Provided by: tcl-itcl4-doc_4.2.4-1_all 
NAME
itcl::widget - create a widget class of objects
WARNING!
This is new functionality in [incr Tcl] where the API can still change!!
SYNOPSIS
itcl::widget widgetName {
inherit baseWidget ?baseWidget...?
constructor args ?init? body
destructor body
public method name ?args? ?body?
protected method name ?args? ?body?
private method name ?args? ?body?
public proc name ?args? ?body?
protected proc name ?args? ?body?
private proc name ?args? ?body?
public variable varName ?init? ?config?
protected variable varName ?init? ?config?
private variable varName ?init? ?config?
public common varName ?init?
protected common varName ?init?
private common varName ?init?
public command ?arg arg ...?
protected command ?arg arg ...?
private command ?arg arg ...?
<delegation info> see delegation page
<option info> see option page
set varName ?value?
array option ?arg arg ...?
}
widgetName objName ?arg arg ...?
objName method ?arg arg ...?
widgetName::proc ?arg arg ...?
________________________________________________________________________________________________________________
DESCRIPTION
One of the fundamental constructs in [incr Tcl] is the widget definition. A widget is like a class with
some additional features. Each widget acts as a template for actual objects that can be created. The
widget itself is a namespace which contains things common to all objects. Each object has its own unique
bundle of data which contains instances of the "variables" defined in the widget definition. Each object
also has a built-in variable named "this", which contains the name of the object. Widgets can also have
"common" data members that are shared by all objects in a widget.
Two types of functions can be included in the widget definition. "Methods" are functions which operate
on a specific object, and therefore have access to both "variables" and "common" data members. "Procs"
are ordinary procedures in the widget namespace, and only have access to "common" data members.
If the body of any method or proc starts with "@", it is treated as the symbolic name for a C procedure.
Otherwise, it is treated as a Tcl code script. See below for details on registering and using C
procedures.
A widget can only be defined once, although the bodies of widget methods and procs can be defined again
and again for interactive debugging. See the body and configbody commands for details.
Each namespace can have its own collection of objects and widgets. The list of widgets available in the
current context can be queried using the "itcl::find widgets" command, and the list of objects, with the
"itcl::find objects" command.
A widget can be deleted using the "delete widget" command. Individual objects can be deleted using the
"delete object" command.
WIDGET DEFINITIONS
widget widgetName definition
Provides the definition for a widget named widgetName. If the widget widgetName already exists,
or if a command called widgetName exists in the current namespace context, this command returns an
error. If the widget definition is successfully parsed, widgetName becomes a command in the
current context, handling the creation of objects for this widget.
The widget definition is evaluated as a series of Tcl statements that define elements within the widget.
The following widget definition commands are recognized:
inherit baseWidget ?baseWidget...?
Causes the current widget to inherit characteristics from one or more base widgets.
Widgets must have been defined by a previous widget command, or must be available to the
auto-loading facility (see "AUTO-LOADING" below). A single widget definition can contain
no more than one inherit command.
The order of baseWidget names in the inherit list affects the name resolution for widget
members. When the same member name appears in two or more base widgets, the base widget
that appears first in the inherit list takes precedence. For example, if widgets "Foo" and
"Bar" both contain the member "x", and if another widget has the "inherit" statement:
inherit Foo Bar
then the name "x" means "Foo::x". Other inherited members named "x" must be referenced
with their explicit name, like "Bar::x".
constructor args ?init? body
Declares the args argument list and body used for the constructor, which is automatically
invoked whenever an object is created.
Before the body is executed, the optional init statement is used to invoke any base widget
constructors that require arguments. Variables in the args specification can be accessed
in the init code fragment, and passed to base widget constructors. After evaluating the
init statement, any base widget constructors that have not been executed are invoked
automatically without arguments. This ensures that all base widgets are fully constructed
before the constructor body is executed. By default, this scheme causes constructors to be
invoked in order from least- to most-specific. This is exactly the opposite of the order
that widgets are reported by the info heritage command.
If construction is successful, the constructor always returns the object name-regardless of
how the body is defined-and the object name becomes a command in the current namespace
context. If construction fails, an error message is returned.
destructor body
Declares the body used for the destructor, which is automatically invoked when an object is
deleted. If the destructor is successful, the object data is destroyed and the object name
is removed as a command from the interpreter. If destruction fails, an error message is
returned and the object remains.
When an object is destroyed, all destructors in its widget hierarchy are invoked in order
from most- to least-specific. This is the order that the widgets are reported by the "info
heritage" command, and it is exactly the opposite of the default constructor order.
method name ?args? ?body?
Declares a method called name. When the method body is executed, it will have automatic
access to object-specific variables and common data members.
If the args list is specified, it establishes the usage information for this method. The
body command can be used to redefine the method body, but the args list must match this
specification.
Within the body of another widget method, a method can be invoked like any other
command-simply by using its name. Outside of the widget context, the method name must be
prefaced an object name, which provides the context for the data that it manipulates.
