Many people have admired the paged question selection facility in the
OpenDoors 5.00 EZVote example door. EZVote allows the user to choose
questions from a list of up to 200 questions, by displaying one page
of questions at a time. The user is able to page up or down in the list
of quesitons, select a question from the list, or return to the main
menu. A prompt at the bottom of the screen shows the current page
number, and a list of options that are currently available. For
instance, when displaying the first of two pages, this prompt indicates
that the user can move to the next page, but not the previous page.

This OpenDoors Tip shows a generic function that provides the same sorts
of capabilities that are seen in the EZVote example door, in a form that
can be re-used in many different programs. This function, named
PagedViewer(), can be used for displaying multi-paged messages, text
files, or for permitting selection from a potentially very long list of
items. To use the PagedViewer() in a program, you should add the
pageview.c file to your project file / makefile, and #include the
pageview.h file in any source file that calls PagedViewer().

The prototype for the PagedViewer() function is as follows:

          int PagedViewer(
             int nInitialLine,
             int nTotalLines,
             void (*pDisplayCallback)(int nLine, void *pCallbackData),
             void *pCallbackData,
             BOOL bAllowSelection,
             char *pszTitle,
             int nPageSize);

The nInitialLine parameter specifies the line to begin viewing at.
Normally this would be 0, but you may wish to pass a different value to
this function to force the viewer to begin on a page other than the
first. Using this parameter, you can have the user return to
the page they were previously viewing, rather than returning to the
first page and having to again find their original location.

The nTotalLines parameter specifies the total number of lines that can
be viewed, and can be any value greater than or equal to 0.

The pDisplayCallback parameter must be a pointer to a function that the
PagedViewer will call to display a particular line of the file at the
current location. When PagedViewer() calls your function, it will pass
the line number to be displayed, along with the value originally passed
to PagedViewer() in pCallbackData. The provided function should simply
display the text for the specified line number, without a trailing CR/LF
sequence, and then return.

The pCallbackData can point to any data that you wish PagedViewer() to
pass to your callback function, or may be NULL if you do not wish to use
this feature.

The bAllowSelection parameter should be TRUE if the user should be able
to make a selection from the list, and FALSE if they should not. If
bAllowSelection is TRUE, PagedViewer() will display a letter beside each
line, and allow the user to select a line by pressing the corresponding
letter. If you are using PagedViewer() to display a text file or
message, you will want to set bAllowSelection to FALSE. If you are using
PagedViewer() to display a list of items from which the user can select,
you will want to set bAllowSelection to TRUE.

The pszTitle parameter can point to a title to be displayed at the top
of each page, and could be something like ("Select a message"). If you
do not wish to have a title displayed, set this parameter to NULL.

The nPageSize parameter specifies the number of lines that should be
displayed on each page. If you do not wish to have the local screen
(with a two line status line) to be scrolled while displaying the list,
the maximum page size you should use is 21 if no title is being
displayed, and 19 if a title is being displayed.

The included fileview.c text file viewing program demonstrates the use
of PagedViewer(). The fileview.c door can be setup to allow the user to
view one or more files. If setup to view multiple files, the program
first displays a list of files that the user can select to view.
The fileview.c program uses PagedViewer() in two places - for providing
the list of files and for displaying the file itself. As such, the user
can page up or down in the list of files, select a file to view, and
then page up or down in the file. When the user selects quit while
viewing the file, they are returned to the list of files to possibly
select another file. When the user selects quit from the list of files,
the door returns control to the calling BBS software.