Windows Message Routing
The system maintains a single system message queue and one thread-specific message queue for each graphical user interface (GUI) thread. To avoid the overhead of creating a message queue for non–GUI threads, all threads are created initially without a message queue. The system creates a thread-specific message queue only when the thread makes its first call to one of the User or Windows Graphics Device Interface (GDI) functions.
Whenever the user moves the mouse, clicks the mouse buttons, or types on the keyboard, the device driver for the mouse or keyboard converts the input into messages and places them in the system message queue. The system removes the messages, one at a time, from the system message queue, examines them to determine the destination window, and then posts them to the message queue of the thread that created the destination window. A thread’s message queue receives all mouse and keyboard messages for the windows created by the thread. The thread removes messages from its queue and directs the system to send them to the appropriate window procedure for processing.
With the exception of the WM_PAINT message, the WM_TIMER message, and the WM_QUIT message, the system always posts messages at the end of a message queue. This ensures that a window receives its input messages in the proper first in, first out (FIFO) sequence. The WM_PAINT message, the WM_TIMER message, and the WM_QUIT message, however, are kept in the queue and are forwarded to the window procedure only when the queue contains no other messages. In addition, multiple WM_PAINT messages for the same window are combined into a single WM_PAINT message, consolidating all invalid parts of the client area into a single area. Combining WM_PAINT messages reduces the number of times a window must redraw the contents of its client area.
typedef struct _W32THREAD
{
struct _USER_MESSAGE_QUEUE MessageQueue;
LIST_ENTRY WindowListHead;
LIST_ENTRY W32CallbackListHead;
struct _KBDTABLES KeyboardLayout;
struct _DESKTOP_OBJECT Desktop;
HANDLE hDesktop;
DWORD MessagePumpHookValue;
BOOLEAN IsExiting;
SINGLE_LIST_ENTRY ReferencesList;
PW32THREADINFO ThreadInfo;
} W32THREAD, PW32THREAD;
The CreateDialog function uses the function to create the dialog box. CreateDialog then sends a message (and a message if the template specifies the or style) to the dialog box procedure. The function displays the dialog box if the template specifies the WS_VISIBLE style. Finally, CreateDialog returns the window handle to the dialog box.
After CreateDialog returns, the application displays the dialog box (if it is not already displayed) by using the function. The application destroys the dialog box by using the function. To support keyboard navigation and other dialog box functionality, the message loop for the dialog box must call the function.
BOOL bRet;while ((bRet = GetMessage(&msg, NULL, 0, 0)) != 0)
{
if (bRet == -1)
{
// Handle the error and possibly exit
}
else if (!IsWindow(hwndGoto) || !IsDialogMessage(hwndGoto, &msg))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
The IsDialogMessage function determines whether a message is intended for the specified dialog box and, if it is, processes the message.
Although the IsDialogMessage function is intended for modeless dialog boxes, you can use it with any window that contains controls, enabling the windows to provide the same keyboard selection as is used in a dialog box.
modal dialog box
The DialogBoxParam function creates a modal dialog box from a dialog box template resource. Before displaying the dialog box, the function passes an application-defined value to the dialog box procedure as the lParam parameter of the message. An application can use this value to initialize dialog box controls.
The DialogBoxParam function uses the function to create the dialog box. DialogBoxParam then sends a WM_INITDIALOG message (and a message if the template specifies the or style) to the dialog box procedure. The function displays the dialog box (regardless of whether the template specifies the WS_VISIBLE style), disables the owner window, and starts its own message loop to retrieve and dispatch messages for the dialog box.
When the dialog box procedure calls the EndDialog function, DialogBoxParam destroys the dialog box, ends the message loop, enables the owner window (if previously enabled), and returns the nResult parameter specified by the dialog box procedure when it called EndDialog.