Adapted from Josh Brobst on Stack Overflow

Usage

1. #include this script. This creates two object declarations, WindowChangeDetector and Debug_Gui, and one super global variable, ChangeDetector.
2. Create a new instance of a WindowChangeDetector. This takes two parameters:
   1. A declared AHK function that will be the callback for when a window changes
   2. An optional boolean debug that controls whether a window will be created for logging debug messages.
3. [optional] if debug == True, log debugging messages to the debug window by calling the instance method debug(msg) Not working, see limitations

Example:

#include %A_ScriptDir%\ahk-detect_window_change\onWindowChange.ahk
changeDetector := new WindowChangeDetector("msgboxActiveWindow", True)
msgboxActiveWindow(){
    global changeDetector
    WinGetActiveTitle, ThisWindow
    changeDetector.debug("Active: '" ThisWindow "'")
    msgbox % ThisWindow
}

Adds three global objects to script:

• WindowChangeDetector object declaration
• Debug_Gui object declaration
• ChangeDetector superglobal object (available to all functions by default, can be overridden).

Causes the including script to become:

• persistent
• single-instance

Known limitations

Code run by the callback does not have access to the instance variables, so using the debugging window fails. Use a msgbox to confirm the callback works.