How-to-trap-Windows-Messages-in-a-Component


            Title: How to trap Windows Messages in a Component
procedure HookMainWindow(Hook : TWindowHook);
{
 HookMainWindow() takes one parameter (Hook of type TWindowHook).
 TWindowHook is a method pointer type that's defined like so:
}
type
 TWindowHook = function(var Message : TMessage) : Boolean of object;
{
 Since TWindowHook is a method pointer, you can define your own method as
 the hook function as long as it follows the nomenclature defined for TWindowHook.
 The return value of the function is of type Boolean.
 This is the equivalent of the "Handled" parameter of OnMessage.
 If your function handles a particular message, you'd return true.
 This will be passed back to the Application's WndProc and message processing for that
 message will be terminated. Otherwise, you'd return False.
 The Component "MessageReceiver" intercepts user-definded messages
 ( WM_USER) und as example the message WM_TIMECHANGE.
}
{*************************************************************************}
{
 TApplication besitzt eine Methode HookMainWindow.
 Damit kann man in die Windows Prozedur (WndProc) "einhaken" und Nachrichten,
 welche an die Applikation geschickt werden, abfangen und behandeln.
 HookMainWindow is wie folgt deklariert:
}
procedure HookMainWindow(Hook : TWindowHook);
{ Und der Parameter TWindowHook (Methoden Pointer) so: }
type
 TWindowHook = function(var Message : TMessage) : Boolean of object;
{
 Der R¨¹ckgabewert der Funktion ist vom Type Boolean.
 Dieser enspricht dem "Handled" Parameter von OnMessage.
 Wenn nun eine bestimmte Nachricht behandlet wird, kann True zur¨¹ckgegeben
 werden, sonst False.
 Die Komponente "MessageReceiver" f?ngt einerseits benuterdefinierte Nachrichten
 ( WM_USER) und als Beispiel die Nachricht WM_TIMECHANGE ab.
 Im OnCreate() wird Application.HookMainWindow aufgerufen und im OnDestroy() dann
 - falls n?tig - Application.UnhookMainWindow.
}
{*************************************************************************}
unit MessageReceiver;
interface
uses
 Windows, Messages, SysUtils, Classes, Forms, Dialogs;
type
 TOnReceiveUserMessage = procedure(Msg: Cardinal; wParam, lParam: Integer;
 var Handled: Boolean) of object;
 TOnReceiveOtherMessage = procedure(var Handled: Boolean) of object;
 TMessageReceiver = class(TComponent)
 private
 { Private declarations }
 FHooked: Boolean;
 FOnReceiveUserMessage: TOnReceiveUserMessage;
 FOnDateTimeChange: TOnReceiveOtherMessage;
 function MessageHook(var Msg: TMessage): Boolean;
 protected
 function DoDateTimeChange(Msg: TMessage): Boolean; dynamic;
 function DoUserMessage(Msg: TMessage): Boolean; dynamic;
 public
 { Public declarations }
 constructor Create(AOwner: TComponent); override;
 destructor Destroy; override;
 published
 { Published declarations }
 property OnDateTimeChange: TOnReceiveOtherMessage
 read FOnDateTimeChange write FOnDateTimeChange;
 property OnReceiveUserMessage: TOnReceiveUserMessage
 read FOnReceiveUserMessage write FOnReceiveUserMessage;
 end;
 
procedure Register;
implementation
procedure Register;
begin
 RegisterComponents('System', [TMessageReceiver]);
end;
function TMessageReceiver.MessageHook(var Msg: TMessage): Boolean;
begin
 Result := False;
 // User defined messages
 if (Msg.Msg = WM_USER) then
 Result := DoUserMessage(Msg)
 else
 // Other messages
 case Msg.Msg of
 WM_TIMECHANGE: Result := DoDateTimeChange(Msg);
 // ...
 end;
end;
function TMessageReceiver.DoDateTimeChange(Msg : TMessage): Boolean;
begin
 Result := False;
 if Assigned(FOnDateTimeChange) then
 FOnDateTimeChange(Result);
end;
function TMessageReceiver.DoUserMessage(Msg: TMessage): Boolean;
begin
 Result := False;
 if Assigned(FOnReceiveUserMessage) then
 FOnReceiveUserMessage(Msg.Msg, Msg.wParam, Msg.LParam, Result);
end;
constructor TMessageReceiver.Create(AOwner: TComponent);
begin
 inherited Create(AOwner);
 FHooked := False;
 if not (csDesigning in ComponentState) then
 begin
 Application.HookMainWindow(MessageHook);
 FHooked := True;
 end;
end;
destructor TMessageReceiver.Destroy;
begin
 if FHooked then Application.UnhookMainWindow(MessageHook);
 inherited Destroy;
end;
end.
System Delphi
