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                                                652
      CHAPTER 8                                                           Interactive Features



      • A notion of location—that is, an indication of where on the screen the device is
        pointing. Location is typically denoted by a screen cursor.
      • A notion of focus—that is, which element in the document is currently interact-
        ing with the user. In many systems, this element is denoted by a blinking caret,
        a focus rectangle, or a color change.

      PDF viewer applications must ensure the presence of such a device for the corre-
      sponding actions to be executed correctly. Mouse-related trigger events are sub-
      ject to the following constraints:

      • An E (enter) event can occur only when the mouse button is up.
      • An X (exit) event cannot occur without a preceding E event.
      • A U (up) event cannot occur without a preceding E and D event.
      • In the case of overlapping or nested annotations, entering a second annotation’s
        active area causes an X event to occur for the first annotation.

      Note: The field-related trigger events K (keystroke), F (format), V (validate), and C
      (calculate) are not defined for button fields (see “Button Fields” on page 685). The
      effects of an action triggered by one of these events are limited only by the action it-
      self and can occur outside the described scope of the event. For example, even
      though the F event is used to trigger actions that format field values prior to display,
      it is possible for an action triggered by this event to perform a calculation or make
      any other modification to the document.
      These field-related trigger events can occur either through user interaction or pro-
      grammatically, such as in response to the NeedAppearances entry in the interactive
      form dictionary (see Section 8.6.1, “Interactive Form Dictionary”), importation of
      FDF data (Section 8.6.6, “Forms Data Format”), or JavaScript actions (“JavaScript
      Actions” on page 709). For example, the user’s modifying a field value can trigger a
      cascade of calculations and further formatting and validation for other fields in the
      document.

8.5.3 Action Types

      PDF supports the standard action types listed in Table 8.48. The following sec-
      tions describe each of these types in detail. Plug-in extensions may add new
      action types.

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