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1 | package NEXT; |
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2 | $VERSION = '0.60'; |
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3 | use Carp; |
4 | use strict; |
5 | |
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6 | sub NEXT::ELSEWHERE::ancestors |
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7 | { |
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8 | my @inlist = shift; |
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9 | my @outlist = (); |
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10 | while (my $next = shift @inlist) { |
11 | push @outlist, $next; |
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12 | no strict 'refs'; |
13 | unshift @inlist, @{"$outlist[-1]::ISA"}; |
14 | } |
15 | return @outlist; |
16 | } |
17 | |
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18 | sub NEXT::ELSEWHERE::ordered_ancestors |
19 | { |
20 | my @inlist = shift; |
21 | my @outlist = (); |
22 | while (my $next = shift @inlist) { |
23 | push @outlist, $next; |
24 | no strict 'refs'; |
25 | push @inlist, @{"$outlist[-1]::ISA"}; |
26 | } |
27 | return sort { $a->isa($b) ? -1 |
28 | : $b->isa($a) ? +1 |
29 | : 0 } @outlist; |
30 | } |
31 | |
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32 | sub AUTOLOAD |
33 | { |
34 | my ($self) = @_; |
35 | my $caller = (caller(1))[3]; |
36 | my $wanted = $NEXT::AUTOLOAD || 'NEXT::AUTOLOAD'; |
37 | undef $NEXT::AUTOLOAD; |
38 | my ($caller_class, $caller_method) = $caller =~ m{(.*)::(.*)}g; |
39 | my ($wanted_class, $wanted_method) = $wanted =~ m{(.*)::(.*)}g; |
40 | croak "Can't call $wanted from $caller" |
41 | unless $caller_method eq $wanted_method; |
42 | |
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43 | local ($NEXT::NEXT{$self,$wanted_method}, $NEXT::SEEN) = |
44 | ($NEXT::NEXT{$self,$wanted_method}, $NEXT::SEEN); |
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45 | |
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46 | |
47 | unless ($NEXT::NEXT{$self,$wanted_method}) { |
48 | my @forebears = |
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49 | NEXT::ELSEWHERE::ancestors ref $self || $self, |
50 | $wanted_class; |
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51 | while (@forebears) { |
52 | last if shift @forebears eq $caller_class |
53 | } |
54 | no strict 'refs'; |
55 | @{$NEXT::NEXT{$self,$wanted_method}} = |
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56 | map { *{"${_}::$caller_method"}{CODE}||() } @forebears |
57 | unless $wanted_method eq 'AUTOLOAD'; |
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58 | @{$NEXT::NEXT{$self,$wanted_method}} = |
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59 | map { (*{"${_}::AUTOLOAD"}{CODE}) ? "${_}::AUTOLOAD" : ()} @forebears |
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60 | unless @{$NEXT::NEXT{$self,$wanted_method}||[]}; |
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61 | $NEXT::SEEN->{$self,*{$caller}{CODE}}++; |
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62 | } |
63 | my $call_method = shift @{$NEXT::NEXT{$self,$wanted_method}}; |
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64 | while ($wanted_class =~ /^NEXT\b.*\b(UNSEEN|DISTINCT)\b/ |
65 | && defined $call_method |
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66 | && $NEXT::SEEN->{$self,$call_method}++) { |
67 | $call_method = shift @{$NEXT::NEXT{$self,$wanted_method}}; |
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68 | } |
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69 | unless (defined $call_method) { |
70 | return unless $wanted_class =~ /^NEXT:.*:ACTUAL/; |
71 | (local $Carp::CarpLevel)++; |
72 | croak qq(Can't locate object method "$wanted_method" ), |
73 | qq(via package "$caller_class"); |
74 | }; |
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75 | return $self->$call_method(@_[1..$#_]) if ref $call_method eq 'CODE'; |
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76 | no strict 'refs'; |
77 | ($wanted_method=${$caller_class."::AUTOLOAD"}) =~ s/.*::// |
78 | if $wanted_method eq 'AUTOLOAD'; |
79 | $$call_method = $caller_class."::NEXT::".$wanted_method; |
80 | return $call_method->(@_); |
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81 | } |
82 | |
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83 | no strict 'vars'; |
