Fixing a FireMonkey TCheckBox/TAction bug

Try this:

• Create a new FireMonkey HD application, before adding a check box and an action list to the form.
• Double click the action list and add an action to it; set the action’s AutoCheck property to True.
• Assign the action just created to the check box’s Action property.
• Double click the form to create a handler for its OnCreate event. In the handler, set the action’s DisableIfNoHandler property to False:

procedure TForm1.FormCreate(Sender: TObject);
begin
  Action1.DisableIfNoHandler := False;
end;

• Run the application; once showing, click the check box. Expected: its checked state is toggled. Actual: nothing happens
• In fact, toggling the check box does work, however you need to double click the control for this to work. This is patently a bug caused by the IsChecked property setter getting called twice in the course of a single mouse click

To fix the problem, you can at a pinch create a suitable interposer class. Doing that won’t be very ‘clean’ however because the IsChecked property setter isn’t virtual, so do this instead:

• Take a copy of FMX.Controls.pas, add it to your project, and open it up.
• Find your way to the implementation of TCheckBox.MouseUp.
• Ignoring the reams of ‘Live’Bindings support code, add a check for IsCheckedStored in front of IsChecked being toggled:

if IsCheckedStored then IsChecked := not IsChecked;

Rebuild the application, and the bug should be fixed.