three-state

Arguments: check-box

Determines whether :gray is a third meaningful value for the argument check-box, which should be a check-box instance.

The default value is nil, in which case the check-box will be blank when its value is nil and otherwise will show a check mark. Clicking on the check-box will toggle its value between nil and t (unchecked and checked). Other equivalent action such as selecting and pressing the Space bar will also toggle between checked and unchecked.

If this method returns a non-nil value other than the value :except-interactively, then the check-box will show a third grayed-out state when its value is :gray. This intermediate state is typically used either to indicate that a true-or-false value is unknown or undetermined, or that some but not all of a set of subchoices are selected. Clicking on the check-box will cycle its value between nil, t, and :gray (in that order).

If the value returned by the method is the keyword :except-interactively, then the check-box's value may be set to :gray programmatically by calling (setf value), but interactive gestures (clicking the check-box or pressing the spacebar when it has the keyboard focus) will only toggle the value between t and nil. If the value is :gray, clicking will change it to t (checked) and then toggle between t and nil. (This value may be useful when you want the gray state before any user action on the check box at all, but once the user acts, the only choices are checked and unchecked.)

This property cannot be changed on an existing check-box (except in the inspector for a check-box on a form designing a dialog). The only way to specify the value programmatically is to pass the :three-state argument when calling make-instance to create the check-box.

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This page was not revised from the 10.0 page.
Created 2019.8.20.