Using Dialog Windows (1.2)
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Using Dialog Windows
The simplest dialog window is the message box. Most message boxes present
simple messages and offer the user one, two, or three responses (represented
by buttons). A message box is easy to use and is appropriate when an
application requires a clearly defined response to a static message.
However, message boxes lack flexibility in size and placement on the screen,
and they are limited in the choices they offer the user. Applications that
require more control over size, position, and content should use regular
dialog boxes instead of message boxes.
Message Boxes
Message boxes provide an easy way for applications to display simple
messages without creating dialog templates or writing dialog procedures.
Message boxes are intended mainly for conveying information to users,
although they do have limited input capabilities.
There are several different kinds of predefined message boxes. There are
three parts to a message box: the icon, the message, and buttons.
Applications specify the icons and buttons using message-box style
constants. Message text is specified by a null-terminated string.
To create a message box, the application calls the WinMessageBox function,
which displays the message box and processes user input until the user
selects a button in the message box. The return value of the WinMessageBox
function indicates which button was selected.
The following code fragment illustrates how to create a message box with a
default Yes button, a No button, and a question-mark (?) icon. This example
assumes that you have defined a string resource with the MY_MESSAGESTR_ID
identifier in the resource file.
CHAR szMessageString[255];
USHORT cch;
USHORT usResult;
cch = WinLoadString(hab,
(HMODULE) NULL,
MY_MESSAGESTR_ID,
sizeof(szMessageString),
szMessageString);
usResult = WinMessageBox(hwndFrame, /* parent */
hwndFrame, /* owner */
szMessageString, /* text */
(PSZ) "", /* caption */
MY_MESSAGEWIN, /* window ID */
MB_YESNO |
MB_ICONQUESTION |
MB_DEFBUTTON1); /* style */
if (usResult == MBID_YES)
/* do yes case */
else
/* do no case */
The WinMessageBox function returns predefined values indicating which button
has been selected.
Note that strings for message boxes should be defined as string resources to
facilitate program translation for other countries. However, there is a
danger in using string resources in message boxes that are called in
low-memory situations. Loading a string resource in these situations could
cause severe memory problems and cause an application to fail. One way to
solve this problem is to preload the string resource and make it
nondiscardable so it will be available when the message box must be
displayed.
System-Modal Message Boxes
Message boxes are always modal. The default style for a message box is
application-modal. With this style, a user cannot select another window in
the same application until the message box is dismissed. However, the user
can switch to a different application.
It is possible to create a message box that is system-modal. A system-modal
message box prevents a user from selecting another window in the current
application or switching to a different application until responding to the
message box. A system-modal message box is useful when displaying a warning
to the user that there may be serious problems with the system, such as
insufficient memory.
There are two levels of modality for system-modal message boxes──soft modal
and hard modal. A soft-modal message box does not allow keystrokes or mouse
input to reach any other window, but does allow other messages, such as
deactivation and timer messages, to reach other windows. A hard-modal
message box does not allow any messages to reach other windows. A hard-model
message box is appropriate for serious system warnings.
A hard-modal message box is created by combining the MB_ICONHAND style with
the MB_SYSTEMMODAL style. A soft-modal message box is created by using the
MB_SYSTEMMODAL style with any style other than MB_ICONHAND. The
MB_SYSTEMMODAL icon is always in memory and is available even in low-memory
situations.
Dialog Boxes
When using dialog boxes, an application must load the dialog box, process
user input, and destroy the dialog box when the user finishes the task. The
process for handling dialog boxes varies, depending on whether the dialog
box is modal or modeless. A modal dialog box requires the user to dismiss
the dialog box before activating another window in the application. However,
the user can activate windows in different applications. A modeless dialog
box does not require an immediate response from the user. It is similar to a
frame window containing control windows. The following sections discuss how
to use both types of dialog boxes.
Modal Dialog Boxes
Modal dialog boxes present users with information and questions in such a
way that they must respond before proceeding with other operations in the
application.
