MacFormat 1995 May/macformat-024.iso/Shareware City/HyperCard/HyperCard Instance Variables/Instance Variable text

Text File | 1993-04-01 | 11.4 KB | 272 lines | [TEXT/GEOL]

Instance Variables for HyperCard

by Ted Kaehler, Apple Computer, [e-mail]

Allows you to have variables attached to a button, shared field, or other HyperCard object.  Each variable can have any value that any variable in a script can have.  Names of variables may contain spaces, commas, or other special characters (enclose the variable name in double-quotes in your script).

Instance variables are useful for storing more data with a button or field.  A puzzle piece button wants to carry its home location with it.  A phone dialing button needs a place to put the local area code, without it being a constant in the middle of its script.  A background may need to carry some data useful to its scripts.

An object may have any number of variables.  Values are preserved when you quit.  They stay with your stack when you drag it to another disk.  The objects that can have variables are: a card button, a card field, the card itself, a bkgnd button, a shared field, or the background itself.  Instance variables for background fields are not supported!  This is because a background field is partly shared and partly local to the card.  When you have the urge to put a variable into a background field, attach it instead to the either the card or the current background.

How to call:

Call  allowInstVars  once in each new background to initialize instance variables.  It creates two background fields to keep the values in.   To store into a variable, say
  putVar cdBk, myID, variable, value, cardID
For example, to store “East” into the variable “Direction” in the button “Engine”, say
  putVar "cd", the id of card btn “Engine”, "Direction", “East”

Use the function getVar to retrieve the value of a variable:
  getVar (cdBk, myID, variable, cardID)
In this case is looks like:
  put getVar("cd",the id of card btn “Engine”, "Direction") into dir

The last argument, cardID is optional.  Leaving it out, as we did above, means the button on the current card.  See the script for more detail on the arguments.


The Fine Print

The total size of all names and values or a single card cannot exceed 32K bytes.  The characters { and } may not appear in any value or variable name name.  Use with HyperCard 2.1 or later.


The Script

This belongs in the stack script of your stack. (Just putting it in the Home script is not the best.  If you give your stack to someone, instance variables will stop working.)

-- Allow Instance Variables for buttons, fields, cards, and backgrounds.
-- (c) 1991 copyright Apple Computer, all rights reserved.
-- Version 1.0 for HyperCard 2.1

-- This belongs in the stack script of your stack.
-- (Just putting it in the Home script is not the best.  If you then give
--  your stack to someone, instance variables will stop working.)

-- You call these: putVar, getVar, allowInstVars
-- Bonus handlers: removeVar, hasThisVar, allVarsOf
-- Internal use only: instVar, createBkgndFld

on putVar cdBk,myID,variable,value,cardID
  -- Store the value of this variable in this object
  -- Example:  putVar cd,the id of me,fred,45
  --    a missing cardID means on the current card
  -- cdBk is cd or bk
  --    (where is this object? cd is card, bk is background)
  -- myID is: the short id of me
  -- variable is the name under which you want to store this value
  -- value is what you want to store
  --   value may not contain { or } characters
  -- cardID (optional) the id of the card that contains the object

  put instVar(cdBk,myID,variable,value,cardID) into dummy
end putVar


function getVar cdBk,myID,variable,cardID
  -- Retrieve the value of this variable in this object
  -- Example:  getVar cd,the id of me,fred
  --    a missing cardID means on the current card
  -- cdBk is cd or bk
  --    (where is this object? cd is card, bk is background)
  -- myID is: the short id of me
  -- variable is the name under which you want to store this value.
  --   variable may not contain { or } characters
  --   it may contain spaces or other characters
  -- cardID (optional) the id of the card that contains the object

  return instVar(cdBk,myID,variable,"{}",cardID)  --retrieval
end getVar


function instVar cdBk,myID,variable,value,cardID
  -- Get or put the value of a variable that belongs to a
  -- card button, card field, the card itself, a bkgnd button,
  -- shared field, or the bkgnd itself.

  -- User should not call this. Called from getVar, putVar and removeVar.

  -- Instance variables for bkgnd fields are not supported!  This is
  -- because it a background field is part shared and part local to
  -- the card.  Attach the variable to the card or to a shared
  -- field instead.

  -- Works for both storing and retireving, for both objects on a
  -- card and object shared across a background.

  -- If value is "{}" then we are retrieving a value from the
  -- dictionary.

  -- Note that myID is not enough information to actually find the
  -- button or field (we don't know which it is).  But we don't
  -- care.  We don't have to get at the button or field itself,
  -- just find it's variables in the dictionary!

  -- The search keys
  put "{" & variable & "{" & myID & "{" into startKey    --{Fred{21{
  put "}" & variable & "}" & myID & "}" into endKey      --}Fred}21}

  if cardID is empty
  then put the short id of this card into holder  -- on current card
  else put cardID into holder   --on a specific card

  if cdBk = "bk"
  then put "call allowInstVars first" into theFld  --background objects
  else put "you must call allowInstVars" into theFld  --card specific
  --  The names of these fields are funny so that the user will know
  --  what to do when he gets and error because the fields are not there.

