Title: How to add a new event to a component?
Question: How to implement an event...
Answer:
You can communicate with components via properties, methods and events. Here is a small example how to add an event to DBGrid when the user clicks the right mouse button:
type
TExtDBGrid = class(TDBGrid)
private
FOnRightClick : TNotifyEvent;
procedure WMRButtonDown(var Message : TWMRButtonDown);
message WM_RBUTTONDOWN;
protected
public
published
property OnRightClick: TNotifyEvent read FOnRightClick
write FOnRightClick;
end;
procedure TExtDBGrid.WMRButtonDown(var Message: TWMRButtonDown);
begin
if Assigned(FOnRightClick) then FOnRightClick(Self);
end;
It's always the same procedure. We need to add a property for the event (in the published section when the event should appear under events in the object-inspector - otherwise in the public section) from the type that defines the parameters of the event. In the above example we used TNotifyEvent
type TNotifyEvent = procedure (Sender: TObject) of object;
which is defined in the unit Classes and therefore always available!
When you want to fire the event you must be sure that the main-program did assign a procedure (assign statement) - then you can call it useing FOnEventName(Self, Params...);
In this example we want to react on the right mouse button - this works with the reserved word message and the windows constant for this event: WM_RBUTTONDOWN
Of course we can also define custom-events with various parameters. One thing to mention is that you can also use VAR-Parameters, so that the component can get input from the main program. For example:
type TMyEvent = procedure (Sender: TObject;
VAR aAbort: boolean) of object;
procedure TMyComp.AnyFunc;
var abort: boolean;
begin
while (...) do
begin
if Assigned(FMyEvent) then FMyEvent(Self, abort);
if abort then exit;
end;
end;
In this case the event would be fired on every step through the loop and the main program could interrupt assigning aAbort to false !