Callback functions allow a developer to create customized code for ‘standard calls’ from within an application. Callback functions are implemented throughout Windows applications and can be used to perform various tasks. The same callback function can be called with different ‘custom function’ references throughout an application. Two examples of callback functions are the EnumWindows and EnumChildProc Functions.
First create the functions that are passed to the callback function. For example:
//from MSDN:
// GetClassName// The GetClassName function retrieves the name of the class to which the
// specified window belongs.
// GetWindowText// The GetWindowText function copies the text of the specified window's title
// bar (if it has one) into a buffer. If the specified window is a control,
// the text of the control is copied. However, GetWindowText cannot retrieve
// the text of a control in another application.
// The IsWindowVisible// The IsWindowVisible function retrieves the visibility state of the// specified window.function EnumWindowsProc(Handle: HWND; AppVal: LongInt): BOOL;
stdcall;varWindowName:
Array[
0..255]
of Char;
ClassName:
Array[
0..255]
of Char;
beginif IsWindowVisible(Handle)
thenbeginGetClassName(Handle,ClassName,SizeOf(ClassName));
GetWindowText(Handle,WindowName,SizeOf(WindowName));
//you could do something more with the information here
end;
end;
function EnumChildWindowsProc(Handle: HWND; AppVal: LongInt): BOOL;
stdcall;
varWindowName:
Array[
0..255]
of Char;
ClassName:
Array[
0..255]
of Char;
beginif IsWindowVisible(Handle)
then
begin
GetClassName(Handle,ClassName,SizeOf(ClassName));
GetWindowText(Handle,WindowName,SizeOf(WindowName));
//you could do something more with the information hereend;end; In order to you the callback functions you pass your ‘custom functions’:
EnumWindows(@EnumWindowsProc,LPARAM(Self));
// Handle is the parent window whose child windows are to be enumerated.
EnumChildWindows(Handle,@EnumChildWindowsProc,LPARAM(Self));
Having a little fun I put together a little application that enumerates through visible parent
and child windows. These windows are added to a TreeView, which then the text of the window retrieved or set. The text is retrieved and set by sending a
WM_GETTEXT or
WM_SETTEXT to the window handle. As you can see from the screenshot you can have a little fun with this. You can even have your own finish button.
Using callback functions in classes can also be accomplished. The LPARAM parameter is application defined. This parameter can be used to pass
‘whatever you want’ into the callback function. For the above program I created a class that enumerated the windows and stored them in a list. The class listing is as follows:
type
TMyWindowObject= class(TObject)
private
FClassName: string;
FWindowHandle: HWND;
FWindowName: string;
public
property WindowClassName: string read FClassName write FClassName;
property WindowHandle: HWND read FWindowHandle write FWindowHandle;
property WindowName: string read FWindowName write FWindowName;
function GetWindowText:string;
procedure SetWindowText(text: string);
procedure CloseWindow;
end;
TMyWindows = class(TObject)
Public
WindowList: TList;
procedure AddWindowObject(Handle: HWND);
constructor Create;
destructor Destroy;override;
procedure ClearList;
function GetChildWindows(Handle: Integer): Boolean;
function GetWindows: Boolean;
end;
implementation
function EnumChildWindowsProc2(Handle: HWND; ctrlHWND: Integer): BOOL; stdcall;
begin
if IsWindowVisible(Handle) then
begin
TMyWindows(ctrlHWND).AddWindowObject(Handle);
end;
end;
function EnumWindowsProc2(Handle: HWND; ctrlHWND: Integer): BOOL; stdcall;
begin
if IsWindowVisible(Handle) then
begin
TMyWindows(ctrlHWND).AddWindowObject(Handle);
end;
end;
{ TMyWindows }
procedure TMyWindows.ClearList;
var
i: Integer;
begin
for i := WindowList.Count - 1 downto 0 do
begin
TMyWindowObject(WindowList.Items[i]).Free;
end;
WindowList.Clear;
end;
constructor TMyWindows.Create;
begin
inherited;
WindowList:= Tlist.Create;
end;
destructor TMyWindows.Destroy;
begin
ClearList;
WindowList.Free;
inherited;
end;
procedure TMyWindows.AddWindowObject(Handle: HWND);
var
WindowName: Array[0..255] of Char;
ClassName: Array[0..255] of Char;
MyWindowObject: TMyWindowObject;
begin
MyWindowObject:= TMyWindowObject.Create;
GetClassName(Handle,ClassName,SizeOf(ClassName));
GetWindowText(Handle,WindowName,SizeOf(WindowName));
MyWindowObject.WindowHandle:= Handle;
MyWindowObject.WindowClassName:= ClassName;
MyWindowObject.WindowName:= WindowName;
WindowList.Add(MyWindowObject);
end;
function TMyWindows.GetChildWindows(Handle: Integer): Boolean;
begin
ClearList;
GetChildWindows:= EnumChildWindows(Handle,@EnumChildWindowsProc2,LPARAM(Self));
end;
function TMyWindows.GetWindows: Boolean;
begin
ClearList;
GetWindows:= EnumWindows(@EnumWindowsProc2,LPARAM(Self));
end;
{ TMyWindowObject }
procedure TMyWindowObject.CloseWindow;
begin
PostMessage(FWindowHandle,WM_CLOSE,0,0);
end;
function TMyWindowObject.GetWindowText: string;
var
iLen: Integer;
strText: string;
begin
iLen:= SendMessage(FWindowHandle,WM_GETTEXTLENGTH,0,0);
SetLength(strText,iLen);
SendMessage(FWindowHandle,WM_GETTEXT, iLen + 1,lParam(PChar(strText)));
Result:= strText;
end;
procedure TMyWindowObject.SetWindowText(text: string);
begin
SendMessage(FWindowHandle,WM_SETTEXT,0,Longint(text));
end;
end.