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Configuration Methods and
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Declaring and Using
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Type Checker
Public Variable Configuration Methods
After a widget instance is created, the user can assign new values
to the public variables using the configure method. For example, the
following code changes the
-direction
variable of the
.arr
instance to
n
.
.arr configure -direction n
In order for configuration to work, you have to define a
configuration method that does what the programmer expects. The
configuration method of a public variable is invoked whenever the
user calls the configure method to change the value of this
variable. The name of a configuration method must be the name of the
public variable prefixed by the creation command of the class and
:config
. For example, the name configuration method for the
-direction
variable of the TixArrowButton class is
tixArrowButton:config-direction
. The following code implements
this method:
proc tixArrowButton:config-direction {w value} {
upvar #0 $w data
$data(w:button) config -bitmap @$value.xbm
}
Notice that when
tixArrowButton:config-direction
is called,
the
value
parameter contains the new value of the
-direction
variable but
data(-direction)
contains the
old
value. This is useful when the configuration method needs to
check the previous value of the variable before taking in the new
value.
If a type checker is defined for a variable, it will be called
before the configuration method is called. Therefore, the
configuration method can assume that the type of the
value
parameter is got is always correct.
Sometimes it is necessary to override the value supplied by the
user. The following code illustrates this idea:
proc tixArrowButton:config-direction {w value} {
upvar #0 $w data
if {$value == "n"} {
set value s
set data(-direction) $value
}
$data(w:button) config -bitmap @$value.xbm
return $data(-direction)
}
Notice the above code always overrides values of
n
to
s
. If you need to override the value, you must do the following two
things:
Explicitly set the instance variable inside the configuration
method (the
set data(-direction) $value
line).
Return the modified value from the configuration method.
If you do not need to override the value, you don't need to return
anything from the configuration method. In this case, the Tix
Intrinsics will assign the new value to the instance variable for
you.
http://tix.sourceforge.net
http://tix.sourceforge.net