84 | package NEXT::UNSEEN; @ISA = 'NEXT'; |
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85 | package NEXT::DISTINCT; @ISA = 'NEXT'; |
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86 | package NEXT::ACTUAL; @ISA = 'NEXT'; |
87 | package NEXT::ACTUAL::UNSEEN; @ISA = 'NEXT'; |
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88 | package NEXT::ACTUAL::DISTINCT; @ISA = 'NEXT'; |
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89 | package NEXT::UNSEEN::ACTUAL; @ISA = 'NEXT'; |
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90 | package NEXT::DISTINCT::ACTUAL; @ISA = 'NEXT'; |
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91 | |
92 | package EVERY::LAST; @ISA = 'EVERY'; |
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93 | package EVERY; @ISA = 'NEXT'; |
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94 | sub AUTOLOAD |
95 | { |
96 | my ($self) = @_; |
97 | my $caller = (caller(1))[3]; |
98 | my $wanted = $EVERY::AUTOLOAD || 'EVERY::AUTOLOAD'; |
99 | undef $EVERY::AUTOLOAD; |
100 | my ($wanted_class, $wanted_method) = $wanted =~ m{(.*)::(.*)}g; |
101 | |
102 | local $NEXT::ALREADY_IN_EVERY{$self,$wanted_method} = |
103 | $NEXT::ALREADY_IN_EVERY{$self,$wanted_method}; |
104 | |
105 | return if $NEXT::ALREADY_IN_EVERY{$self,$wanted_method}++; |
106 | |
107 | my @forebears = NEXT::ELSEWHERE::ordered_ancestors ref $self || $self, |
108 | $wanted_class; |
109 | @forebears = reverse @forebears if $wanted_class =~ /\bLAST\b/; |
110 | no strict 'refs'; |
111 | my %seen; |
112 | my @every = map { my $sub = "${_}::$wanted_method"; |
113 | !*{$sub}{CODE} || $seen{$sub}++ ? () : $sub |
114 | } @forebears |
115 | unless $wanted_method eq 'AUTOLOAD'; |
116 | |
117 | my $want = wantarray; |
118 | if (@every) { |
119 | if ($want) { |
120 | return map {($_, [$self->$_(@_[1..$#_])])} @every; |
121 | } |
122 | elsif (defined $want) { |
123 | return { map {($_, scalar($self->$_(@_[1..$#_])))} |
124 | @every |
125 | }; |
126 | } |
127 | else { |
128 | $self->$_(@_[1..$#_]) for @every; |
129 | return; |
130 | } |
131 | } |
132 | |
133 | @every = map { my $sub = "${_}::AUTOLOAD"; |
134 | !*{$sub}{CODE} || $seen{$sub}++ ? () : "${_}::AUTOLOAD" |
135 | } @forebears; |
136 | if ($want) { |
137 | return map { $$_ = ref($self)."::EVERY::".$wanted_method; |
138 | ($_, [$self->$_(@_[1..$#_])]); |
139 | } @every; |
140 | } |
141 | elsif (defined $want) { |
142 | return { map { $$_ = ref($self)."::EVERY::".$wanted_method; |
143 | ($_, scalar($self->$_(@_[1..$#_]))) |
144 | } @every |
145 | }; |
146 | } |
147 | else { |
148 | for (@every) { |
149 | $$_ = ref($self)."::EVERY::".$wanted_method; |
150 | $self->$_(@_[1..$#_]); |
151 | } |
152 | return; |
153 | } |
154 | } |
155 | |
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156 | |
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157 | 1; |
158 | |
159 | __END__ |
160 | |
161 | =head1 NAME |
162 | |
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163 | NEXT.pm - Provide a pseudo-class NEXT (et al) that allows method redispatch |
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164 | |
165 | |
166 | =head1 SYNOPSIS |
167 | |
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168 | use NEXT; |
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169 | |
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170 | package A; |
171 | sub A::method { print "$_[0]: A method\n"; $_[0]->NEXT::method() } |
172 | sub A::DESTROY { print "$_[0]: A dtor\n"; $_[0]->NEXT::DESTROY() } |
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173 | |
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174 | package B; |
175 | use base qw( A ); |
176 | sub B::AUTOLOAD { print "$_[0]: B AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() } |
177 | sub B::DESTROY { print "$_[0]: B dtor\n"; $_[0]->NEXT::DESTROY() } |
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178 | |
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179 | package C; |
180 | sub C::method { print "$_[0]: C method\n"; $_[0]->NEXT::method() } |
181 | sub C::AUTOLOAD { print "$_[0]: C AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() } |
182 | sub C::DESTROY { print "$_[0]: C dtor\n"; $_[0]->NEXT::DESTROY() } |
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183 | |
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184 | package D; |
185 | use base qw( B C ); |