Methods in a base widget that are redefined in the current widget, or hidden by another
base widget, can be qualified using the "widgetName::method" syntax.
proc name ?args? ?body?
Declares a proc called name. A proc is an ordinary procedure within the widget namespace.
Unlike a method, a proc is invoked without referring to a specific object. When the proc
body is executed, it will have automatic access only to common data members.
If the args list is specified, it establishes the usage information for this proc. The
body command can be used to redefine the proc body, but the args list must match this
specification.
Within the body of another widget method or proc, a proc can be invoked like any other
command-simply by using its name. In any other namespace context, the proc is invoked
using a qualified name like "widgetName::proc". Procs in a base widget that are redefined
in the current widget, or hidden by another base widget, can also be accessed via their
qualified name.
variable varName ?init? ?config?
Defines an object-specific variable named varName. All object-specific variables are
automatically available in widget methods. They need not be declared with anything like
the global command.
If the optional init string is specified, it is used as the initial value of the variable
when a new object is created. Initialization forces the variable to be a simple scalar
value; uninitialized variables, on the other hand, can be set within the constructor and
used as arrays.
The optional config script is only allowed for public variables. If specified, this code
fragment is executed whenever a public variable is modified by the built-in "configure"
method. The config script can also be specified outside of the widget definition using the
configbody command.
common varName ?init?
Declares a common variable named varName. Common variables reside in the widget namespace
and are shared by all objects belonging to the widget. They are just like global
variables, except that they need not be declared with the usual global command. They are
automatically visible in all widget methods and procs.
If the optional init string is specified, it is used as the initial value of the variable.
Initialization forces the variable to be a simple scalar value; uninitialized variables, on
the other hand, can be set with subsequent set and array commands and used as arrays.
Once a common data member has been defined, it can be set using set and array commands
within the widget definition. This allows common data members to be initialized as arrays.
For example:
itcl::widget Foo {
protected common boolean
set boolean(true) 1
set boolean(false) 0
}
Note that if common data members are initialized within the constructor, they get
initialized again and again whenever new objects are created.
public command ?arg arg ...?
protected command ?arg arg ...?
private command ?arg arg ...?
These commands are used to set the protection level for widget members that are created
when command is evaluated. The command is usually method, proc, variable orcommon, and the
remaining arg's complete the member definition. However, command can also be a script
containing many different member definitions, and the protection level will apply to all of
the members that are created.
WIDGET USAGE
Once a widget has been defined, the widget name can be used as a command to create new objects belonging
to the widget.
widgetName objName ?args...?
Creates a new object in widget widgetName with the name objName. Remaining arguments are passed
to the constructor of the most-specific widget. This in turn passes arguments to base widget
constructors before invoking its own body of commands. If construction is successful, a command
called objName is created in the current namespace context, and objName is returned as the result
of this operation. If an error is encountered during construction, the destructors are
automatically invoked to free any resources that have been allocated, the object is deleted, and
an error is returned.
If objName contains the string "#auto", that string is replaced with an automatically generated
name. Names have the form widgetName<number>, where the widgetName part is modified to start with
a lowercase letter. In widget "Toaster", for example, the "#auto" specification would produce
names like toaster0, toaster1, etc. Note that "#auto" can be also be buried within an object
name:
fileselectiondialog .foo.bar.#auto -background red
This would generate an object named ".foo.bar.fileselectiondialog0".
OBJECT USAGE
Once an object has been created, the object name can be used as a command to invoke methods that operate
on the object.
objName method ?args...?
Invokes a method named method on an object named objName. Remaining arguments are passed to the
argument list for the method. The method name can be "constructor", "destructor", any method name
appearing in the widget definition, or any of the following built-in methods.
BUILT-IN METHODS
objName cget option
Provides access to public variables as configuration options. This mimics the behavior of the
usual "cget" operation for Tk widgets. The option argument is a string of the form "-varName",
and this method returns the current value of the public variable varName.
objName configure ?option? ?value option value ...?
Provides access to public variables as configuration options. This mimics the behavior of the
usual "configure" operation for Tk widgets. With no arguments, this method returns a list of
lists describing all of the public variables. Each list has three elements: the variable name,
its initial value and its current value.
If a single option of the form "-varName" is specified, then this method returns the information
for that one variable.
Otherwise, the arguments are treated as option/value pairs assigning new values to public
variables. Each variable is assigned its new value, and if it has any "config" code associated
with it, it is executed in the context of the widget where it was defined. If the "config" code
generates an error, the variable is set back to its previous value, and the configure method
returns an error.
objName isa widgetName
Returns non-zero if the given widgetName can be found in the object's heritage, and zero
otherwise.
objName info option ?args...?
Returns information related to a particular object named objName, or to its widget definition.
The option parameter includes the following things, as well as the options recognized by the usual
Tcl "info" command:
objName info widget
Returns the name of the most-specific widget for object objName.
objName info inherit
Returns the list of base widgets as they were defined in the "inherit" command, or an empty
string if this widget has no base widgets.
objName info heritage
Returns the current widget name and the entire list of base widgets in the order that they
are traversed for member lookup and object destruction.
objName info function ?cmdName? ?-protection? ?-type? ?-name? ?-args? ?-body?
