ID Number: Q35957
3.00
WINDOWS
Summary:
Although the concepts of "input focus," "active application," and
"active window" are very closely related, there are differences among
them.
1. The "input focus" determines which window receives keyboard input.
2. The "active window" is the window that receives the user's
attention. If it's an overlapped window or a pop-up window with a
caption bar, the caption bar is highlighted. If it's a dialog
window, the frame is highlighted. Either the active window or one
of its child windows has the focus.
3. The "active application" is the application that created the window
that has the input focus.
More Information:
The following discussion contains detailed definitions of the terms
"input focus," "active window," and "active application," as well as a
demonstration of those differences that you can perform using Windows
Write.
The following definition is from the glossary of "Programmer's Guide
to Windows, Second Edition," by David Durant, Geta Carlson, and Paul
Yao, published by Sybex, page 647:
Active Application
The active application is the application that created the window
that currently has the keyboard input focus. Applications do not
need to be the active application in order to receive and process
messages. Applications are notified by message whenever they are
gaining or losing the status of "the active application." The user
normally determines the active application, but applications can
override this decision.
For more information on the active application, refer to the
documentation for the WM_ACTIVATEAPP message in the "Microsoft Windows
Software Development Kit (SDK) Reference, Volume 1."
The following is also from "Programmer's Guide to Windows, Second
Edition," page 655:
Focus
Keyboard input is transferred to the application as messages. Since
several windows may be visible on the screen simultaneously, there
must be a method for determining which of these windows should
receive the keyboard input messages. In Windows, the window that
has the focus is the window that will receive the keyboard
messages. Normally, the user controls which window has the focus by
use of the mouse, but applications can transfer the focus from
window to window themselves.
For more information on focus, refer to the documentation for the
WM_SETFOCUS and WM_KILLFOCUS messages and for the GetFocus, SetFocus,
and EnableWindow functions in the "SDK Reference, Volume 1."
The following is from the book Programming Windows, Second Edition by
Charles Petzold, published by Microsoft Press, pages 89-90:
Focus, Focus, Who's Got the Focus?
The keyboard must be shared by all applications running under
Windows. Some applications may have more than one window, and the
keyboard must be shared by these windows within the same
application. When a key on the keyboard is pressed, only one window
procedure can receive a message that the key has been pressed. The
window that receives this keyboard message is the window with the
"input focus."
The concept of input focus is closely related to the concept of
"active window." The window with the input focus is either the
active window or a child window of the active window. The active
window is usually easy to identify. If the active window has a
caption bar, Windows highlights the caption bar. If the active
window has a dialog frame (a form most commonly seen in dialog
boxes) instead of a caption bar, Windows highlights the frame. If
the active window is an icon, Windows highlights the window's
caption bar text below the icon.
The most common child windows are controls such as push buttons,
radio buttons, check boxes, scroll bars, edit boxes, and list boxes
that usually appear in a dialog box. Child windows are never
themselves active windows. If a child window has the input focus,
then the active window is its parent. Child window controls
indicate that they have the input focus generally by using a
flashing cursor or caret.
If the active window is an icon, then no window has the input
focus. Windows continues to send keyboard messages to the icon, but
these messages are in a different form from keyboard messages sent
to active windows that are not icons.
A window procedure can determine when it has the input focus by
trapping WM_SETFOCUS and WM_KILLFOCUS messages. WM_SETFOCUS
indicates that the window is receiving the input focus, and
WM_KILLFOCUS signals that the window is losing the input focus.
Petzold also indicates that Windows sends keyboard messages to icons
differently than it does to windows with the focus. This difference is
noted in the following excerpt from the "Microsoft Windows Software
Development Kit Reference, Volume 1," page 4-381:
If a window is active but doesn't have the focus (that is, no
window has the focus), any key pressed will produce the WM_SYSCHAR,
WM_SYSKEYDOWN, or WM_SYSKEYUP message.
For more information on the active window, refer to the documentation
for the WM_ACTIVATE message and for the GetActiveWindow and
SetActiveWindow functions in the "Microsoft Windows Software
Development Kit Reference, Volume 1."
Demonstration of Differences Between
Focus, Active Application, and Active Window
--------------------------------------------
The following is an experiment you can do that will clarify the
differences among active application, active window, and input focus,
and show you how the user can specify which attribute is assigned to
which window:
Open Windows Write and choose Find from the Find menu. You'll now have
a pop-up window that is a child of Write. Arrange your screen so that
the Write, Find, and Program Manager windows are all on the screen
together.
If you click in the Program Manager window, both the Write window's
and the Find dialog window's caption bars will be set to the inactive
color, and the Program Manager window's caption bar will be set to the
active color. At this point, Program Manager is the active
application; it is also the active window and has the input focus.
Click in the Write window; it will become the active application, as
well as the active window, and will get the input focus.
Click in the Find window; although it is now the active window, it is
not the active application (Write is) nor does it have the input
focus: the focus is in the Find What box, which is an edit control
window that is a child of the Find dialog window, which is itself a
pop-up-style window that is a child of Write. (Note that in this
discussion the term "child windows" is used in terms of parent-child
relationships rather than the child window style WS_CHILD.)
At this point, you can move the input focus from box to box within the
Find dialog window by using the TAB key (or by clicking with the
mouse). There are three or four controls you can move between: Find
What, Whole Word, Match Upper/Lowercase, and Find Next (available only
if there is some text in the Find What box).
Now press ALT+F6. This will make Write the active window instead of
Find, and the focus will be set to the Write window as well; this is
indicated by the flashing vertical caret. Press ALT+F6 a second time.
Find will be the active window again, and the input focus will be set
to whichever control it was last set to (Find What, Whole Word, Match
Upper/Lowercase, or Find Next).
Finally, you can use ALT+TAB to return to the Program Manager, making
it the active application, the active window, and the one with the
input focus. If you press ALT+TAB again, you will make Write the
active application, Find the active window, and set the input focus to
one of Find's controls (whichever one last had the focus).
Additional reference words: 3.00 3.0 SR# G880919-3424