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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 invocation 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 not invoked when processing an invocation  of  the  unknown
       method  because of a failure to locate a method implementation, or when
       invoking either constructors or destructors.

       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::define obj method example args {
		  puts "per-object method, args = $args"
		  next x {*}$args y
	      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 = pureSynthesis
	      after chaining from subclass
	      before chaining from subclass, args =
	      in the superclass, args = a b
	      in the superclass, 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 caching
		      return -level 2 ""

	      oo::object create demo
	      oo::define 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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