Receiving Callback Messages

Each taskbar icon can have an application-defined callback message associated with it. If an icon has a callback message, the system will send the message to the application whenever a mouse event occurs within the icon. In this way, the system can notify an application whenever the user clicks or double-clicks the icon, or moves the mouse cursor into the icon's bounding rectangle.

An application defines an icon's callback message when it adds the icon to the taskbar. The uCallbackMessage member of the NOTIFYICONDATA structure included with the NIM_ADD message specifies the identifier of the callback message. When a mouse event occurs, the system sends the callback message to the window identified by the hWnd member. The message's lParam parameter is the identifier of the mouse message that the system generated as a result of the mouse event. For example, when the mouse cursor moves into a taskbar icon, the lParam parameter of the resulting callback message contains the WM_MOUSEMOVE identifier. The wParam parameter contains the identifier of the taskbar icon in which the mouse event occurred.

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