The easiest way to use a modal dialog box is to define a dialog template in
the resource file and then call the WinDlgBox function, specifying the
dialog-box resource ID and a pointer to the dialog procedure. The WinDlgBox
function loads the dialog-box resource, displays the dialog box, and handles
all user input until the user dismisses the dialog box. The dialog procedure
receives messages when the dialog box is created (WM_INITDLG) and other
messages when the user interacts with each dialog item, such as entering
text in entry fields or selecting buttons.
You must specify both the parent and owner windows when loading a dialog box
using the WinDlgBox function. Generally, the parent window should be
HWND_DESKTOP and the owner should be a client window in your application.
Dialog boxes typically contain buttons that send WM_COMMAND messages when
selected by the user. WM_COMMAND messages passed to the WinDefDlgProc
function result in the WinDismissDlg function being called, with the window
ID of the source button as the return code. Dialog boxes with OK or Cancel
as the only buttons can ignore WM_COMMAND messages, allowing them to be
passed to the WinDefDlgProc function. The WinDefDlgProc function calls the
WinDismissDlg function to dismiss the dialog box and returns the DID_OK or
DID_CANCEL code.
Passing WM_COMMAND messages to the WinDefDlgProc function means that all
button presses in the dialog box will dismiss the dialog box. If you want
particular buttons to initiate operations without closing the dialog box or
if you want to perform some processing without closing the dialog box, you
should handle the WM_COMMAND messages in the dialog procedure.
If you handle WM_COMMAND messages in the dialog procedure, you must call the
WinDismissDlg function to dismiss the dialog box. Your dialog procedure
passes the DID_OK code to the WinDismissDlg function if the user selects the
OK button or the DID_CANCEL code if the user selects the Cancel button.
When you call the WinDismissDlg function or pass the WM_COMMAND message to
the WinDefDlgProc function, the dialog box is dismissed and the WinDlgBox
function returns the value passed to the WinDismissDlg function. This return
value identifies the button selected.
An alternative to using the WinDlgBox function is to call the individual
functions that duplicate its functionality, as shown in the following code
fragment:
dlg = WinLoadDlg(...);
result = WinProcessDlg(dlg);
WinDestroyWindow(dlg);
After calling the WinProcessDlg function, your dialog procedure must call
the WinDismissDlg function to dismiss the dialog box. Although the dialog
box is dismissed (hidden), it still exists. You must call the
WinDestroyWindow function to destroy a dialog box if it was loaded using the
WinLoadDlg function. The WinDlgBox function automatically destroys a dialog
box before returning.
If you want to manipulate individual items in a dialog box or add a menu
after loading the dialog box (but before calling WinProcessDlg), it is
better to make individual calls rather than calling the WinDlgBox function.
Individual calls are also useful for querying individual dialog items, such
as the contents of an entry-field control after a dialog box is closed but
before it is destroyed. Destroying a dialog box also destroys any
dialog-item control windows that are child windows of the dialog box.
Modeless Dialog Boxes
A modeless dialog box, unlike a modal dialog box, does not require user
interaction to activate another window in the current application.
To use a modeless dialog box in an application, you should create a dialog
template in the resource file, just as for a modal dialog box. Because
modeless dialog boxes share the screen equally with other frame windows, it
is a good idea to give modeless dialog boxes a title bar so they can be
moved around the screen. The following Resource Compiler source fragment
shows a dialog template for a dialog box with a title bar, a System menu,
and a Minimize Box.
DLGTEMPLATE IDD_SAMP
BEGIN
DIALOG "Modeless Dialog", IDD_SAMP, 80, 92, 126, 130,
WS_VISIBLE | FS_DLGBORDER,
FCF_TITLEBAR | FCF_SYSMENU | FCF_MINIMIZE
BEGIN
/* Put control-window definitions here. */
END
END
The application loads the dialog resource from the resource file by using
the WinLoadDlg function, receiving in return a window handle to the dialog
box. The application treats the dialog box as if it were an ordinary window.
Messages for the dialog box are dispatched through the event loop the
application uses for its other windows. In fact, an application can have a
modeless dialog box as its only window.
