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                                                    686
          CHAPTER 8                                                              Interactive Features




                             TABLE 8.75 Field flags specific to button fields
BIT POSITION    NAME               MEANING

15              NoToggleToOff      (Radio buttons only) If set, exactly one radio button must be selected at all
                                   times; clicking the currently selected button has no effect. If clear, clicking
                                   the selected button deselects it, leaving no button selected.

16              Radio              If set, the field is a set of radio buttons; if clear, the field is a check box.
                                   This flag is meaningful only if the Pushbutton flag is clear.

17              Pushbutton         If set, the field is a pushbutton that does not retain a permanent value.

26              RadiosInUnison     (PDF 1.5) If set, a group of radio buttons within a radio button field that
                                   use the same value for the on state will turn on and off in unison; that is if
                                   one is checked, they are all checked. If clear, the buttons are mutually ex-
                                   clusive (the same behavior as HTML radio buttons).


          Pushbuttons

          The simplest type of field is a pushbutton field, which has a field type of Btn and
          the Pushbutton flag (see Table 8.75) set. Because this type of button retains no
          permanent value, it does not use the V and DV entries in the field dictionary (see
          Table 8.69 on page 675).

          Check Boxes

          A check box field represents one or more check boxes that toggle between two
          states, on and off, when manipulated by the user with the mouse or keyboard. Its
          field type is Btn and its Pushbutton and Radio flags (see Table 8.75) are both clear.
          Each state can have a separate appearance, which is defined by an appearance
          stream in the appearance dictionary of the field’s widget annotation (see Section
          8.4.4, “Appearance Streams”). The appearance for the off state is optional but, if
          present, must be stored in the appearance dictionary under the name Off. The
          recommended (but not required) name for the on state is Yes.

          The V entry in the field dictionary (see Table 8.69 on page 675) holds a name ob-
          ject representing the check box’s appearance state, which is used to select the ap-
          propriate appearance from the appearance dictionary.

          Example 8.16 shows a typical check box definition.

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