Adding OnMouseDown event-handler to VCL component?

Question

In a Delphi 10.4.2 32-bit VCL Application, I use the component TSVGIconImage from the SVGIconImageList library from the GetIt PackageManager.

Although the component supports the OnDblClick event-handler, it does NOT support the OnMouseDown event-handler! I.e., I can add an OnMouseDown event-handler by double-clicking the OnMouseDown event in the Object Inspector, however that event-handler gets never called at run-time:

procedure TformMain.SVGIconImage1MouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
  CodeSite.Send('called!'); // never called!
end;

The TSVGIconImage component is declared in SVGIconImage.pas as:

TSVGIconImage = class(TCustomControl)

So shouldn't the TSVGIconImage component inherit its OnMouseDown event from TCustomControl?

Anyway, how can I add a working OnMouseDown event for TSVGIconImage in my application's code?

EDIT: After testing this in a separate VCL Application I found out that the TSVGIconImage OnMouseDown event handler is working there at run-time. So it must be something else that blocks the TSVGIconImage OnMouseDown event handler in my application. I have still to find out the cause.

You could report the issue to the developer github.com/EtheaDev/SVGIconImageList — R. Hoek
I have reported it: github.com/EtheaDev/SVGIconImageList/issues/158 But shouldn't it be possible to add a working OnMouseDown event-handler in my application code? — user1580348
I had a look at the repository and really couldn't find anything odd about this TCustomControl that would explain why the OnMouseDown doesn't work. So, I somewhat reluctantly downloaded the library using GetIt. After quite a few errors (like the demo project not being runnable because a published property isn't found), I made a small demo app using the TSVGIconImage. My first impression is that this is not a very good SVG renderer: it is by far the slowest SVG renderer I have ever seen, and it doesn't render my math graphs correctly. Still, the OnMouseDown handler works for me! — Andreas Rejbrand
(Well, to be fair, the library is probably not meant to be a 100% compliant and high-performance SVG 1.1 renderer; it's probably supposed to be used for fairly simple icons. And it probably is a very good library for such applications.) — Andreas Rejbrand

user1580348 user1580348 · Accepted Answer · 2021-03-08T11:01:11

A) Place a TApplicationEvents component on your form.

B) Double-click the ApplicationEvents.OnMessage event in the Object Inspector to create an OnMessage event handler and write a case-filter for WM_LBUTTONDOWN:

procedure TForm1.AppEvents1Message(var Msg: tagMSG; var Handled: Boolean);
begin
  case Msg.message of
    WM_LBUTTONDOWN:
      begin
        if Msg.hwnd = SVGIconImag1.Handle then
          DoSomething;
      end;
  end;
end;

Thanks to @AndreasRejbrand and @fpiette for their constructive and helpful input!

Adding OnMouseDown event-handler to VCL component?

1 Answers