The resource for a modeless dialog box is just like that used for a modal
dialog box. The difference between modal and modeless dialog boxes is in the
way applications handle input to each box. For a modal dialog, the
WinDlgBox and WinProcessDlg functions handle all user input to the dialog
box, preventing access to other windows in the application. For a modeless
dialog box, the application does not call these functions, relying instead
on a normal message loop to dispatch messages to the dialog procedure.
The main difference between a modeless dialog box and a standard frame
window with child control windows is that for a modeless dialog box, an
application can define child windows for the dialog box in a dialog
template, automating the creation process of the window and its child
windows. The same effect can be achieved by creating a standard frame
window, but the child control windows must be created individually.
It is important that an application keep track of all open modeless dialog
boxes so that it can destroy all open windows before terminating.
Initializing a Dialog Box
Generally, an application defines a dialog template in its resource file and
loads the dialog box by calling the WinLoadDlg function or the WinDlgBox
function (which itself calls WinLoadDlg). The dialog box is created as an
invisible window unless the window style WS_VISIBLE is specified in the
dialog template. A WM_INITDLG message is sent to the dialog procedure before
the WinLoadDlg function returns. As each control defined in the template is
created, the dialog procedure may receive various control notifications
before the function returns. A dialog window can be destroyed by using the
WinDestroyWindow function. The WinLoadDlg function returns a handle to the
dialog window immediately after creating a dialog box.
In general, it is a good idea to define a dialog box as invisible since this
allows for optimization. For example, an experienced user may rapidly type
ahead, anticipating the processing of a dialog-box command. In such a case,
there is no need to display the dialog box because the user has finished the
interaction before the window can be displayed. This is how the
WinProcessDlg function works──it does not display a dialog box while there
are still WM_CHAR messages in the input queue. It allows these messages to
be processed first.
As control windows in a dialog box are created from the template, strings in
the template are processed by the WinSubstituteStrings function. Any
WM_SUBSTITUTESTRING messages are sent to the dialog procedure before the
WinLoadDlg function returns.
When child windows of a dialog window are created, the WinSubstituteStrings
function is used so child windows can make substitutions in their window
text. If any child-window text string contains the percent sign (%)
substitution character, the length of the text string is limited to 256
characters after it is returned from the substitution.
Menus in Dialog Boxes
To create a menu bar and menus in a dialog box, an application should first
load the dialog box to get a handle to the dialog-frame window. The
dialog-frame window can be associated with a menu resource by calling the
WinLoadMenu function. This function requires arguments specifying the menu
ID and handle of the parent window for the menu. Finally, the dialog-frame
window must incorporate the menu by sending a WM_UPDATEFRAME message to the
dialog box. The following code fragment illustrates these operations:
/* Get the dialog resource. */
hwndDialog = WinLoadDlg(...);
/* Get the menu resource and attach it to the dialog. */
hwndMenu = WinLoadMenu(hwndDialog, ...);
/* Inform the dialog that it has a new menu. */
WinSendMsg(hwndDialog, WM_UPDATEFRAME, 0L, 0L);
Applications can create menus in modal and modeless dialog boxes. The code
fragment above can be used for either type of dialog box. In the case of a
modal dialog box, your application should call the WinProcessDlg function to
handle user input until the dialog box is dismissed. For a modeless dialog
box, your application should call the WinShowWindow function to display the
dialog box, allowing the message loop to direct messages to the dialog box.
Dialog Procedure
The main difference between a dialog procedure and a window procedure is
that a dialog procedure does not receive WM_CREATE messages. Instead, a
dialog procedure receives WM_INITDLG messages, which are sent after a dialog
box is created but before it is displayed. The WM_INITDLG message can be
used to do the same type of initialization tasks that are handled by
WM_CREATE messages.
For example, if a dialog box contains a list box, you should use the
WM_INITDLG message to fill the list box with items. This procedure can also
be used to enable or disable buttons in a dialog box, depending on your
application.
You can also call the WinSetDlgItemText or WinSetDlgItemShort function
during dialog initialization to set up text items that reflect the current
conditions in your application.