186 | sub D::method { print "$_[0]: D method\n"; $_[0]->NEXT::method() } |
187 | sub D::AUTOLOAD { print "$_[0]: D AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() } |
188 | sub D::DESTROY { print "$_[0]: D dtor\n"; $_[0]->NEXT::DESTROY() } |
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189 | |
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190 | package main; |
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191 | |
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192 | my $obj = bless {}, "D"; |
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193 | |
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194 | $obj->method(); # Calls D::method, A::method, C::method |
195 | $obj->missing_method(); # Calls D::AUTOLOAD, B::AUTOLOAD, C::AUTOLOAD |
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196 | |
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197 | # Clean-up calls D::DESTROY, B::DESTROY, A::DESTROY, C::DESTROY |
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198 | |
199 | |
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200 | |
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201 | =head1 DESCRIPTION |
202 | |
203 | NEXT.pm adds a pseudoclass named C<NEXT> to any program |
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204 | that uses it. If a method C<m> calls C<$self-E<gt>NEXT::m()>, the call to |
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205 | C<m> is redispatched as if the calling method had not originally been found. |
206 | |
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207 | In other words, a call to C<$self-E<gt>NEXT::m()> resumes the depth-first, |
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208 | left-to-right search of C<$self>'s class hierarchy that resulted in the |
209 | original call to C<m>. |
210 | |
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211 | Note that this is not the same thing as C<$self-E<gt>SUPER::m()>, which |
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212 | begins a new dispatch that is restricted to searching the ancestors |
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213 | of the current class. C<$self-E<gt>NEXT::m()> can backtrack |
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214 | past the current class -- to look for a suitable method in other |
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215 | ancestors of C<$self> -- whereas C<$self-E<gt>SUPER::m()> cannot. |
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216 | |
217 | A typical use would be in the destructors of a class hierarchy, |
218 | as illustrated in the synopsis above. Each class in the hierarchy |
219 | has a DESTROY method that performs some class-specific action |
220 | and then redispatches the call up the hierarchy. As a result, |
221 | when an object of class D is destroyed, the destructors of I<all> |
222 | its parent classes are called (in depth-first, left-to-right order). |
223 | |
224 | Another typical use of redispatch would be in C<AUTOLOAD>'ed methods. |
225 | If such a method determined that it was not able to handle a |
226 | particular call, it might choose to redispatch that call, in the |
227 | hope that some other C<AUTOLOAD> (above it, or to its left) might |
228 | do better. |
229 | |
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230 | By default, if a redispatch attempt fails to find another method |
231 | elsewhere in the objects class hierarchy, it quietly gives up and does |
232 | nothing (but see L<"Enforcing redispatch">). This gracious acquiesence |
233 | is also unlike the (generally annoying) behaviour of C<SUPER>, which |
234 | throws an exception if it cannot redispatch. |
235 | |
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236 | Note that it is a fatal error for any method (including C<AUTOLOAD>) |
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237 | to attempt to redispatch any method that does not have the |
238 | same name. For example: |
239 | |
240 | sub D::oops { print "oops!\n"; $_[0]->NEXT::other_method() } |
241 | |
242 | |
243 | =head2 Enforcing redispatch |
244 | |
245 | It is possible to make C<NEXT> redispatch more demandingly (i.e. like |
246 | C<SUPER> does), so that the redispatch throws an exception if it cannot |
247 | find a "next" method to call. |
248 | |
249 | To do this, simple invoke the redispatch as: |
