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docs for ewoc.el, rev 4

From: Thien-Thi Nguyen
Subject: docs for ewoc.el, rev 4
Date: Mon, 22 May 2006 11:27:12 +0200

lots of text clean-up (thanks rms).

changes: added `ewoc-set-data' (also in ewoc.el) and `ewoc-delete' (not
yet in ewoc.el but will be RSN); example code slightly bummed (thanks
address@hidden); new @cindex in example.

one remaining doubt: "much the way" is ok?


@node Abstract Display
@section Abstract Display
@cindex ewoc
@cindex display, abstract
@cindex display, arbitrary objects
@cindex model/view/controller
@cindex view part, model/view/controller

  The Ewoc package constructs buffer text that represents a structure
of Lisp objects, and updates the text to follow changes in that
structure.  This is like the ``view'' component in the the
``model/view/controller'' design paradigm.

  An @dfn{ewoc} is a structure that organizes information required to
construct buffer text that represents certain Lisp data.  The buffer
text of the ewoc has three parts, in order: first, fixed @dfn{header}
text; next, textual descriptions of a series of data elements (Lisp
objects that you specify); and last, fixed @dfn{footer} text.
Specifically, an ewoc contains information on:

@itemize @bullet
The buffer which its text is generated in.

The text's start position in the buffer.

The header and footer strings.

A doubly-linked chain of @dfn{nodes}, each of which contains:

A @dfn{data element}, a single Lisp object.

Links to the preceding and following nodes in the chain.
@end itemize

A @dfn{pretty-printer} function which is responsible for
inserting the textual representation of a data
element value into the current buffer.
@end itemize

  Typically, you define an ewoc with @code{ewoc-create}, and then pass
the resulting ewoc structure to other functions in the Ewoc package to
build nodes within it, and display it in the buffer.  Once it is
displayed in the buffer, other functions determine the correspondance
between buffer positions and nodes, move point from one node's textual
representation to another, and so forth.  @xref{Abstract Display

  A node @dfn{encapsulates} a data element much the way a variable
holds a value.  Normally, encapsulation occurs as a part of adding a
node to the ewoc.  You can use @code{ewoc-data} to both retrieve the
data element and to place a new element in its place, like so:

(ewoc-data @var{node})
@result{} value

(ewoc-set-data @var{node} @var{new-data})
@result{} @var{new-data}
@end lisp

You can also use, as the data element value, a Lisp object (list or
vector) that is a container for the ``real'' value, or an index into
some other structure.  The example (@pxref{Abstract Display Example})
uses the latter approach.

  When the data changes, you will want to update the text in the
buffer.  You can update all nodes by calling @code{ewoc-refresh}, or
just specific nodes using @code{ewoc-invalidate}, or all nodes
satisfying a predicate using @code{ewoc-map}.  Alternatively, you can
delete invalid nodes using @code{ewoc-delete} or @code{ewoc-filter},
and add new nodes in their place.

* Abstract Display Functions::
* Abstract Display Example::
@end menu

@node Abstract Display Functions
@subsection Abstract Display Functions

  In this subsection, @var{ewoc} and @var{node} stand for the
structures described above (@pxref{Abstract Display}), while
@var{data} stands for an arbitrary Lisp object used as a data element.

@defun ewoc-create pretty-printer &optional header footer
This constructs and returns a new ewoc, with no nodes (and thus no data
elements).  @var{pretty-printer} should be a function that takes one
argument, a data element of the sort you plan to use in this ewoc, and
inserts its textual description at point using @code{insert} (and never
@code{insert-before-markers}, because that would interfere with the
Ewoc package's internal mechanisms).

