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                               About Windows
 
This topic describes the portions of MS OS/2 that let you create and use
windows; manage relationships between windows; and size, move, and display
windows in your Presentation Manager application. You should also be
familiar with the following topics:
 
    ◄Messages and message queues►
    ◄Window classes►
    ◄Window procedures►
 
A window is a rectangular area of the screen where an application displays
output and receives input from the user. You might think of a window as a
"graphics terminal" that shares the screen with other terminals. Only one
"terminal" is active at a time, and when it is, a user can use the mouse and
keyboard to interact with the application that owns the terminal.
 
Unlike a graphics terminal, however, a window must be created by an
application before it can be used. MS OS/2 does not create a window by
default. This means that one of the first tasks of a Presentation Manager
application is to create a window.
 
Desktop Windows
 
MS OS/2 automatically creates two windows: the desktop window and the
desktop-object window. The desktop window is the base (bottom-most) window
in the Presentation Manager session. It is the window that paints the
background for this session. It serves as the base for all windows created
and displayed by applications. The desktop-object window is like a desktop
window that is never displayed. It serves as a base for windows that
coordinate the activity of other windows that are not displayed.
 
Application Windows
 
Every application creates at least one window, called the main window, to
serve as the "graphics terminal" for the application. The application also
creates many other windows either directly or indirectly to carry out tasks
related to the main window. In fact, most windows used by an application are
composed of several different windows. Each window plays a part in
displaying output and receiving input from the user.
 
Typically, an application's main window is made up of several windows acting
together as one. The main window is usually a frame window that contains a
client window and one or more control windows, such as a title bar and a
System menu.
 
An application can use several types of windows: frame windows, client
windows, control windows, dialog windows, message boxes, and menus.
 
A frame window is a special window that the application uses as the base
when constructing a main window or other composite window. A frame window
provides basic features, such as borders and system-command processing, that
a main window needs to conform to the MS OS/2 user-interface guidelines.
 
A dialog window is a frame window that contains one or more control windows.
Dialog windows are used almost exclusively for prompting the user for input.
An application usually creates a dialog window when it needs additional
information to complete a command. It then destroys the dialog window when
the requested information has been entered.
 
A message box is a frame window that an application uses to display a note,
caution, or warning to the user. Message boxes are commonly used to inform
the user of problems the application encounters while carrying out a task.
 
A client window is the window in which the application displays the current
document or data. For example, a desktop-publishing application displays the
current page of a document in a client window. Most applications create at
least one client window. The application must process input to the window
and then display output.
 
A control window is any window used in conjunction with another window to
carry out useful input or output tasks, such as displaying messages or
reading text. MS OS/2 provides several predefined control-window classes
that can be used to create control windows. Control windows include buttons,
entry fields, list boxes, menus, scroll bars, static text, and title bars.
 
A menu is a control window that presents a list of commands and other menus
to the user. The user chooses commands from the list by using a mouse or
keyboard. The application then carries out the chosen task.
 
Many simple applications create only a main window. The application manages
the client window and allows the frame and control windows to operate as
defined by MS OS/2.
 
Window Creation
 
An application creates windows by using a window-creation function, such as
WinCreateWindow, and supplying information about the window to be created.
An application can create one or more windows in any thread for which it has
created a message queue. An application creates a message queue for a thread
by using the WinCreateMsgQueue function after initializing the application
for Presentation Manager by using the WinInitialize function.
 
The following information must be supplied when creating a window:
 
♦  Window class
♦  Window name
♦  Parent window
♦  Window position relative to the parent window
♦  Window position relative to its sibling windows (Z order)
♦  Window width and height
♦  Window styles
♦  Owner window
♦  Window identifier
♦  Class-specific data
 
Every window belongs to a window class. The window class defines how the
window behaves and appears when operating. The chief component of the window
class is the window procedure. The window procedure is a function that
receives and processes all input and requests for action sent to the window
by the system. The window class also defines the class styles. These tell MS
OS/2 what initial window styles to give a window created with this class.
 
A window can have a name. A window name is a text string that identifies the
window for the user. The window name is typically displayed in the window or
in a title bar within the window. How the name is used depends on the window
class.
 
Every window created has a parent window. The parent window provides the
coordinate system used for positioning the window and defines the
relationship the new window has with other windows in the system. The parent
window also affects the behavior and appearance of the window. For example,
when the parent window is hidden, the child window is also hidden.
 
