NAME

       VistaIOtype - registering Vista object types

DESCRIPTION

   Introduction
       An  object  such  as  an  image  or  edge  set  is  represented in a Vista data file by an
       attribute. That attribute's value includes a type name, an attribute  list,  and  possibly
       some binary data. Although the object is easily stored and communicated in that form, some
       other representation is often more useful when working  with  the  object  in  memory.  An
       image,  for  example,  is  best  represented in memory by a structure with fixed fields to
       record the properties of the image that are frequently accessed.

       The Vista library allows you to define a type that has its own internal representation for
       objects. By registering your type with the library you can arrange to have objects of that
       type automatically translated to and from your internal representation whenever data files
       are  read  and  written.   And  when you use attribute lists to organize your objects, the
       objects will be correctly copied and released by routines such  as  VistaIOCopyAttrList(3)
       and VistaIODestroyAttrList(3).

       Since  Vista  allows you to invent your own types of objects and to store those objects in
       data files, it must provide some way for standard programs to  gracefully  handle  objects
       that  are  unfamiliar to them. A program must at least be able to copy your custom objects
       intact, binary data and all, from input to  output.  Unfamiliar  objects  are  represented
       internally by the Vista library using the VistaIOBundle data structure, which is described
       separately in VistaIOBundle(3). You should be read it before reading this.

       This manual page explains how to register a type, what routines you must supply to support
       a type you are registering, and how to find out about a registered type.

   Registering a Type
       typedef struct {
              VistaIOCopyMethod *copy; /* copy object's value */
              VistaIODestroyMethod *destroy;/* destroy object's value */
              VistaIODecodeMethod *decode;/* decode object's binary data */
              VistaIOEncodeAttrMethod *encode_attr;/* encode object's attr list */
              VistaIOEncodeDataMethod *encode_data;/* encode object's binary data */
       } VistaIOTypeMethods;

       VistaIORepnKind VistaIORegisterType (VistaIOStringConst *name, VistaIOTypeMethods *methods)

       Before registering a custom object type you will need to establish

         • a  unique  name  identifying  the  type.  The  name  must match the regular expression
           [a-zA-Z0-9_.+-]+ (see grep(1)), and it must not be bit,  ubyte,  sbyte,  short,  long,
           float,  double, attr-list, boolean, bundle list, pointer, string, edges, image, or any
           type name already registered.

         • a data structure for representing instances of the type in memory.  A  single  pointer
           must  suffice  for  referring to an instance, and the memory for each instance must be
           dynamically allocated.

         • a set of routines for copying, destroying, encoding, and  decoding  instances  of  the
           type. These routine are called methods. They're described in more detail below.

         • any  other  routines  or  macros  your  programs will be using to access or manipulate
           instances of the type.

       The type is registered with a call to VistaIORegisterType prior to any  use  of  the  type
       within  a  program.  Pass  to  VistaIORegisterType  the  type's  name  and  a pointer to a
       VistaIOTypeMethods structure listing the type's methods.  VistaIORegisterType will  return
       a  unique code from the VistaIORepnKind series, which can subsequently be used to refer to
       the  type  whenever  a  VistaIORepnKind  value  is  called  for  (e.g.,  in  a   call   to
       VistaIOSetAttr(3)).

   Methods
       The type you register must be accompanied by these five methods:

       typedef VistaIOPointer VistaIOCopyMethod (VistaIOPointer value);

              The  Copy  method  is  passed  an  instance of your type; it returns a copy of that
              object made with newly-allocated storage.

       typedef void VistaIODestroyMethod (VistaIOPointer value);

              The Destroy method releases all storage occupied by the object value.

       typedef VistaIOPointer VistaIODecodeMethod (VistaIOStringConst name,
                 VistaIOBundle bundle);

            The  Decode  method creates an instance of your type. Data for the object is supplied
            via bundle in the form of an attribute list and an optional  block  of  binary  data,
            both of which may be modified by the method. An attribute name, name, is supplied for
            use in any error messages produced by decode (e.g.,  ``The  name  attribute  has  the
            wrong  amount of binary data''). The method returns the newly-created object, or NULL
            if an error is encountered.

            Storage for the new object can be freshly allocated, or it can be taken  from  bundle
            provided  bundle  itself  is  left  in  a state such that it can be destroyed without
            releasing storage used by the new object. For example, the new object can incorporate
            the  attribute  list  bundle->list  provided bundle->list is then set to a new, empty
            attribute  list.   It  can  incorporate  the   data   block   bundle->data   provided
            bundle->length is then set to zero.

       typedef VistaIOAttrList VistaIOEncodeAttrMethod (VistaIOPointer value, size_t *length);

       typedef VistaIOPointer VistaIOEncodeDataMethod (VistaIOPointer value,
                 VistaIOAttrList list, size_t length, VistaIOBoolean *free_it);

            These two methods produce an attribute list value and a block of binary data from  an
            instance of your type. For any particular object the two methods are always called in
            sequence. First the EncodeAttr method is called with value identifying the object. It
            returns the attribute list value while setting length to the number of bytes required
            for the block of binary data. Later, the  EncodeData  method  is  called  with  value
            identifying the same object, and list and length supplying the values returned by the
            EncodeAttr method. It returns a pointer to memory containing the block of binary data
            while  setting  free_it to TRUE if the memory should be released once the binary data
            has been recorded.  Setting free_it to FALSE indicates that the memory may be part of
            the  object,  value,  and  the  memory  contents should be recorded before taking any
            action that might modify the object.

            The attribute list returned by the EncodeAttr method can be  assumed  to  exist  only
            until  the  subsequent  call to the EncodeData method. If this list is created by the
            EncodeAttr method, then the EncodeData method is responsible  for  destroying  it.  A
            practice adopted for the image and edges types is for the EncodeAttr method to return
            a list that belongs to the object being encoded, but with  some  standard  attributes
            prepended  to it; the EncodeData method then removes the prepended attributes so that
            the object is left in the correct form.

            If either method encounters an error, it signals the error by  returning  NULL.  Note
            that  both methods are called even if the first indicates, by setting length to zero,
            that there is no binary data associated with value. When there is no binary  data  to
            return,  the  EncodeData  method  should return a non-NULL pointer and set free_it to
            FALSE to avoid signalling an error.

   Querying Registered Types
       The following routine and macro provide information about registered types:

       VistaIORepnKind VistaIOLookupType (VistaIOStringConst name)

              VistaIOLookupType returns the representation code associated with  the  type  named
              name. (This will be the same as that returned by VistaIORegisterObjectType when the
              type was registered.) If name is not the name of a  known  type,  VistaIOLookupType
              returns VistaIOUnknownRepn.

       VistaIOTypeMethods *VistaIORepnMethods (VistaIORepnKind repn)

              The  macro  VistaIORepnMethods  provides a handle to the methods for the registered
              type identified by repn.

   Standard Object Types
       The library implements some standard object types using the mechanism  described  by  this
       manual page. Currently, these standard object types are:

         edges     Set of edges. See VistaIOEdges(3).

         image     Multi-band two-dimensional array of pixels. See VistaIOImage(3).

       If  you  are  developing your own custom type, you're encouraged to first consult these as
       examples; their methods  are  implemented  in  the  files  /usr/src/vista/EdgesType.c  and
       /usr/src/vista/ImageType.c.

SEE ALSO

       VistaIOBundle(3), VistaIOattribute(3),

AUTHOR

       Art Pope <pope@cs.ubc.ca>

       Adaption to vistaio: Gert Wollny <gw.fossdev@gmail.com>