How does this AHK script work?

Question

The following AHK script automatically brings up the menu that you would normally get when you right-click on the uTorrent icon in the system task-bar tray:

DetectHiddenWindows, On
Process, Exist, utorrent.exe
WinGet, W, List, ahk_pid %ErrorLevel%
Loop %W%
 {
 WinGetClass, Class, % "ahk_id" W%A_Index%
 If InStr( Class, "µTorrent" ) {
   hWnd := W%A_Index%
   Break
 }}
PostMessage, 0x8001, 0,0x204,, ahk_id %hWnd% ; Right Click down
PostMessage, 0x8001, 0,0x205,, ahk_id %hWnd% ; Right Click Up

I understand most of it except the last two lines. 0x8001 is WM_APP+1. Why is a message in the WM_APP range used, and what exactly happens when these two messages are issued? Many thanks for the explanation.

Answer 1

According to MSDN (http://msdn.microsoft.com/en-us/library/windows/desktop/ms644927(v=vs.85).aspx#system_defined)

If your application is marked version 4.0, you can use message-identifier values in the range 0x8000 (WM_APP) through 0xBFFF for private messages.

So this would indicate that 0x8001 is an identifier that utorrent has chosen for a message. The 0x204 and 0x205 values are for the Right-Click Down and Up events respectively. My guess is that this code is intended to simulate a Right-Click on utorrent's icon in the Windows tray.

Also, if you're using Autohotkey_L the code can probably be simplified to three lines and perform the same:

DetectHiddenWindows, On
PostMessage, 0x8001, 0,0x204,, ahk_exe utorrent.exe ahk_class µTorrent ; Right Click down
PostMessage, 0x8001, 0,0x205,, ahk_exe utorrent.exe ahk_class µTorrent ; Right Click Up

Answer 2

When you right click on something, the context menu is shown. The context menu can also be shown by pressing the appskey button on the keyboard. When you press it, Windows sends the application a message, i.e., the WM_APP message. Most applications respond by showing a context menu. In the case of the tray icon, the response is the same as the response to a right-click.

Answer 3

Seems that the WM_APP+1 message number is a choice made in the development of the application, in this case utorrent. Could have been another message #, nothing magic. I believe the code I presented (which is from the AHK forums) figured out the message to send through reverse engineering.