Every window has a position, size, and Z-order position. The position
specifies the location on the screen of the window's lower-left corner. This
position is relative to the lower-left corner of the parent window (in
pels). A window's size is the width and height in the window (in pels). A
window's Z-order position specifies the position of the window in the stack
of overlapping windows. The window at the top of the Z order overlaps all
sibling windows (that is, windows having the same parent window). A window
at the bottom of the Z order is overlapped by all sibling windows. An
application sets a window's Z-order position by placing it behind a given
sibling window.
 
Every window can have a style. The window style specifies how the window
behaves or appears. For example, a window style can specify whether the
window is visible or invisible when first created. A few window styles apply
to all windows, but most apply to windows of specific window classes. The
window procedure for that class interprets the style.
 
A window can be owned by another window. An owner window is similar to a
parent window, but it does not affect the behavior or appearance of the
window in the same way. The owner window usually coordinates the activity of
a window so that it can operate in conjunction with other windows. The
window sends messages about its state to its owner window; the owner window
sends messages about what action to carry out next.
 
A window can have a window identifier. A window identifier is a way to
uniquely identify a window that operates in conjunction with other windows.
A window identifier is especially useful if a window sends information to
the owner window.
 
A window can have class-specific data. This data further defines how the
window behaves and appears when first created. The system passes
class-specific data to the window procedure. The window procedure then
applies the data to the new window.
 
Window-Creation functions 1.2
 
The basic window-creation function is WinCreateWindow. The WinCreateWindow
function takes window class, style, size, and position information and
creates a new window. All other window-creation functions, such as
WinCreateStdWindow and WinCreateDlg, supply some of this information by
default and create windows of a specific class or style.
 
Although WinCreateWindow provides the most direct means of creating a
window, most applications do not use it. Instead, they typically use the
WinCreateStdWindow function to create a main window and use the WinDlgBox or
WinCreateDlg function to create dialog windows.
 
The WinCreateMenu, WinLoadMenu, WinLoadDlg, WinMessageBox, and
WinCreateFrameControls functions also create windows. Each of these
functions substitutes for one or more calls to the WinCreateWindow function
required to create a given window. For example, you can create a frame
window, one or more control windows, and a client window, all in a single
call to WinCreateStdWindow.
 
Window-Creation Messages
 
The system sends messages to the window procedure as it creates a window.
Each window procedure receives a WM_CREATE message, specifying that the
window is being created.
 
The system also sends a WM_ADJUSTWINDOWPOS message, specifying the size and
position for the window. This message allows the window procedure to adjust
the size and position before they are actually applied to the window.
 
The system also sends other messages. The number and order of these messages
depend on the window class and style and on the function used to create the
window.
 
Window Handles
 
When a window is created, the creation function returns a window handle. A
window handle uniquely identifies the window and can be used in functions to
direct the action of the function to the window. Window handles have the
data type HWND; applications must use this type when declaring the variables
that hold window handles.
 
There are several special constants that can be used in place of a window
handle in certain functions. For example, HWND_DESKTOP can be used in the
WinCreateWindow function to specify the desktop window as the new window's
parent window. Similarly, HWND_OBJECT represents the desktop-object window.
HWND_TOP and HWND_BOTTOM represent the top and bottom positions when setting
the Z-order position for a window.
 
Although the NULL constant is not a window handle, it can be used in some
functions to specify that no window is affected. For example, NULL can be
used in the WinCreateWindow function to specify that there is no owner
window. Some functions may return NULL, indicating that the given action
applies to no window.
 
Window Size and Position
 
A window's size and position can be expressed as a bounding rectangle, given
in coordinates relative to its parent window. The window's size and position
can be explicitly specified when it is created, or the system can use
default values. The window's size and position can be changed at any time.
 
The default coordinate system for a window specifies that the point (0,0) is
at the window's lower-left corner; coordinates increase upward and to the
right.
 
When two sibling windows overlap, the system must specify which window is
displayed in front. This ordering of sibling windows is known as the Z
order.
 
Window Styles
 
A window style is a value that specifies how a window behaves or appears in
a given situation. Window styles let applications adapt windows of a given
class for special circumstances. For example, an application can give a
window the style WS_SYNCPAINT to cause it to paint immediately whenever any
portion of the window becomes invalid. A window normally paints only if
there are no messages waiting in the message queue.
 
