allocate stuff
end;

destructor TframeEditCustomer.Destroy;
begin
   //cleanup stuff

   inherited Destroy;
end;

The problem with this is that by the time my destructor runs, controls on the frame have been destroyed and are no longer valid.

The reason for this is in the containing form's destructor, which it uses to fire an OnDestroy event:

destructor TCustomForm.Destroy;
begin
   ...
   if OldCreateOrder then DoDestroy; //-->fires Form's OnDestroy event; while controls are still valid
   ...
   if HandleAllocated then DestroyWindowHandle; //-->destroys all controls on the form, and child frames
   ...
   inherited Destroy; //--> calls destructor of my frame
   ...
end;

The destructor of my frame object is being called when the form's destructor runs. The problem with this is that it's too late. The form calls DestroyWindowHandle, which asks Windows to destroy the form's window handle. This recursively destroys all child windows - including those on my frame.

So when my frame's destructor runs, I attempt to access controls that are no longer in a valid state.

How can I simulate an OnDestroy event for a TFrame in Delphi?

Solution

You need to add a WM_DESTROY handler and check for csDestroying in the ComponentState so it's only caught when actually destroying, and not when recreating the handle.

type
  TCpFrame = class(TFrame)
  private
    FOnDestroy: TNotifyEvent;
    procedure WMDestroy(var Msg: TWMDestroy); message WM_DESTROY;
  published
    property OnDestroy: TNotifyEvent read FOnDestroy write FOnDestroy;
  end;

procedure TCpFrame.WMDestroy(var Msg: TWMDestroy);
begin
  if (csDestroying in ComponentState) and Assigned(FOnDestroy) then
    FOnDestroy(Self);
  inherited; 
end;

That will only work if the frame's window handle has actually been created. There isn't another good hook point, so if you want to ensure it's always called you'll need to set a flag in WMDestroy and fall back to calling it in the destructor if that isn't hit.

The window handles themselves are all cleared in WM_NCDESTROY, which is called after all of the descendant WM_DESTROY messages return, so the form and all of its childens' handles should still be valid at this point (ignoring any that were freed in the form's OnDestroy).