Another typical task for the WM_INITDLG message handler is centering a
dialog on the screen or within its owner window. The following code fragment
illustrates how to center a dialog box on screen using the WM_INITDLG
message:
case WM_INITDLG:
/* Center the dialog box and get the screen rectangle. */
WinQueryWindowRect(HWND_DESKTOP, &rclScreenRect);
/* Get the dialog-box rectangle. */
WinQueryWindowRect(hwnd, &rclDialogRect);
/* Get the dialog-box width. */
sWidth = (SHORT) (rclDialogRect.xRight - rclDialogRect.xLeft);
/* Get the dialog-box height. */
sHeight = (SHORT) (rclDialogRect.yTop - rclDialogRect.yBottom);
/* Set the lower-left corner horizontal coordinate. */
sBLCx = ((SHORT) rclScreenRect.xRight - sWidth) / 2;
/* Set the lower-left corner vertical coordinate. */
sBLCy = ((SHORT) rclScreenRect.yTop - sHeight) / 2;
/* Move, size, and show the window. */
WinSetWindowPos(hwnd,
HWND_TOP,
sBLCx, sBLCy,
0, 0, /* Ignores size arguments. */
SWP_MOVE);
return 0L;
The dialog procedure receives notification messages from each control-window
item in a dialog box whenever a user clicks an item or enters text in an
entry field. Most dialog procedures wait for the user to select one or more
dialog-window buttons to signal that he or she has finished with the dialog
box. When the dialog procedure receives one of these messages, it should
call the WinDismissDlg function, as shown in the following code fragment.
The second argument to the WinDismissDlg function is the value returned by
the WinDlgBox or WinProcessDlg function. Generally, the ID of the button
that was pressed is returned.
MRESULT FAR PASCAL SampDialogProc(hwnd, usMessage, mp1, mp2)
HWND hwnd;
USHORT usMessage;
MPARAM mp1;
MPARAM mp2;
{
switch (usMessage) {
case WM_COMMAND:
switch (SHORT1FROMMP(mp1)) {
case DID_OK:
/*
* Final dialog-item queries,
* dismiss the dialog.
*/
WinDismissDlg(hwnd, DID_OK);
return 0L;
}
break;
}
return (WinDefDlgProc(hwnd, usMessage, mp1, mp2));
}
Other dialog-box items send notification messages specific to the type of
control window. Your dialog procedure should respond to notification
messages from each dialog item. Any messages that a dialog procedure does
not handle should be passed to the WinDefDlgProc function for default
processing. The default dialog procedure is exactly the same as the default
frame-window procedure.
The WM_COMMAND message from the OK button indicates that the user has
selected the OK button and is finished with the dialog box. If the dialog
box has other controls, such as entry fields or check boxes, your dialog
procedure should query the contents or state of each control when it
receives a message from the OK button. Before dismissing a dialog box, your
dialog procedure should collect input from each dialog-box control before
closing the dialog box.
Manipulating Dialog Items
Dialog items are control windows, and as such they can be manipulated using
standard window-management function calls. The window handle is obtained for
each dialog item by calling the WinWindowFromID function and passing the
window handle for the dialog box and the window ID for the dialog item as
defined in the dialog template. For example, the following Resource Compiler
source-code fragment should be included in your dialog template:
DLGTEMPLATE IDD_ABOUT
BEGIN
DIALOG "", IDD_ABOUT, 80, 92, 126, 130, FS_DLGBORDER, 0
BEGIN
PUSHBUTTON "My Button", ITEMID_MYBUTTON, 37, 107, 56, 12
/* Other item definitions ... */
END
END
Based on the above code fragment, your application will receive the
button-item handle by initiating the following call to the WinWindowFromID
function:
hwndItem = WinWindowFromID(hwndDialog, ITEMID_MYBUTTON);
Applications often change the contents, enabled state, or position of dialog
items at run time. For example, in a dialog box that contains a list box of
filenames and an Open button, the Open button should be disabled until the
user selects a file from the list. To do this, the button should be defined
as disabled in the dialog resource so that it is disabled when the dialog
box is first displayed. At run time, the dialog procedure receives a
notification message from the list box when the user selects a file. At that
time, the dialog procedure calls the WinEnableWindow function to enable the
Open button.
Applications can also change the text in static dialog items and buttons.
This is done by calling the WinSetWindowText function and using the window
handle of particular dialog item.
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