An application usually sets the window style when it creates the window. It
can also set the window style after creation by using the WinShowWindow and
WinSetWindowULong functions. MS OS/2 provides several standard window styles
that apply to all windows. It also provides many styles for the predefined
frame and control windows. The frame and control styles are unique to each
predefined window class and can be used only for windows belonging to the
corresponding class.
 
Initially, the class styles of the window class used to create the window
determine the window styles of the new window. If the window class has the
style CS_SYNCPAINT, all windows created using that class have the style
WS_SYNCPAINT by default.
 
MS OS/2 has the following standard window styles:
 
Style            Description
────────────────────────────────────────────────────────────────────────────
WS_VISIBLE       Makes the window visible. MS OS/2 draws the window on the
                 screen unless overlapping windows completely obscure it.
                 Windows without this style are hidden. If overlapping
                 windows completely obscure the window, the window is still
                 considered to be visible. Visibility simply means that MS
                 OS/2 draws the window if it can.
 
WS_DISABLED      Disables mouse and keyboard input to the window. This style
                 is used to temporarily prevent the user from using the
                 window.
 
WS_CLIPCHILDREN  Prevents a window from painting over its child windows.
 
WS_CLIPSIBLINGS  Prevents a window from painting over its sibling windows.
 
WS_PARENTCLIP    Prevents a window from painting over its parent window.
 
WS_SAVEBITS      Saves the image under the window as a bitmap. When the
                 window is moved or hidden, the system restores the image by
                 copying the bits.
 
WS_SYNCPAINT     Causes the window to immediately receive WM_PAINT messages
                 after a part of the window becomes invalid. Without this
                 style, the window receives WM_PAINT messages only if no
                 other message is waiting to be processed.
 
WS_MINIMIZED     Reduces the window to the minimum size.
 
WS_MAXIMIZED     Enlarges the window to the maximum size.
 
WS_GROUP         Identifies the window as the first dialog item in a group
                 of dialog items. This style is used with controls in dialog
                 windows to permit the user to move among the controls by
                 pressing the ARROW keys.
 
WS_TABSTOP       Identifies the window as a tabstop window. This style is
                 used with controls in dialog windows to permit the user to
                 move to the control by pressing the TAB key.
 
Window Destruction
 
An application can destroy the windows it has created. When a window is
destroyed, the system hides the window if it is visible, and then removes
any internal data associated with the window. This invalidates the window
handle; it can no longer be used in functions. An application destroys a
window by using the WinDestroyWindow function.
 
Most applications destroy the windows they create soon after creating them.
For example, an application usually destroys any dialog windows as soon as
the application has sufficient input from the user to continue its task. An
application eventually destroys the main window of the application (before
terminating). In general, an application must destroy all the windows it
creates.
 
Destroying a window does not affect the window class from which the window
is created. New windows can still be created using that class and any
existing windows of that class continue to operate.
 
Destroying a window also destroys that window's child and owned windows. The
WinDestroyWindow function sends a WM_DESTROY message to the window, which in
turn sends the same message to all its child and owned windows. Each child
and owned window passes the message on to other child and owned windows. In
this way, all descendant windows of the window being destroyed are also
destroyed.
 
Before destroying a window, an application should save or remove any data
associated with the window and release any resources. For example, a
presentation space created for the window by the WinGetPS function must be
released by calling the WinReleasePS function. This must be done before
calling the WinDestroyWindow function. If a presentation space is associated
with the device context for the window, the application should disassociate
or destroy the presentation space by using the GpiAssociate or GpiDestroyPS
function before calling WinDestroyWindow. Failing to release a resource can
cause an error.
 
The WinDestroyWindow function may send several messages to a window. The
following is a list of possible messages sent by WinDestroyWindow:
 
Message                  Description
────────────────────────────────────────────────────────────────────────────
WM_DESTROY               Always sent to the window being destroyed after it
                         has been hidden, but before its child windows have
                         been destroyed.
 
WM_ACTIVATE              Sent with the first message parameter equal to
                         FALSE if the window being destroyed is the active
                         window.
 
WM_OTHERWINDOWDESTROYED  Sent to all main windows of the window being
                         destroyed and to its descendant windows, if the
                         window being destroyed has been registered with the
                         WinRegisterWindowDestroy function.
 
WM_RENDERALLFMTS         Sent if the clipboard owner is being destroyed and
                         there are unrendered formats on the clipboard.
 
