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next(n)				TclOO Commands			       next(n)

       next, nextto - invoke superclass	method implementations

       package require TclOO

       next ?arg ...?
       nextto class ?arg ...?

       The  next  command  is  used  to	 call implementations of a method by a
       class, superclass or mixin that are overridden by the  current  method.
       It  can	only be	used from within a method. It is also used within fil-
       ters to indicate	the point where	a filter calls the actual  implementa-
       tion  (the filter may decide to not go along the	chain, and may process
       the results of going along the chain of methods	as  it	chooses).  The
       result  of  the	next  command  is the result of	the next method	in the
       method chain; if	there are no further methods in	the method chain,  the
       result  of  next	 will be an error. The arguments, arg, to next are the
       arguments to pass to the	next method in the chain.

       The nextto command is the same as the  next  command,  except  that  it
       takes an	additional class argument that identifies a class whose	imple-
       mentation of the	current	method chain (see info object call) should  be
       used;  the  method  implementation selected will	be the one provided by
       the given class,	and it must refer to an	existing non-filter invocation
       that lies further along the chain than the current implementation.

       When a method of	an object is invoked, things happen in several stages:

       [1]    The  structure  of the object, its class,	superclasses, filters,
	      and mixins, are examined to build	a method chain,	which contains
	      a	list of	method implementations to invoke.

       [2]    The first	method implementation on the chain is invoked.

       [3]    If that method implementation invokes the	next command, the next
	      method implementation is invoked (with its arguments being those
	      that were	passed to next).

       [4]    The  result  from	the overall method call	is the result from the
	      outermost	method implementation;	inner  method  implementations
	      return their results through next.

       [5]    The method chain is cached for future use.

       When constructing the method chain, method implementations are searched
       for in the following order:

       [1]    In the classes mixed into	the object, in class traversal	order.
	      The list of mixins is checked in natural order.

       [2]    In  the  classes	mixed  into  the  classes  of the object, with
	      sources of mixing	in being searched in  class  traversal	order.
	      Within  each  class,  the	list of	mixins is processed in natural

       [3]    In the object itself.

       [4]    In the object's class.

       [5]    In the superclasses of the class,	following each superclass in a
	      depth-first fashion in the natural order of the superclass list.

       Any  particular	method	implementation	always	comes  as  late	in the
       resulting list of implementations as possible; this means that if  some
       class, A, is both mixed into a class, B,	and is also a superclass of B,
       the instances of	B will always treat A as a superclass  from  the  per-
       spective	 of  inheritance.  This	is true	even when the multiple inheri-
       tance is	processed indirectly.

       When an object has a list of  filter  names  set	 upon  it,  or	is  an
       instance	of a class (or has mixed in a class) that has a	list of	filter
       names set upon it, before every invokation of any  method  the  filters
       are  processed.	Filter	implementations	 are  found in class traversal
       order, as are the lists of filter names (each of	which is traversed  in
       natural list order). Explicitly invoking	a method used as a filter will
       cause that method to be invoked twice, once as a	filter and once	 as  a
       normal method.

       Each filter should decide for itself whether to permit the execution to
       go forward to the proper	implementation of the method (which it does by
       invoking	the next command as filters are	inserted into the front	of the
       method call chain) and is responsible for returning the result of next.

       Filters are invoked when	processing an invokation of the	unknown	method
       because	of  a  failure to locate a method implementation, but not when
       invoking	either constructors or destructors.  (Note  however  that  the
       destroy	method	is  a  conventional method, and	filters	are invoked as
       normal when it is called.)

       This example demonstrates how to	use  the  next	command	 to  call  the
       (super)class's implementation of	a method. The script: oo::class	create
       theSuperclass {
	   method example {args} {
	       puts "in	the superclass,	args = $args"
	   } } oo::class create	theSubclass {
	   superclass theSuperclass
	   method example {args} {
	       puts "before chaining from subclass, args = $args"
	       next a {*}$args b
	       next pureSynthesis
	       puts "after chaining from subclass"
	   } } theSubclass create obj oo::objdefine obj	method example args {
	   puts	"per-object method, args = $args"
	   next	x {*}$args y
	   next	} obj example 1	2 3 prints the following:  per-object  method,
       args  =	1  2  3	before chaining	from subclass, args = x	1 2 3 y	in the
       superclass, args	= a x 1	2 3 y b	in the superclass, args	=  pureSynthe-
       sis  after chaining from	subclass before	chaining from subclass,	args =
       in the superclass, args = a b in	the superclassm	args  =	 pureSynthesis
       after chaining from subclass

       This  example  demonstrates  how	 to  build  a  simple cache class that
       applies memoization to all the method calls of the objects it is	 mixed
       into, and shows how it can make a difference to computation times:

       oo::class create	cache {
	   filter Memoize
	   method Memoize args {
	       # Do not	filter the core	method implementations
	       if {[lindex [self target] 0] eq "::oo::object"} {
		   return [next	{*}$args]

	       # Check if the value is already in the cache
	       my variable ValueCache
	       set key [self target],$args
	       if {[info exist ValueCache($key)]} {
		   return $ValueCache($key)

	       # Compute value,	insert into cache, and return it
	       return [set ValueCache($key) [next {*}$args]]
	   method flushCache {}	{
	       my variable ValueCache
	       unset ValueCache
	       # Skip the cacheing
	       return -level 2 ""
	   } }

       oo::object create demo oo::objdefine demo {
	   mixin cache
	   method compute {a b c} {
	       after 3000 ;# Simulate deep thought
	       return [expr {$a	+ $b * $c}]
	   method compute2 {a b	c} {
	       after 3000 ;# Simulate deep thought
	       return [expr {$a	* $b + $c}]
	   } }

       puts  [demo  compute   1	2 3]	  -_ prints "7"	after delay puts [demo
       compute2	4 5 6]	    -_ prints "26" after delay puts [demo compute  1 2
       3]	-_  prints  "7"	 instantly  puts [demo compute2	4 5 6]	    -_
       prints "26" instantly puts [demo	compute	 4 5 6]	      -_  prints  "34"
       after  delay  puts  [demo compute  4 5 6]      -_ prints	"34" instantly
       puts [demo compute  1 2 3]      -_ prints "7" instantly demo flushCache
       puts [demo compute  1 2 3]      -_ prints "7" after delay

       oo::class(n), oo::define(n), oo::object(n), self(n)

       call, method, method chain

TclOO				      0.1			       next(n)


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