[comment {-*- tcl -*- doctools manpage}]
[manpage_begin switched n 2.2.1]
[keywords C++]
[keywords class]
[keywords object]
[keywords {object oriented}]
[moddesc   {Simple Tcl Only Object Oriented Programming}]
[titledesc {switch/option management.}]
[category  {Programming tools}]
[require Tcl 8.3]
[require switched [opt 2.2.1]]
[description]
[para]

The [class switched] class serves as base class for user classes with
switch / option configuration procedures. It provides facilities for
managing options through a simple interface.

[para] For example:

[example {
set vehicle [new car -length 4.5 -width 2 -power 100 -fuel diesel]
puts "my car was running on [switched::cget $vehicle -fuel]"
switched::configure $vehicle -power 40 -fuel electricity
puts "but is now running on clean [switched::cget $vehicle -fuel]"
}]

[para] Of course, as you might have guessed, the [class car] class is
derived from the [class switched] class. Let us see how it works:

[example {
class car {
    proc car {this args} switched {$args} {
        # car specific initialization code here
        switched::complete $this
    }
    ...
}
}]

[para] The switched class constructor takes the optional configuration
option / value pairs as parameters.

The switched class layer then completely manages the switched options:
it checks their validity, stores their values and provides a clean
interface to the user layer configuration setting procedures.

[para] The switched class members available to the programmer are:

[list_begin definitions]

[comment ---------------------------------------------------------------------]
[call [cmd <switched>] [method complete] [arg this]]

This procedure is used to tell the switched layer that the derived
class object (a car in the examples) is completely built.

At that time, the initial configuration of the switched object occurs,
using default option values (see procedure [method options])
eventually overridden by construction time values, passed at the time
of the [method new] operator invocation.

This procedure must be called only once, usually around or at the end
of the derived class constructor.

([emph Note]: Also check the [var complete] data member later in this
chapter).

[comment ---------------------------------------------------------------------]
[call [cmd <switched>] [method options] [arg this]]

This procedure must return the configuration description for
[emph all] options that the switched object will accept.

It is a pure virtual member procedure and therefore its implementation
is [emph mandatory] in the derived class layer.

The procedure must return a list of lists.

Each list pertains to a single option and is composed of the switch
name, the default value for the option and an optional initial value.

For example:

[para][example {
class car {
    ...
    proc options {this} {
        return [list\
            [list -fuel petrol petrol]\
            [list -length {} {}]\
            [list -power {} {}]\
            [list -width {} {}]\
        ]
    }
    proc set-fuel {this value} {
        ...
    }
    ...
}
}]

[para] In this case, 4 options are specified:
[option fuel], [option length], [option power] and [option width].

The default and initial values for the [option fuel] option are
identical and set to [option petrol].

For the other options, values are all empty.

[para] For each option, there must be a corresponding
[method set-[option option]] procedure defined in the derived class
layer.

For example, since we defined a [option fuel] option, there is a
[method set-fuel] procedure in the car class.

The parameters always are the object identifier (since this is not a
static procedure, but rather a dynamically defined virtual one),
followed by the new value for the option.

A [method set-[option option]] procedure is only invoked if the new
value differs from the current one (a caching scheme for improving
performance), or if there is no initial value set in the
[method options] procedure for that option.

[para] In this procedure, if the initial value differs from the
default value or is omitted, then initial configuration is forced and
the corresponding [method set-[option option]] procedure is invoked by
the switched [method complete] procedure located at the end of the
derived class constructor.

For example:

[example {
class car {
    ...
    proc options {this} {
        return [list\
            [list -fuel petrol]\
            [list -length {} {}]\
            [list -power 100 50]\
            [list -width {} {}]\
        ]
    }
    ...
}
}]

In this case, configuration is forced on the [option fuel] and
[option power] options, that is the corresponding

[method set-[option option]] procedures will be invoked when the
switched object is constructed (see [method set-[option option]]
procedures documentation below).

