delegation

Section: [incr\ Tcl] (n)
Updated: 4.0
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NAME

itcl::delegation - delegate methods, procs or options to other objects

Parts of this description are "borrowed" from Tcl extension [snit], as the functionality is mostly identical.

WARNING!

This is new functionality in [incr Tcl] where the API can still change!!

SYNOPSIS

delegate method methodName to componentName ?as targetName?
delegate method methodName ?to componentName? using pattern
delegate method * ?to componentName? ?using pattern? ?except methodName methodName ...?

delegate proc procName to componentName ?as targetName?
delegate proc procName ?to componentName? using pattern
delegate proc * ?to componentName? ?using pattern? ?except procName procName ...?

delegate option optionSpec to componentName
delegate option optionSpec to componentName as targetname?
delegate option * to componentName
delegate option * to componentName except optionName optionname ...

DESCRIPTION

The delegate command is used inside an [incr Tcl] extendedclass/widget/widgetadaptor definition to delegate methods/procs/options to other objects for handling.

delegate method methodName to componentName ?as targetName?

This form of delegate method delegates method methodName to component componentName. That is, when method methdoNameame is called on an instance of this type, the method and its arguments will be passed to the named component's command instead. That is, the following statement


delegate method wag to tail

is roughly equivalent to this explicitly defined method:


method wag {args} {
    uplevel $tail wag $args
}

The optional as clause allows you to specify the delegated method name and possibly add some arguments:


delegate method wagtail to tail as "wag briskly"

A method cannot be both locally defined and delegated.

delegate method methodName ?to componentName? using pattern

In this form of the delegate statement, the using clause is used to specify the precise form of the command to which method name name is delegated. The to clause is optional, since the chosen command might not involve any particular component.

The value of the using clause is a list that may contain any or all of the following substitution codes; these codes are substituted with the described value to build the delegated command prefix. Note that the following two statements are equivalent:


delegate method wag to tail
delegate method wag to tail using "%c %m"

Each element of the list becomes a single element of the delegated command --it is never reparsed as a string.

Substitutions:

%%: This is replaced with a single "%". Thus, to pass the string "%c" to the command as an argument, you'd write "%%c".
%c: This is replaced with the named component's command.
%j: This is replaced by the method name; if the name consists of multiple tokens, they are joined by underscores ("_").
%m: This is replaced with the final token of the method name; if the method name has one token, this is identical to %M.
%M: This is replaced by the method name; if the name consists of multiple tokens, they are joined by space characters.
%n: This is replaced with the name of the instance's private namespace.
%s: This is replaced with the name of the instance command.
%t: This is replaced with the fully qualified type name.
%w: This is replaced with the original name of the instance command; for Itcl widgets and widget adaptors, it will be the Tk window name. It remains constant, even if the instance command is renamed.

delegate method * ?to componentName? ?using pattern? ?except methodName methodName ...?

In this form all unknown method names are delegeted to the specified component. The except clause can be used to specify a list of exceptions, i.e., method names that will not be so delegated. The using clause is defined as given above. In this form, the statement must contain the to clause, the using clause, or both.

In fact, the "*" can be a list of two or more tokens whose last element is "*", as in the following example:


delegate method {tail *} to tail

This implicitly defines the method tail whose subcommands will be delegated to the tail component.

The definitions for delegate proc ... are the same as for method, the only difference being, that this is for procs.

delegate option namespec to comp

delegate option namespec to comp as target

delegate option * to comp

delegate option * to comp except exceptions

Defines a delegated option; the namespec is defined as for the option statement. When the configure, configurelist, or cget instance method is used to set or retrieve the option's value, the equivalent configure or cget command will be applied to the component as though the option was defined with the following -configuremethod and -cgetmethod:


method ConfigureMethod {option value} {
    $comp configure $option $value
}

method CgetMethod {option} {
    return [$comp cget $option]
}

Note that delegated options never appear in the itcl_options array. If the as clause is specified, then the target option name is used in place of name.

delegate option * ?except optionName optionName ...?

This form delegates all unknown options to the specified component. The except clause can be used to specify a list of exceptions, i.e., option names that will not be so delegated.

Warning: options can only be delegated to a component if it supports the configure and cget instance methods.

An option cannot be both locally defined and delegated. TBD: Continue from here.

KEYWORDS

delegation, option, method, proc