How to use the EnumWindows call back function? - windows

Question

I would like to have a single neat (close and self contained) function (let's call it GetDesktopHandle) that returns a handle to the Desktop window. I use the code below. But it only works in the DeskHandle is a global var.
How to get rid of this global variable? If I make it local I get an AV in getDesktopWnd when I try to DeskHandle := hChild
VAR DeskHandle : HWND;
function GetDesktopHandle: HWND;
function getDesktopWnd (Handle: HWND; NotUsed: Longint): bool; stdcall; { Callback function }
VAR hChild : HWND;
begin
if handle <> 0 then
begin
hChild := FindWindowEx(handle, 0, 'SHELLDLL_DefView', nil);
if hChild <> 0 then
begin
hChild := FindWindowEx(hChild, 0, 'SysListView32', nil);
if hChild <> 0
then DeskHandle := hChild;
end;
end;
Result:= TRUE;
end;
begin
DeskHandle := 0;
EnumWindows(#getDesktopWnd, 0);
Result:= DeskHandle;
end;
The main question is: can I write this code as a single function or AT LEAST, can I get rid of the external/global var?
Possible solution:
The documentation says that the second parameter is only a IN parameter.
lParam [in]
Type: LPARAM
An application-defined value to be passed to the callback function.
https://msdn.microsoft.com/en-us/library/windows/desktop/ms633497%28v=vs.85%29.aspx
Would it be wrong to use it to pass the result back?

Answer 1

Local functions cannot be used as callbacks. If you hadn't used the # operator to pass your function, the compiler would have told you that. (Using the operator turns the argument into an ordinary untyped pointer, so the compiler can't check anymore.)
You'll have to make your callback be a standalone function.
To pass data between the callback and the caller, use the second parameter, which you've currently named NotUsed. For example, you could pass a pointer to a handle variable, and then the callback could dereference the pointer to return a result.

Answer 2

type
TMyData = record
Handle: HWND;
Pid: DWORD;
Caption: String;
ClassName: String;
end;
PMyData = ^TMyData;
function GetWindowClass(const Handle: HWND): String;
begin
SetLength(Result, MAX_PATH);
SetLength(Result, GetClassName(Handle, PChar(Result), Length(Result)));
end;
function GetWindowCaption(const Handle: HWND): String;
begin
SetLength(Result, MAX_PATH);
SetLength(Result, GetWindowText(Handle, PChar(Result), Length(Result)));
end;
function EnumChildWindowsProc(Handle: THandle; MyData: PMyData): BOOL; stdcall;
var
ClassName: String;
Caption: String;
Pid: DWORD;
begin
ClassName := GetWindowClass(Handle);
Caption := GetWindowCaption(Handle);
Result := (ClassName = 'SysListView32') and (Caption = 'FolderView');
if Result then
begin
MyData.Handle := Handle;
GetWindowThreadProcessId(Handle, MyData.Pid);
MyData.Caption := Caption;
MyData.ClassName := ClassName;
end;
// To continue enumeration, the callback function must return TRUE;
// to stop enumeration, it must return FALSE
Result := not Result;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
MyData: TMyData;
begin
ZeroMemory(#MyData, SizeOf(MyData));
EnumChildWindows(GetDesktopWindow, #EnumChildWindowsProc, NativeInt(#MyData));
if MyData.Handle > 0 then
begin
ShowMessageFmt('Found Window in Pid %d', [MyData.Pid]);
end
else begin
ShowMessage('Window not found!');
end;
end;