An ewoc maintains its text in the buffer that is current when
you create it, so switch to the intended buffer before calling
@end defun

@defun ewoc-buffer ewoc
This returns the buffer where @var{ewoc} maintains its text.
@end defun

@defun ewoc-get-hf ewoc
This returns a cons cell @code{(@var{header} . @var{footer})}
made from @var{ewoc}'s header and footer.
@end defun

@defun ewoc-set-hf ewoc header footer
This sets the header and footer of @var{ewoc} to the strings
@var{header} and @var{footer}, respectively.
@end defun

@defun ewoc-enter-first ewoc data
@defunx ewoc-enter-last ewoc data
These add a new node encapsulating @var{data}, putting it, respectively,
at the beginning or end of @var{ewoc}'s chain of nodes.
@end defun

@defun ewoc-enter-before ewoc node data
@defunx ewoc-enter-after ewoc node data
These add a new node encapsulating @var{data}, adding it to
@var{ewoc} before or after @var{node}, respectively.
@end defun

@defun ewoc-prev ewoc node
@defunx ewoc-next ewoc node
These return, respectively, the previous node and the next node of @var{node}
in @var{ewoc}.
@end defun

@defun ewoc-nth ewoc n
This returns the node in @var{ewoc} found at zero-based index @var{n}.
A negative @var{n} means count from the end.  @code{ewoc-nth} returns
@code{nil} if @var{n} is out of range.
@end defun

@defun ewoc-data node
This extracts the data encapsulated by @var{node} and returns it.
@end defun

@defun ewoc-set-data node data
This sets the data encapsulated by @var{node} to @var{data}.
@end defun

@defun ewoc-locate ewoc &optional pos guess
This determines the node in @var{ewoc} which contains point (or
@var{pos} if specified), and returns that node.  If @var{ewoc} has no
nodes, it returns @code{nil}.  If @var{pos} is before the first node,
it returns the first node; if @var{pos} is after the last node, it returns
the last node.  The optional third arg @var{guess}
should be a node that is likely to be near @var{pos}; this doesn't
alter the result, but makes the function run faster.
@end defun

@defun ewoc-location node
This returns the start position of @var{node}.
@end defun

@defun ewoc-goto-prev ewoc arg
@defunx ewoc-goto-next ewoc arg
These move point to the previous or next, respectively, @var{arg}th node
in @var{ewoc}.  @code{ewoc-goto-prev} does not move if it is already at
the first node or if @var{ewoc} is empty, whereas @code{ewoc-goto-next}
moves past the last node, returning @code{nil}.  Excepting this special
case, these functions return the node moved to.
@end defun

@defun ewoc-goto-node ewoc node
This moves point to the start of @var{node} in @var{ewoc}.
@end defun

@defun ewoc-refresh ewoc
This function regenerates the text of @var{ewoc}.  It works by
deleting the text between the header and the footer, i.e., all the
data elements' representations, and then calling the pretty-printer
function for each node, one by one, in order.
@end defun

@defun ewoc-invalidate ewoc &rest nodes
This is similar to @code{ewoc-refresh}, except that only @var{nodes} in
@var{ewoc} are updated instead of the entire set.
@end defun

@defun ewoc-delete ewoc &rest nodes
This deletes each node in @var{nodes} from @var{ewoc}.
@end defun

@defun ewoc-filter ewoc predicate &rest args
This calls @var{predicate} for each data element in @var{ewoc} and
removes those nodes for which @var{predicate} returns @code{nil}.  Any
@var{args} are passed to @var{predicate}.
@end defun

@defun ewoc-collect ewoc predicate &rest args
This calls @var{predicate} for each data element in @var{ewoc}
and returns a list of those elements for which @var{predicate}
returns address@hidden  The elements in the list are ordered
as in the buffer.  Any @var{args} are passed to @var{predicate}.
@end defun

@defun ewoc-map map-function ewoc &rest args
This calls @var{map-function} for each data element in @var{ewoc} and
updates those nodes for which @var{map-function} returns address@hidden
Any @var{args} are passed to @var{map-function}.
@end defun

@node Abstract Display Example
@subsection Abstract Display Example

  Here is a simple example using functions of the ewoc package to
implement a ``color components display'', an area in a buffer that
represents a vector of three integers (itself representing a 24-bit RGB
value) in various ways.