If the window being destroyed is the active window, both the active and
focus states are transferred to another window. The window that becomes the
active window is the next window (as defined by the ALT+ESC key
combination). The new active window determines which window has the input
focus.
 
Locked Windows
 
A window can be locked. An application typically locks a window to prevent
it from being destroyed. This is useful whenever a window needs to access
data that may be lost if the associated window is destroyed. An application
can lock a window by using the WinLockWindow function.
 
Each window has a lock count. When a window is created, its lock count is
set to zero, meaning that the window is unlocked. An application can use the
WinLockWindow function to increment or decrement the lock count. If the lock
count is greater than zero, the window is locked. If the lock count is zero,
the window is unlocked. The lock count can never be less than zero. An
application can retrieve the current lock count by using the
WinQueryWindowLockCount function.
 
The WinQueryWindow, WinQueryActiveWindow, and WinQuerySysModalWindow
functions also lock a window if specified.
 
Disabled Windows
 
A window can be disabled. A disabled window receives no keyboard or mouse
input. An application typically disables a window to prevent the user from
using the window. For example, the application may disable a push button in
a dialog window to prevent the user from choosing it. An application can
enable a disabled window at any time. Enabling a window restores normal
input. An application enables or disables a window by using the
WinEnableWindow function.
 
By default, a window is enabled when created. The WS_DISABLED style can be
specified, however, to disable a new window. If an application uses the
WinEnableWindow function to disable an existing window, that window also
loses the keyboard focus. The keyboard focus is set to NULL, meaning no
window has the focus. If a child window or other descendant window has the
keyboard focus, the descendant window loses it when the window is disabled.
 
An application can determine whether a window is disabled by using the
WinIsWindowEnabled function.
 
System-Modal Windows
 
System-modal windows require the user to respond immediately to warnings
about the state of the system. Because the system-modal window receives all
keyboard and mouse input, all other windows are effectively disabled when
the system-modal window is set; the user cannot continue working in other
windows until the system-modal window has been cleared. An application sets
and clears the system-modal window by using the WinSetSysModalWindow
function.
 
Due to its absolute control of input, applications must use care when
setting a system-modal window. Ideally, an application uses a system-modal
window only when there is danger of losing data if the user does not respond
to the problem immediately.
 
Although an application can destroy a system-modal window, the new active
window will also be the new system-modal window. An application can also
make another window active while the system-modal window exists. Again, the
new active window is also the new system-modal window. In general, once a
system-modal window is set, a system-modal window will continue to exist in
the Presentation Manager session until explicitly cleared.
 
Window Data
 
Every window has an associated data structure. The window data structure
contains all the information specified for the window when it was created
and any additional information supplied for the window since creation.
Although the exact number and meaning of fields in the window data structure
is private to the system, an application can directly access any of the
following fields:
 
♦  Pointer to window-class data structure
♦  Pointer to window procedure
♦  Parent-window handle
♦  Owner-window handle
♦  Handle of first child window
♦  Handle of next sibling window
♦  Window size and position (expressed as a rectangle)
♦  Lock count
♦  Window style
♦  Window identifier
♦  Update-region handle
♦  Message-queue handle
 
An application can examine and modify these fields by using functions such
as WinQueryWindowUShort and WinSetWindowUShort. These functions let an
application access fields, such as the lock count, which are stored as
16-bit integers. Other functions let an application access fields containing
32-bits integers and pointers. There are several fields that indirectly
affect the fields in the window data structure. For example, the
WinLockWindow function modifies the lock-count field; the WinSubclassWindow
function replaces the window-procedure pointer.
 
An application can extend the number of available fields in the window data
structure by specifying a count of extra bytes when it registers the
corresponding window class. The window procedure can then use these bytes to
store information about the window. Functions such as WinQueryWindowUShort
and WinSetWindowUShort give direct access to the extra bytes.
 
If a window needs more than a few bytes of storage added to the window data
structure, using extra bytes alone is not the best solution. One common
alternative is to dynamically allocate some memory and then store a pointer
to that dynamic memory in the extra bytes of the window data structure.
 
Subclassed Windows
 
A subclassed window is any window whose original window procedure has been
replaced with another window procedure. The original window procedure is
specified by the window class used to create the window. An application
typically subclasses a window (replaces the window procedure) so that it can
support additional capabilities in a window created with a given class. For
example, an application may subclass a push-button control so that it can
add sound when the user chooses the button. An application subclasses a
window by using the WinSubclassWindow function.
 
