 
 subsubsection3_8_3_1.html  section3_8.html  subsubsection3_8_2_1.html  tableofcontents3_1.html   Next: subsubsection3_8_3_1.html Declaring Public Methods Up: section3_8.html Tix Object Oriented  Previous: subsubsection3_8_2_1.html Using the tixWidgetClass    Writing Methods  
After we have declared the new widget class, we can write methods
  for this class to define its behavior. Methods are just a special
  type of TCL procedures and they are created by the 
proc  command. There are, however, three requirements for methods. First,
  their names must be prefixed by the command name of their
  class. Second, they must accept at least one argument and the first
  argument that they accept must be called 
w. Third, the first
  command executed inside each method must be:
  upvar #0 $w data
For example, the following is an implementation of the invert method
  for the class TixArrowButton:
  proc tixArrowButton:invert {w} {    upvar #0 $w data
    set curDirection $data(-direction)
    case $curDirection {
        n {
            set newDirection s
        }
        s {
            set newDirection n
        }
        # ....
    }
}
   Notice that the name of the method is prefixed by the command name
  of the class (
tixArrowButton). Also, the first and only
  argument that it accepts is 
w and the first line it executes
  is ``
upvar #0 $wdata''.
   The argument 
w specifies which widget instance this method
  should act upon. For example, if the user has issued the command
  .up invert
   on an instance 
.up of the class tixArrowButton, the method
  
tixArrowButton:invert will be called and the argument w  will have the value 
.up.
   The 
invert method is used to invert the direction of the
  arrow. Therefore, it should examine the variable 
.up(-direction), which stores the current direction of the instance
  
.up, and modify it appropriately. It turns out that in TCL,
  the only clean way to access an array whose name is stored in a
  variable is the ``
upvar #0 $wdata'' technique: essentially
  it tells the intepreter that the array data should be an alias for
  the global array whose name is stored in 
$w. We will soon see
  how the widget's methods use the data array.
   Once the mysterious ``
upvar #0 $wdata'' line is explained,
  it becomes clear what the rest of the 
tixArrowButton:invert  method does: it examines the current direction of the arrow, which
  is stored in 
$data(-direction) and inverts it.
 
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