(setq colorcomp-ewoc nil
      colorcomp-data nil
      colorcomp-mode-map nil
      colorcomp-labels ["Red" "Green" "Blue"])

(defun colorcomp-pp (data)
  (if data
      (let ((comp (aref colorcomp-data data)))
        (insert (aref colorcomp-labels data) "\t: #x"
                (format "%02X" comp) " "
                (make-string (ash comp -2) ?#) "\n"))
    (let ((cstr (format "#%02X%02X%02X"
                        (aref colorcomp-data 0)
                        (aref colorcomp-data 1)
                        (aref colorcomp-data 2)))
          (samp " (sample text) "))
      (insert "Color\t: "
              (propertize samp 'face `(foreground-color . ,cstr))
              (propertize samp 'face `(background-color . ,cstr))

(defun colorcomp (color)
  "Allow fiddling with COLOR in a new buffer.
The buffer is in Color Components mode."
  (interactive "sColor (name or #RGB or #RRGGBB): ")
  (when (string= "" color)
    (setq color "green"))
  (unless (color-values color)
    (error "No such color: %S" color))
   (generate-new-buffer (format "originally: %s" color)))
  (setq major-mode 'colorcomp-mode
        mode-name "Color Components")
  (use-local-map colorcomp-mode-map)
  (let ((data (apply 'vector (mapcar (lambda (n) (ash n -8))
                                     (color-values color))))
        (ewoc (ewoc-create 'colorcomp-pp
                           "\nColor Components\n\n"
    (set (make-local-variable 'colorcomp-data) data)
    (set (make-local-variable 'colorcomp-ewoc) ewoc)
    (ewoc-enter-last ewoc 0)
    (ewoc-enter-last ewoc 1)
    (ewoc-enter-last ewoc 2)
    (ewoc-enter-last ewoc nil)))
@end example

@cindex controller part, model/view/controller
  This example can be extended to be a ``color selection widget'' (in
other words, the controller part of the ``model/view/controller''
design paradigm) by defining commands to modify @code{colorcomp-data}
and to ``finish'' the selection process, and a keymap to tie it all
together conveniently.

(defun colorcomp-mod (index limit delta)
  (let ((cur (aref colorcomp-data index)))
    (unless (= limit cur)
      (aset colorcomp-data index (+ cur delta)))
     (ewoc-nth colorcomp-ewoc index)
     (ewoc-nth colorcomp-ewoc -1))))

(defun colorcomp-R-more () (interactive) (colorcomp-mod 0 255 1))
(defun colorcomp-G-more () (interactive) (colorcomp-mod 1 255 1))
(defun colorcomp-B-more () (interactive) (colorcomp-mod 2 255 1))
(defun colorcomp-R-less () (interactive) (colorcomp-mod 0 0 -1))
(defun colorcomp-G-less () (interactive) (colorcomp-mod 1 0 -1))
(defun colorcomp-B-less () (interactive) (colorcomp-mod 2 0 -1))

(defun colorcomp-copy-as-kill-and-exit ()
  "Copy the color components into the kill ring and kill the buffer.
The string is formatted #RRGGBB (hash followed by six hex digits)."
  (kill-new (format "#%02X%02X%02X"
                    (aref colorcomp-data 0)
                    (aref colorcomp-data 1)
                    (aref colorcomp-data 2)))
  (kill-buffer nil))

(setq colorcomp-mode-map
      (let ((m (make-sparse-keymap)))
        (suppress-keymap m)
        (define-key m "i" 'colorcomp-R-less)
        (define-key m "o" 'colorcomp-R-more)
        (define-key m "k" 'colorcomp-G-less)
        (define-key m "l" 'colorcomp-G-more)
        (define-key m "," 'colorcomp-B-less)
        (define-key m "." 'colorcomp-B-more)
        (define-key m " " 'colorcomp-copy-as-kill-and-exit)
@end example

Note that we never modify the data in each node, which is fixed when the
ewoc is created to be either @code{nil} or an index into the vector
@code{colorcomp-data}, the actual color components.

@c (end of text)

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