Typically, a window procedure used to subclass a window will pass most (if
not all) messages on to the original window procedure. The usual goal of
subclassing is to add capability. The WinSubclassWindow function returns the
address of the original window procedure, making it easy to call the
original function from the new window procedure. The following code fragment
shows the general format of a window procedure used for subclassing:
 
PFNWP pfnwp;
 
main() {
 
    /* Subclass in main function or other window procedure. */
 
    pfnwp = WinSubclassWindow(hwnd, MySubClass);
}
 
MRESULT EXPENTRY MySubClass(hwnd, usMessage, mp1, mp2)
HWND hwnd;
USHORT usMessage;
MPARAM mp1;
MPARAM mp2;
{
    switch (usMessage) {
      .
      . /* Process messages. */
      .
    }
    return (pfnwp(hwnd, usMessage, mp1, mp2));
}
 
Note that the replacement window procedure calls the original window
procedure instead of the WinDefWindowProc function.
 
An application can subclass only one window at a time. It cannot subclass an
entire class.
 
Window Relationships
 
Window relationships define how windows interact with each other on the
screen and through messages. There are parent-child window relationships and
ownership relationships.
 
The parent-child relationship determines how a window looks when drawn on
the screen. It also determines what happens to a window when a related
window is destroyed or hidden. The parent-child rules apply to all windows
at all times and cannot be modified.
 
Ownership determines how windows communicate using messages. Cooperative
windows define the rules of ownership and then carry them out. Although some
windows, such as windows belonging to the preregistered, public window class
WC_FRAME, have quite complex rules of ownership, the application ordinarily
defines the ownership rules.
 
Parent-Child Relationship
 
Most windows have a parent window. (The exceptions to this rule are the
desktop and the desktop-object windows. These windows, created by the system
when it first starts, have no parent windows.) An application sets the
parent window when it creates the window; the system uses the parent window
to determine where and how to draw the new window, as well as when to
destroy the window.
 
A window is drawn relative to its parent window. The coordinates given to
specify the position of a window's lower-left corner are relative to the
lower-left corner of its parent window. For example, a window whose
coordinates are (10,10) is placed 10 pels right and 10 pels up from the
lower-left corner of its parent window. A window is a top-level window if
its parent window is the desktop window. Top-level windows are drawn
relative to the lower-left corner of the screen (the desktop window's
lower-left corner).
 
Windows with the same parent window are called sibling windows. All
top-level windows are sibling windows since they share a common parent
window, the desktop window. Sibling windows can overlap; an application or a
user can arrange the windows so that some appear on top of others. Every
sibling window has a Z-order position that specifies where it lies in the
stack of overlapping windows. The parent window for the sibling windows is
always at the bottom of the stack.
 
A window is clipped to its parent window. This means that no part of a child
window is ever drawn outside of its parent window. If an application creates
a child window that is larger than the parent window or positions a child
window so that some or all of the window extends beyond the edges of the
parent window, the system automatically clips (does not draw) the portion of
the child window that extends beyond the edges. Depending on the window
styles for a window, a window may also be clipped to its child and its
sibling windows. When a window has the style WS_CLIPCHILDREN or
WS_CLIPSIBLINGS, the system clips the window.
 
A window is destroyed when its parent window is destroyed. When the parent
window is destroyed, the system sends WM_DESTROY messages to each child
window. This is convenient for composite windows (for example, the
application's main window) since an application needs to destroy only the
parent window; all the related windows, including the client window, are
destroyed automatically. The parent window is always the last window to be
destroyed. This allows the parent window to use any data saved or left
behind by its child windows.
 
While every window has only one parent window, a window can have any number
of child windows. Any child window can have child windows. Each child window
in this chain of windows is a called a descendant window of the original
parent window. Immediate child windows are child windows directly related to
the parent window, not just descendant windows.
 
An application can change a window's parent window at any time. Changing the
parent window changes where and how the child window is drawn.
 
Ownership
 
Any window can have an owner window. An owner is a window, not necessarily a
parent window, that controls some aspect of another window. Applications
typically use ownership to establish a connection between windows so that
together they can carry out useful tasks. For example, the title bar in an
application's main window is owned by the frame window. Together they let
the user move the entire main window by clicking the mouse in the title bar.
An application can set the owner window when it creates the window, or it
can set the owner window at a later time.
 
