Window Procedure Subclassing
When an application creates a window, the system allocates a block of memory for storing information specific to the window, including the address of the window procedure that processes messages for the window. When the system needs to pass a message to the window, it searches ................
Subclassing is a technique that allows an application to intercept and process messages sent or posted to a particular window before the window has a chance to process them. By subclassing a window, an application can augment, modify, or monitor the behavior of the window. An application can subclass a window belonging to a system global class, such as an edit control or a list box. For example, an application could subclass an edit control to prevent the control from accepting certain characters. However, you can not subclass a window or .
An application subclasses a window by replacing the address of the window's original window procedure with the address of a new window procedure, called the subclass procedure. Thereafter, the subclass procedure receives any messages sent or posted to the window.
The subclass procedure can take three actions upon receiving a message: it can pass the message to the original window procedure, modify the message and pass it to the original window procedure, or process the message and not pass it to the original window procedure If. the subclass procedure processes a message, it can do so before, after, or both before and after it passes the message to the original window procedure.The system provides two types of subclassing:. instance and global In instance subclassing, an application replaces the window procedure address of a single instance of a window. An application must use instance subclassing to subclass an existing window. in global subclassing, an application replaces the address of the window procedure in the WNDCLASS structure of a window class. All subsequent windows created with The Class Have The Address of The Subclass Procedure, But Existing Windows of The Class Are Not Affected.
Instance Subclassing
An application subclasses an instance of a window by using the SetWindowLong function. The application passes the GWL_WNDPROC flag, the handle to the window to subclass, and the address of the subclass procedure to SetWindowLong. The subclass procedure can reside in either the application's executable or A Dynamic-Link Library (DLL).
SetWindowLong returns the address of the window's original window procedure. The application must save the address, using it in subsequent calls to the CallWindowProc function, to pass intercepted messages to the original window procedure. The application must also have the original window procedure address to remove the subclass from the window. to remove the subclass, the application calls SetWindowLong again, passing the address of the original window procedure with the GWL_WNDPROC flag and the handle to the window.The system owns the system global classes, and aspects of the controls might change from one version of the system to the next. If the application must subclass a window that belongs to a system global class, the developer may need to update the application when a new version of the system is released.
Because instance subclassing occurs after a window is created, you can not add any extra bytes to the window. Applications that subclass a window should use the window's property list to store any data needed for an instance of the subclassed window. For more information, see Window Properties.
When An Application Subclasses A Subclassed Window, IT Must Remove The Subclasses in The Reverse Order, WERE Performed. If The Removal ORDER IS NOT, AN Unrecoverable System Error May Occur.
Global Subclassing
To globally subclass a window class, the application must have a handle to a window of the class. The application also needs the handle to remove the subclass. To get the handle, an application typically creates a hidden window of the class to be subclassed. After obtaining the handle, the application calls the SetClassLong function, specifying the handle, the GCL_WNDPROC flag, and the address of the subclass procedure. SetClassLong returns the address of the original window procedure for the class.The original window procedure address is used in global subclassing in the same way it is used in instance subclassing. The subclass procedure passes messages to the original window procedure by calling CallWindowProc. The application removes the subclass from the window class by calling SetClassLong again, specifying the address of the original window procedure, the GCL_WndProc Flag, And The Class Being Subclassed. An Application That Globally Subclasses A Control Class Must Remove The Subclass When the Application Terminates; OtherWise, An Unrecoverable System Error May Occur.
Global subclassing has the same limitations as instance subclassing, plus some additional restrictions. An application should not use the extra bytes for either the class or the window instance without knowing exactly how the original window procedure uses them. If the application must associate data with a Window, IT Should Use Window Properties
