Delphi - Getting Window Handles

I wanted to develop my own print screen function - but instead of capturing the image to the clipboard, I wanted to write the window image directly to a jpeg or png file.

To do that, I needed a way to obtain the handle of the application window.

Windows handles | Windows commands | Using a mouse

Windows handles

Windows uses handles to identify many objects. These include application windows and the controls (buttons, menus, and so forth) displayed on the screen.

In windows.pas, handles are defined as

THandle = LongWord;    // The comments say these are from WINDEF.H
PHandle = ^THandle;    // but they are really from winnt.h

HWND = type LongWord;  // from winnt.h

where a LongWord is an unsigned 32-bit value. In winnt.h, these are defined as pointers to void - which amounts to approximately the same thing .. in a 32-bit operating system.

C++ is a case sensitive language. In it, all the native variable types are lower case and all the Microsoft defined types are uppercase. In Delphi (a case insensitive language), defined types begin with an uppercase-T (by convention).

Note that windows.pas contains data from many c++ header files and is used to define the Windows API as described in the Windows SDK. Specifically, it includes data types, constants, and function calls.

Windows commands

From the Windows SDK help (Window Handles)

A window handle has the HWND data type; an application must use this type when declaring a variable that holds a window handle.

An application can use the FindWindow function to discover whether a window with the specified class name or window name exists in the system. If such a window exists, FindWindow returns the handle of the window. To limit the search to the child windows of a particular application, use the FindWindowEx function. The IsWindow function determines whether a window handle identifies a valid, existing window.

Delphi 5 ships with a tool - WinSight - that will show you all the current windows, their children, and their handles. Double click an entry for additional information.

Warning - selecting the following option crashed the application and required a full reboot to
run WinSight a second time.

If you select Messages / Selected windows you will be able to see all the windows messages sent to the selected window. Unfortunately, clicking Stop! crashes the application and it will not restart until you reboot the system. I have tried using both Process Explorer and Alt-ctrl-del to terminate the application, but it still won't restart. (You've been warned!!)

These are some of the available Windows functions

FindWindow	Retrieves the handle to the top-level window whose class name and window name match the specified strings. This function does not search child windows.
EnumWindows	Passes the handle of each window, one at a time, to an application-defined callback. Use GetWindowText to get the text in the title bar.
GetWindow	Returns the handle of the window in the specified z-position. Should not be used in a loop - use EnumWindows instead.
WindowFromPoint	Returns the handle of the window that contains the specified point.

Using a mouse

It is also possible to get a window handle by using the mouse.

The WindowFromPoint function retrieves the handle of the window that contains the specified point.

When the mouse button is released, OnMouseUp gets x/y values relative to the origin of the control where mouse button was pressed. To get the position relative to the form, the control's position must be added. Then ClientToScreen will provide the appropriate coordinates.

procedure TForm1.Some_UIButtonMouseUp(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
var
 hnd        : HWND;
 SourceRect : TRect;   // Apparently, TRect is automatically allocated
 DestRect   : TRect;   // and these variables are actually pointers
begin
 DestRect.TopLeft := ClientToScreen( 
     point(Some_UIButton.Left + x, Some_UIButton.Top + y));

 hnd := WindowFromPoint(DestRect.TopLeft);

 GetWindowRect(hnd, SourceRect); // Get SourceRect

  // other code to capture an image here

end;

Even though GetWindowRect requires a pointer, the syntax above works.

In ClientToScreen I hardcoded the name of the button attached to the method. Generally, that is considered bad form. A more generic technique is

 DestRect.TopLeft := (Sender as TControl).ClientToScreen( point(x, y) );

However, the help files were far from clear on how that works. Once I read the source files it was clear.

The code recursively calls ClientToScreen for each parent until a windowed control is found. That control returns its screen position and the offsets of all its children are added.

The first example above called ClientToScreen for the form, not the control, and I had to add the position of the button to make it work.

GetCursorPos

The Windows API function GetCursorPos provides another way to get the mouse position.

The Delphi 5 help on this is a bit confusing.

GetCursorPos is a Windows command and documented in the Win32 Developer's Reference

However, from the Delphi help

GetCursorPos Method

Applies to
  TChart, TDBChart components

Declaration
  function GetCursorPos:TPoint;

Description

The GetCursorPos function returns a TPoint record containing current mouse cursor position coordinates in pixels. It calls Windows GetCursorPos and adjusts coordinates to TChart origin position using ScreenToClient method.

Mouse Cursor

Once the button is pressed, the mouse cursor should change to indicate its new function. In the code below, I have simply selected the hand cursor. It was important to only change the cursor when the left mouse button was pressed - without that test, the cursor would change when any button was pressed, but would not change back when either the middle or the right button was used. Apparently, windows does not send a button up message when the cursor is no longer in the application's window.

procedure TForm1.Some_UIButtonMouseUp(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin

 if button <> mbLeft then exit;

 screen.Cursor := crDefault;

  // other code to identify the window and capture an image here

end;

procedure TForm1.Some_UIButtonMouseDown(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
   if button = mbLeft then screen.Cursor := crHandPoint;
end;

screen.Cursor keeps the cursor image from changing when selecting a window outside the application's border.

Author: Robert Clemenzi