Keyboard Focus, Keyboard Navigation, and the Default Button


Keyboard focus
determines which window or control will receive information typed at the keyboard. Windowing systems show which control has focus by highlighting it. MS Windows uses various means to highlight the control that has keyboard focus. The 'FOCUS' mnemonic can be used to force the focus to a particular window or control.

Keyboard navigation allows the use of standard keys to change keyboard focus. Without keyboard navigation, it is almost always necessary to use the mouse to operate controls. (The exception is if hot keys are provided, or if keypresses are detected and acted upon. In effect, personalized keyboard navigation is being created when this is done.)

If keyboard navigation is enabled, and a window contains any buttons, there is always a default button. The default button is identifiable by a black rectangle drawn around the outside. Normally the default button is the button with the lowest ID number, but if another button has focus, that button temporarily becomes the default button and shows the black rectangle. (If a button is inside a group box, the black rectangle will not be visible. However, the user can still tell which button is the default because it will also have keyboard focus.)

If the keyboard navigation flag was specified when a window or child window was created, key pressed ("t") events for arrow keys and the tab key will not be reported.

Standard keyboard navigation proceeds as follows:

Key

Action

[Tab]

Move to the next focusable control. Controls that are disabled or invisible cannot receive keyboard focus, nor can controls that do absolutely nothing, like static text or group boxes. Also, grouped controls are treated as a set.

[BackTab]

Move to the previous focusable control.

[Enter]

Operate default button (often OK).

[Space]

Operate the control that has focus.

Arrow Keys

Move among grouped controls.

[Esc]

Operate button with ID=2, if present (usually Cancel).

Standard Button IDs

If ID 1 id used for OK buttons and ID 2 for Cancel buttons, they will behave in standard ways when keyboard navigation is enabled. Pressing [Enter] will choose OK, provided that no other button has focus. Initially the OK button will have focus since it has the lowest ID. Pressing [Esc] will choose Cancel.

Note that the [Enter] key will always operate the default button, regardless of its ID, (provided that keyboard navigation is enabled), but the [Escape] key will only work if a button with ID 2 exists.

The title of the button with ID 1 does not have to read "OK", nor does the one with ID 2 have to read "Cancel", nor does the code have to behave in any particular way when these buttons are pressed. The [Enter] key simply operates the default button, and [Esc] operates the button with ID 2, if there is one. ID 1 is suggested for the default button if it is used as an OK type operation, but this is not the only way it can be used.

Hot Keys

Whether or not keyboard navigation is enabled, it is possible to assign hot keys to push buttons, check boxes, and radio buttons and menus. To do this, specify an ampersand, "&", before the letter or symbol in the title text which is to be the hot key. It is possible to have a hot key that is not in the title text. To do this, create the control with one title, then change titles. Only the initial title determines the hot key used for a control. Ampersands in subsequent titles merely appear in the displayed title.

Pressing the hot key character when the window has keyboard focus will operate the control. It will not work, however, if some other control in the window has keyboard focus. This is necessary, since some controls accept text. Under MS Windows, holding down the [Alt] key while pressing the hot key will work any time the window has focus, even if another control currently has focus.

Top-level menu mnemonics can work the same way. For example, to access the File menu, [Alt]+[F] could be used. If this is the case, avoid assigning F as the hot key for a control. It may operate both the menu and the control (but the behavior is not guaranteed).



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