Microsoft KB Archive/35957
INFO: Active Application, Active Window, Input Focus Definition
The information in this article applies to:
- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1
Although the concepts of "input focus," "active application," and "active window" are very closely related, there are differences among them.
- The "input focus" determines which window receives keyboard input.
- 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.
- The "active application" is the application that created the window that has the input focus.
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:
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:
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 query words:
Keywords : kb16bitonly kbSDKPlatform kbGrpDSUser kbWndw
Issue type : kbinfo
Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Last Reviewed: November 12, 1999