[para] For the [option fuel] option, since there is no initial value,
the [method set-[option fuel]] procedure is called with the default
value ([const petrol]) as argument.

For the [option power] option, since the initial value differs from
the default value, the [method set-[option power]] procedure is called
with the initial value as argument ([const 50]).

[para] For the other options, since the initial values (last elements
of the option lists) are identical to their default values, the
corresponding [method set-[option option]] procedures will not be
invoked. It is the programmer's responsibility to insure that the
initial option values are correct.

[comment ---------------------------------------------------------------------]
[call [cmd <switched>] [method set-[option option]] [arg this] [arg value]]

These procedures may be viewed as dynamic virtual functions.

There must be one implementation per supported option, as returned by
the [method options] procedure.

For example:

[example {
class car {
    ...
    proc options {this} {
        return [list\
            ...
            [list -width {} {}]\
        ]
    }
    ...
    proc set-width {this value} {
        ...
    }
    ...
}
}]

Since the [option -width] option was listed in the [method options]
procedure, a [method set-width] procedure implementation is provided, which
of course would proceed to set the width of the car (and would modify
the looks of a graphical representation, for example).

[para] As you add a supported [option option] in the list returned by
the [method options] procedure, the corresponding

[method set-[option option]] procedure may be called as soon as the
switched object is complete, which occurs when the switched level
[method complete] procedure is invoked.

For example:

[para][example {
class car {
    proc car {this args} switched {args} {
        ...
        switched::complete $this
   }
    ...
    proc options {this} {
        return [list\
            [list -fuel petrol]\
            [list -length 4.5]\
            [list -power 350]\
            [list -width 1.8]\
        ]
    }
    proc set-fuel {this value} {
        ...
    }
    proc set-length {this value} {
        ...
    }
    proc set-power {this value} {
        ...
    }
    proc set-width {this value} {
        ...
    }
}

new car
}]

[para] In this case, a new car is created with no options, which
causes the car constructor to be called, which in turns calls the
switched level [method complete] procedure after the car object layer
is completely initialized.

At this point, since there are no initial values in any option list in
the options procedure, the [method set-fuel] procedure is called with
its default value of [const petrol] as parameter, followed by the
[method set-length] call with [const 4.5] value, [method set-power]
with [const 350] value and finally with [method set-width] with
[const 1.8] as parameter.

This is a good way to test the [method set-[option option]] procedures
when debugging, and when done, just fill-in the initial option values.

[para] The switched layer checks that an option is valid (that is,
listed in the [method options] procedure) but obviously does not check
the validity of the value passed to the [method set-[option option]]
procedure, which should throw an error (for example by using the Tcl
error command) if the value is invalid.

[para] The switched layer also keeps track of the options current
values, so that a [method set-[option option]] procedure is called
only when the corresponding option value passed as parameter is
different from the current value (see [variable -option] data members
description).

[def [variable -option]]

[para] The [variable -option] data member is an options current value.

There is one for each option listed in the options procedure. It is a
read-only value which the switched layer checks against when an option
is changed.

It is rarely used at the layer derived from switched, except in the
few cases, such as in the following example:

[para][example {
...
proc car::options {this} {
    return {
        ...
        {-manufacturer {} {}}
        ...
    }
}

proc car::set-manufacturer {this value} {}

proc car::printData {this} {
    puts "manufacturer: $switched::($this,-manufacturer)"
    ...
}
}]

[para] In this case, the manufacturer's name is stored at the switched
layer level (this is why the set-manufacturer procedure has nothing to
do) and later retrieved in the printData procedure.

[def [variable complete]]

[para] The [variable complete] data member (not to be confused with
the [method complete] procedure) is a boolean.

Its initial value is [const false] and it is set to [const true] at
the very end of the switched [method complete] procedure.

It becomes useful when some options should be set at construction time
only and not dynamically, as the following example shows:

[para][example {
proc car::set-width {this value} {
    if {$switched::($this,complete)} {
        error {option -width cannot be set dynamically}
    }
    ...
}
}]

[list_end]

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[manpage_end]