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 filters to indicate the point where a filter calls the actual implementation (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 implementation 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:
When constructing the method chain, method implementations are searched for in the following order:
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 perspective of inheritance. This is true even when the multiple inheritance 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 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 = 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::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