Ownership establishes a relationship between windows that is independent of
the parent-child relationship. Unlike parent and child windows, there are no
predefined rules for how the owner and owned windows interact. The window
procedures for the owner and owned windows must carry out any special
interactions specified.
 
The preregistered, public window classes provided by MS OS/2 recognize
ownership. Control windows, created with classes such as WC_TITLEBAR and
WC_SCROLLBAR, notify their owners of events; frame windows, created using
the WC_FRAME class, receive and process notification messages from the
control windows they own. For example, a title-bar control sends a
notification message to its owner when it receives a mouse click. If the
owner is a frame window, the frame window receives the notification message
and prepares to move the frame window and its child windows.
 
Owner and owned windows must be created by the same thread; that is, they
must belong to the same message queue. Since ownership is independent of the
parent-child relationship, the owner and owned windows do not have to be
descendants of the same parent window. This means one window can be a
descendant of the desktop window and the other a descendant of the
desktop-object window. This can affect how windows are destroyed. Destroying
the owner window does not necessarily destroy the owned window. An
application must explicitly destroy any owned window that is not a
descendant window of the owner.
 
Frame windows often have owned windows that are not descendants (they are
sibling windows instead). A frame window has the following special
properties:
 
♦  Destroys all owned windows, even if they are not descendants, when the
   frame window is destroyed.
 
♦  Moves owned windows when the frame window moves. The owned windows that
   are not descendants maintain their position relative to the upper-left
   (not the usual lower-left) corner of the owner window. Any owned window
   with the style FS_NOMOVEWITHOWNER does not move.
 
♦  Changes the Z-order position of all owned windows when the frame window
   changes.
 
♦  Hides all owned windows when the frame window is minimized or hidden.
   Owned windows hidden in this way are restored when the frame window is
   restored.
 
If an application needs the same special processing for its own window
classes, it must provide that support in the window procedures for those
classes.
 
Object Windows
 
Any descendant of the desktop-object window is called an object window. An
object window is like any other window but it is not displayed. Applications
typically use object windows to provide services for windows. For example,
an application might use an object window to manage a shared database. The
advantage of using an object window in this way is that a window can request
information from the database by sending a message to the object window and
receive a reply as a message.
 
Because object windows are not displayed, the window procedure for an object
window does not have to process input and paint messages. This means that an
application can use object windows just as it would other objects in
object-oriented environments. The object window processes messages that
affect the data belonging to the object.
 
The rules for parent-child relationship and ownership also apply to object
windows. In particular, changing the parent window of an object window to
the desktop window or to a descendant of the desktop window causes the
system to display the window if it is visible.
 
Visibility
 
A window can be visible or invisible. The system displays visible windows on
the screen. It hides invisible windows by not drawing them. If a window is
visible, the user can supply input to the window and view output. If a
window is invisible, the window is effectively disabled. An application sets
a window's visibility state when it creates the window. Later, a user or the
application can change these initial values.
 
A window is visible if the style WS_VISIBLE is set for the window. An
application can set this style when it creates the window. By default, the
WinCreateWindow function creates invisible windows unless the WS_VISIBLE
style is given. After a window is created, an application typically hides a
window to hide the details of operation from the user. For example, an
application may keep a new window invisible while it customizes the window's
appearance.
 
Even if a window is visible, the user may not be able to see the window on
the screen. Other windows may completely overlap the window or the window
may have been moved beyond the edge of the screen. The window is considered
visible but it cannot be seen.
 
A visible window is subject to the clipping rules established by its
parent-child relationship. If the window's parent window is not visible, the
window will not be visible. Since a child window is drawn relative to the
parent's lower-left corner, if the parent window is moved beyond the edge of
the screen, the child window will also move.
 
A user may move only part of the parent window containing the child window
off the edge of the screen, so although the window and its parent window are
visible, the user may not be able to see them. An application determines
whether the user can actually see a visible window by checking the window's
current position.
 
Size
 
Every window has a size (width and height) given in pels, in the range 0
through 65,535. A window can have zero width and/or height. A window with
zero width or height is not drawn on the screen even though it may be
visible.
 
Although an application can create very large windows, it should consider
the size of the screen when choosing a window size. One way to choose an
appropriate size is to use the WinGetMaxPosition function to retrieve the
size of the maximized window. A window that is larger than its maximized
size will also be larger than the screen.
 
An application can retrieve the current size of the window by using the
WinQueryWindowRect function.
 