With no arguments, this command returns a list of all widgets methods and procs. If
cmdName is specified, it returns information for a specific method or proc. If no flags
are specified, this command returns a list with the following elements: the protection
level, the type (method/proc), the qualified name, the argument list and the body. Flags
can be used to request specific elements from this list.
objName info variable ?varName? ?-protection? ?-type? ?-name? ?-init? ?-value? ?-config?
With no arguments, this command returns a list of all object-specific variables and common
data members. If varName is specified, it returns information for a specific data member.
If no flags are specified, this command returns a list with the following elements: the
protection level, the type (variable/common), the qualified name, the initial value, and
the current value. If varName is a public variable, the "config" code is included on this
list. Flags can be used to request specific elements from this list.
CHAINING METHODS/PROCS
Sometimes a base widget has a method or proc that is redefined with the same name in a derived widget.
This is a way of making the derived widget handle the same operations as the base widget, but with its
own specialized behavior. For example, suppose we have a Toaster widget that looks like this:
itcl::widget Toaster {
variable crumbs 0
method toast {nslices} {
if {$crumbs > 50} {
error "== FIRE! FIRE! =="
}
set crumbs [expr {$crumbs+4*$nslices}]
}
method clean {} {
set crumbs 0
}
}
We might create another widget like SmartToaster that redefines the "toast" method. If we want to access
the base widget method, we can qualify it with the base widget name, to avoid ambiguity:
itcl::widget SmartToaster {
inherit Toaster
method toast {nslices} {
if {$crumbs > 40} {
clean
}
return [Toaster::toast $nslices]
}
}
Instead of hard-coding the base widget name, we can use the "chain" command like this:
itcl::widget SmartToaster {
inherit Toaster
method toast {nslices} {
if {$crumbs > 40} {
clean
}
return [chain $nslices]
}
}
The chain command searches through the widget hierarchy for a slightly more generic (base widget)
implementation of a method or proc, and invokes it with the specified arguments. It starts at the
current widget context and searches through base widgets in the order that they are reported by the "info
heritage" command. If another implementation is not found, this command does nothing and returns the
null string.
AUTO-LOADING
Widget definitions need not be loaded explicitly; they can be loaded as needed by the usual Tcl auto-
loading facility. Each directory containing widget definition files should have an accompanying
"tclIndex" file. Each line in this file identifies a Tcl procedure or [incr Tcl] widget definition and
the file where the definition can be found.
For example, suppose a directory contains the definitions for widgets "Toaster" and "SmartToaster". Then
the "tclIndex" file for this directory would look like:
# Tcl autoload index file, version 2.0 for [incr Tcl]
# This file is generated by the "auto_mkindex" command
# and sourced to set up indexing information for one or
# more commands. Typically each line is a command that
# sets an element in the auto_index array, where the
# element name is the name of a command and the value is
# a script that loads the command.
set auto_index(::Toaster) "source $dir/Toaster.itcl"
set auto_index(::SmartToaster) "source $dir/SmartToaster.itcl"
The auto_mkindex command is used to automatically generate "tclIndex" files.
The auto-loader must be made aware of this directory by appending the directory name to the "auto_path"
variable. When this is in place, widgets will be auto-loaded as needed when used in an application.
C PROCEDURES
C procedures can be integrated into an [incr Tcl] widget definition to implement methods, procs, and the
"config" code for public variables. Any body that starts with "@" is treated as the symbolic name for a
C procedure.
Symbolic names are established by registering procedures via Itcl_RegisterC(). This is usually done in
the Tcl_AppInit() procedure, which is automatically called when the interpreter starts up. In the
following example, the procedure My_FooCmd() is registered with the symbolic name "foo". This procedure
can be referenced in the body command as "@foo".
int
Tcl_AppInit(interp)
Tcl_Interp *interp; /* Interpreter for application. */
{
if (Itcl_Init(interp) == TCL_ERROR) {
return TCL_ERROR;
}
if (Itcl_RegisterC(interp, "foo", My_FooCmd) != TCL_OK) {
return TCL_ERROR;
}
}
C procedures are implemented just like ordinary Tcl commands. See the CrtCommand man page for details.
Within the procedure, widget data members can be accessed like ordinary variables using Tcl_SetVar(),
Tcl_GetVar(), Tcl_TraceVar(), etc. Widget methods and procs can be executed like ordinary commands using
Tcl_Eval(). [incr Tcl] makes this possible by automatically setting up the context before executing the
C procedure.
This scheme provides a natural migration path for code development. Widgets can be developed quickly
using Tcl code to implement the bodies. An entire application can be built and tested. When necessary,
individual bodies can be implemented with C code to improve performance.
KEYWORDS
widget, object, object-oriented
itcl 4.0 widget(3itcl)