  -- If the field is missing, you must call allowInstVars (just once in
  -- each background) to initialize some hidden fields.
  put offset(startKey,bkgnd fld theFld of cd id holder) ¬
  into startLoc    --loc of {Fred{21{

  if startLoc is 0 then
    --the key wasn't found
    if (cdBk is not "cd") and (cdBk is not "bk")
    then answer "First arg of putVar must be bk or cd" with "OK"

    if value = "{}" then   --getting and it was not found
      put cdBK && "id" && myID && variable && "on card id" && holder
      answer "You must putVar into a variable" & return &¬
      "before using getVar to read it."
      return "no value in:" && variable
    else
      --putting a value, create a new variable
      put return & startKey & value & endKey after bkgnd fld ¬
      theFld of cd id holder
      return value
    end if
  end if

  put offset(endKey,bkgnd fld theFld of cd id holder) ¬
  into endLoc    --loc of }Fred}21}

  add length(startKey) to startLoc   --loc after key, start of value
  subtract 1 from endLoc
  if value = "{}" then   --getting a value
    -- Retrieve the value of the variable
    return char startLoc to endLoc of bkgnd fld theFld of cd id holder
  else
    if value = "re{}move" then
      --remove the variable to save space
      subtract length(startKey) from startLoc   --loc before key
      add length(startKey) to endLoc   --loc after end key
      put empty into char startLoc to endLoc of bkgnd fld ¬
      theFld of cd id holder  -- remove keys and value
    else
      -- Store the value in the dictionary over old value
      put value into char startLoc to endLoc of bkgnd fld ¬
      theFld of cd id holder
    end if
    return value
  end if
end instVar

on allowInstVars
  -- Create a field for the instance variable dictionary.
  -- It has a funny name so the user will know what to do
  -- when he gets and error because the field is not there.
  createBkgndFld "you must call allowInstVars"  --card variables
  createBkgndFld "call allowInstVars first"  --bkgnd variables
  set sharedText of bkgnd fld "call allowInstVars first" to true
  set loc of bkgnd fld "call allowInstVars first" to 160,250
end allowInstVars

on createBkgndFld  theName
  -- Create hidden background field if not there already.
  -- Use this field for storing the dictionary of names and values.
  repeat with ii = 1 to the number of bkgnd fields
    put the short name of bkgnd field ii into myName
    if myName is theName
    then
      put "already there!"
      exit createBkgndFld  --already exists
    end if
  end repeat

  -- Create a background field for the instance variable dictionary.
  doMenu "background"
  doMenu "New Field"
  put the number of bkgnd fields into num
  set the name of field num to theName
  hide field theName   -- important!
  choose browse tool
  doMenu "Background"    --get out of Background mode
  put "new is " & num  --debugging
end createBkgndFld


-- Bonus handlers: removeVar, hasThisVar, allVarsOf

on removeVar cdBk,myID,variable,cardID
  -- Remove this variable in this object
  -- Is not necessary.  Only do it to save space when destroying
  -- a button or other object.
  -- Example:  removeVar cd,the id of me,fred
  --    a missing cardID means on the current card
  -- see getVar for the meaning of the arguments

  put instVar(cdBk,myID,variable,"re{}move",cardID) into data
  -- this specal value means remove the variable from the dictionary
end removeVar

function hasThisVar cdBk,myID,variable,cardID
  -- Return true if this object has a variable of this name
  -- So a script can ask if the object has one before trying to read it.
  -- Example:  hasThisVar(cd,the id of me,"fred")
  --    a missing cardID means on the current card
  -- cdBk is cd or bk
  --    (where is this object? cd is card, bk is background)
  -- myID is: the short id of me
  -- variable is the name under which you want to store this value
  -- cardID (optional) the id of the card that contains the object

  put "{" & variable & "{" & myID & "{" into startKey    --{Fred{21{

  if cardID is empty
  then put the short id of this card into holder  -- on current card
  else put cardID into holder   --on a specific card

  if cdBk = "bk"
  then put "call allowInstVars first" into theFld  --background objects
  else put "you must call allowInstVars" into theFld  --card specific

  return offset(startKey,bkgnd fld theFld of cd id holder) is not 0
end hasThisVar

function allVarsOf cdBk,myID,cardID
  -- Return a list of the names of the variables in this object now.
  -- Returns a list of items separated by commas
  -- Example:  allVarsOf(cd,the id of me)
  --    a missing cardID means on the current card
  -- cdBk is cd or bk
  --    (where is this object? cd is card, bk is background)
  -- myID is: the short id of me
  -- cardID (optional) the id of the card that contains the object

  put "{" & myID & "{" into startKey    --{21{

  if cardID is empty
  then put the short id of this card into holder  -- on current card
  else put cardID into holder   --on a specific card

  if cdBk = "bk"
  then put "call allowInstVars first" into theFld  --background objects
  else put "you must call allowInstVars" into theFld  --card specific

  put bkgnd fld theFld of cd id holder into rawData
  put empty into list
  repeat
    put offset(startKey,rawData) into index    --{21{
    if index = 0 then return list
    put index-1 into ii
    repeat until ii = 1    -- will not leave this way
      if char ii of rawData = "{" then
        if list is not empty then put "," after list
        put (char ii+1 to index-1 of rawData) after list   --fred
        delete char 1 to index+5 of rawData   -- remove start of this
        --     variable so the next one will be the first one found
      else
        subtract 1 from ii  --backup one char
      end if
    end repeat
  end repeat
end allVarsOf