Custom Controls

Applications can create custom controls to perform tasks not supported by predefined controls. Windows provides the following ways to create custom controls:

• Use owner-drawn buttons, list boxes, and combo boxes.

• Subclass an existing control-window class.

• Register and implement from scratch an application-defined window class.

Buttons, list boxes, and combo boxes have owner-drawn styles available that direct the control to send a message to the parent window whenever the control must be drawn. This feature permits an application to alter the appearance of a control. For buttons, the owner-drawn style affects how the system draws the entire control. For list boxes and combo boxes, the parent window draws the items within the control, and the control draws its own outline. For example, an application can customize a list box so that it displays a small bitmap next to each item in the list.

An application can designate list boxes, combo boxes, and buttons as owner-drawn controls by creating them with the appropriate style. When a control has the owner-drawn style, Windows handles the user's interaction with the control as usual, performing such tasks as detecting when a user has chosen a button and notifying the button's owner of the event. However, because the control is owner drawn, the parent window of the control is responsible for the visual appearance of the control. For more information about owner-drawn controls, see the individual topics for buttons, list boxes, and combo boxes.

Subclassing an existing control is another way to create a custom control. The subclass procedure can alter selected behaviors of the control by processing those messages that affect the selected behaviors. All other messages pass to the original window procedure for the control. For example, an application can display a small bitmap next to the text in a read-only, single-line edit control by subclassing the control and processing the WM_PAINT message. For more information about subclassing, see Window Classes.

Although an application may subclass a predefined control, it relies on the window procedure of the control to provide all other aspects of the control's behavior. For more information about a control's behavior, see the individual topics for the predefined controls.

An application can create custom controls by registering an application-defined window class and specifying the name of the window class in the CreateWindowEx function or in the dialog box template. The process for registering an application-defined window class for a custom control is the same as for registering a class for an ordinary window. Each class must have a unique name, a corresponding window procedure, and other information.

At a minimum, the window procedure draws the control. If an application uses the control to let the user type information, the window procedure also processes input messages from the keyboard and mouse and sends notification messages to the parent window. In addition, if the control supports control messages, the window procedure processes messages sent to it by the parent window or other windows. For example, controls often process the WM_GETDLGCODE message sent by dialog boxes to direct a dialog box to process keyboard input in a given way.

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