Position
 
Every window has a position. The position is specified as the coordinates of
the window's lower-left corner. The coordinates, sometimes called window
coordinates, are always relative to the lower-left corner of the parent
window.
 
To improve drawing performance, a frame window may adjust its horizontal
position so that it is a multiple of 8, relative to the screen origin (the
lower-left corner of the screen). Coordinates that are multiples of 8
correspond to byte boundaries in the screen-memory bitmap. It is usually
faster to draw starting at a byte boundary. An application can override this
action by using the FCF_NOBYTEALIGN style when creating the window.
 
Size and Position Messages
 
A window receives messages when it changes size or position. Before a change
is actually made, the system may send a WM_ADJUSTWINDOWPOS message to allow
the window procedure to make final adjustments to the window's size and
position. This message includes an SWP structure that contains the width,
height, and position requested. If the window procedure adjusts these values
in the structure, the system uses the adjusted values to draw the new
window. The WM_ADJUSTWINDOWPOS message is not sent if the change is a result
of a call to the WinSetWindowPos function and the SWP_NOADJUST constant is
specified.
 
After a change has been made to a window, the system sends a WM_SIZE message
to specify the new size of the window. If the window has the class style
CS_MOVENOTIFY, the system also sends a WM_MOVE message. The WM_MOVE message
includes the new position for the window. The system sends a WM_SHOW message
if the visibility of the window has changed.
 
Z Order
 
Every window has a Z-order position. Imagine an axis extends outward from
the screen toward the viewer. A window at the top of the Z order is
displayed in front of its sibling windows when the windows overlap. A window
at the bottom of the Z order is displayed behind its sibling windows when
the windows overlap.
 
Maximized and Minimized Windows
 
A maximized window is a window that has been enlarged so it fills the
screen. Although a window's size can be set so it exactly fills the screen,
a maximized window is slightly different; the system automatically moves the
window's title bar to the top of the screen and sets the WS_MAXIMIZED style
for the window.
 
A minimized window is a window whose size has been reduced so that it is
exactly the size of an icon. Like a maximized window, a minimized window is
more than just a window of a given size. The system typically moves the
minimized window to the lower part of the screen and sets the WS_MINIMIZED
style for that window. The lower part of the screen is sometimes call the
icon area. The system moves a minimized window into the first available icon
position in the icon area if no other position is specified.
 
If a window is created with the styles WS_MAXIMIZED or WS_MINIMIZED, the
system draws the window as a maximized or minimized window.
 
An application can restore a maximized or minimized window to its previous
size and position.
 
Redrawing Windows
 
After the system moves or changes the size of a window, it may invalidate
all or part of the window. If at all possible, the system tries to preserve
the contents of the window and simply copy them to the new position. But if
a window's size has increased, the window must fill the area exposed by the
size change. If a window has moved from behind an overlapping window, any
area that was formerly obscured by the other window must be drawn. In these
cases, the system invalidates the exposed areas and the window receives a
WM_PAINT message.
 
An application can require that the system invalidate the entire window for
each move or size change by setting the CS_SIZEREDRAW class style in the
corresponding window class. This class style is typically used for
applications that use the window's current size and position to determine
how to draw the window. For example, a clock application may always draw the
face of the clock so that it exactly fills the window.
 
An application can also explicitly specify which parts of the window to
preserve during a move or size change. Before any change is made, the system
sends a WM_CALCVALIDRECTS message to windows that do not have the style
CS_SIZEREDRAW. This allows the window procedure to specify what part of the
window to save and where to align it after the move or size change.
 
System Commands
 
An application that has a window with a System menu can change the size and
position of that window by sending system commands. The system commands are
usually generated by the user choosing commands from the System menu. An
application can emulate the user action by sending a WM_SYSCOMMAND message
to the window.
 
Some of the system commands are listed here:
 
Command      Description
────────────────────────────────────────────────────────────────────────────
SC_SIZE      Starts a size command. The user can change the size of the
             window by using the mouse or keyboard.
 
SC_MOVE      Starts a move command. The user can move the window by using
             the mouse and keyboard.
 
SC_MINIMIZE  Minimizes the window.
 
SC_MAXIMIZE  Maximizes the window.
 
SC_RESTORE   Restores a minimized or maximized window to its previous size
             and position.
 
SC_CLOSE     Closes the window. This command sends a WM_CLOSE message to the
             window. The window carries out any steps needed to clean up and
             destroy itself.
 
 
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