Active Application, Active Window, Input Focus Definition

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows, versions 3.0 and 3.1

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.

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.

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.

   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.

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).