Modal Dialog Boxes
A modal dialog box should be a pop-up window having a System menu, a title
bar, and a thick border; that is, the dialog box template should specify the
WS_POPUP, WS_SYSMENU, WS_CAPTION, and DS_MODALFRAME styles. Although an application
can designate the WS_VISIBLE style, Windows always displays a modal dialog box
regardless of whether the dialog box template specifies the WS_VISIBLE style.
An application must not create a modal dialog box having the WS_CHILD style. A
modal dialog box with this style disables itself, preventing any subsequent
input from reaching the application.
An application creates a modal dialog box by using either the DialogBox
requires the name or identifier of a resource containing a dialog box
requires the handle of a memory object containing a dialog box template. The DialogBoxParam
functions also create modal dialog boxes; they are identical to the
previously mentioned functions but pass a specified parameter to the dialog box
procedure when the dialog box is created.
When creating the modal dialog box, Windows makes it the active window. The
dialog box remains active until the dialog box procedure calls the EndDialog
function or Windows activates a window in another application. Neither the
user nor the application can make the owner window active until the modal dialog
box is destroyed.
When the owner window is not already disabled, Windows automatically disables
the window and any child windows belonging to it when it creates the modal
dialog box. The owner window remains disabled until the dialog box is destroyed.
Although a dialog box procedure could potentially enable the owner window at any
time, enabling the owner defeats the purpose of the modal dialog box and is not
recommended. When the dialog box procedure is destroyed, Windows enables the
owner window again, but only if the modal dialog box caused the owner to be
As Windows creates the modal dialog box, it sends the WM_CANCELMODE
message to the window (if any) currently capturing mouse input. An
application that receives this message should release the mouse capture so that the user
can move the mouse in the modal dialog box. Because Windows disables the owner
window, all mouse input is lost if the owner fails to release the mouse upon
receiving this message.
To process messages for the modal dialog box, Windows starts its own message
loop, taking temporary control of the message queue for the entire application.
When Windows retrieves a message that is not explicitly for the dialog box, it
dispatches the message to the appropriate window. If it retrieves a WM_QUIT
message, it posts the message back to the application message queue so that
the application's main message loop can eventually retrieve the message.
Windows sends the WM_ENTERIDLE
message to the owner window whenever the application message queue is empty.
The application can use this message to carry out a background task while the
dialog box remains on the screen. When an application uses the message in this
way, the application must frequently yield control (for example, by using the PeekMessage
function) so that the modal dialog box can receive any user input. To prevent
the modal dialog box from sending the WM_ENTERIDLE messages, the application
can specify the DS_NOIDLEMSG style when creating the dialog box.
An application destroys a modal dialog box by using the EndDialog
function. In most cases, the dialog box procedure calls EndDialog
when the user chooses the Close command from the dialog box's System menu or
chooses the OK or Cancel button in the dialog box. The dialog box can return a
value through the DialogBox
function (or other creation functions) by specifying a value when calling the EndDialog
function. Windows returns this value after destroying the dialog box. Most
applications use this return value to determine whether the dialog box completed
its task successfully or was canceled by the user. Windows does not return
control from the function that creates the dialog box until the dialog box
procedure has called the EndDialog
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