250 | |
251 | $self->NEXT::ACTUAL::method(); |
252 | |
253 | rather than: |
254 | |
255 | $self->NEXT::method(); |
256 | |
257 | The C<ACTUAL> tells C<NEXT> that there must actually be a next method to call, |
258 | or it should throw an exception. |
259 | |
260 | C<NEXT::ACTUAL> is most commonly used in C<AUTOLOAD> methods, as a means to |
261 | decline an C<AUTOLOAD> request, but preserve the normal exception-on-failure |
262 | semantics: |
263 | |
264 | sub AUTOLOAD { |
265 | if ($AUTOLOAD =~ /foo|bar/) { |
266 | # handle here |
267 | } |
268 | else { # try elsewhere |
269 | shift()->NEXT::ACTUAL::AUTOLOAD(@_); |
270 | } |
271 | } |
272 | |
273 | By using C<NEXT::ACTUAL>, if there is no other C<AUTOLOAD> to handle the |
274 | method call, an exception will be thrown (as usually happens in the absence of |
275 | a suitable C<AUTOLOAD>). |
276 | |
277 | |
278 | =head2 Avoiding repetitions |
279 | |
280 | If C<NEXT> redispatching is used in the methods of a "diamond" class hierarchy: |
281 | |
282 | # A B |
283 | # / \ / |
284 | # C D |
285 | # \ / |
286 | # E |
287 | |
288 | use NEXT; |
289 | |
290 | package A; |
291 | sub foo { print "called A::foo\n"; shift->NEXT::foo() } |
292 | |
293 | package B; |
294 | sub foo { print "called B::foo\n"; shift->NEXT::foo() } |
295 | |
296 | package C; @ISA = qw( A ); |
297 | sub foo { print "called C::foo\n"; shift->NEXT::foo() } |
298 | |
299 | package D; @ISA = qw(A B); |
300 | sub foo { print "called D::foo\n"; shift->NEXT::foo() } |
301 | |
302 | package E; @ISA = qw(C D); |
303 | sub foo { print "called E::foo\n"; shift->NEXT::foo() } |
304 | |
305 | E->foo(); |
306 | |
307 | then derived classes may (re-)inherit base-class methods through two or |
308 | more distinct paths (e.g. in the way C<E> inherits C<A::foo> twice -- |
309 | through C<C> and C<D>). In such cases, a sequence of C<NEXT> redispatches |
310 | will invoke the multiply inherited method as many times as it is |
311 | inherited. For example, the above code prints: |
312 | |
313 | called E::foo |
314 | called C::foo |
315 | called A::foo |
316 | called D::foo |
317 | called A::foo |
318 | called B::foo |
319 | |
320 | (i.e. C<A::foo> is called twice). |
321 | |
322 | In some cases this I<may> be the desired effect within a diamond hierarchy, |
323 | but in others (e.g. for destructors) it may be more appropriate to |
324 | call each method only once during a sequence of redispatches. |
325 | |
326 | To cover such cases, you can redispatch methods via: |
327 | |
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328 | $self->NEXT::DISTINCT::method(); |
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329 | |
330 | rather than: |
331 | |
332 | $self->NEXT::method(); |
333 | |
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334 | This causes the redispatcher to only visit each distinct C<method> method |
335 | once. That is, to skip any classes in the hierarchy that it has |
336 | already visited during redispatch. So, for example, if the |
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337 | previous example were rewritten: |
338 | |
339 | package A; |
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340 | sub foo { print "called A::foo\n"; shift->NEXT::DISTINCT::foo() } |
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341 | |
342 | package B; |
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343 | sub foo { print "called B::foo\n"; shift->NEXT::DISTINCT::foo() } |
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344 | |
345 | package C; @ISA = qw( A ); |
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346 | sub foo { print "called C::foo\n"; shift->NEXT::DISTINCT::foo() } |
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347 | |
348 | package D; @ISA = qw(A B); |
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349 | sub foo { print "called D::foo\n"; shift->NEXT::DISTINCT::foo() } |
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350 | |
351 | package E; @ISA = qw(C D); |
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352 | sub foo { print "called E::foo\n"; shift->NEXT::DISTINCT::foo() } |
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353 | |
354 | E->foo(); |
355 | |
356 | then it would print: |
357 | |
358 | called E::foo |
359 | called C::foo |
360 | called A::foo |
361 | called D::foo |
362 | called B::foo |
363 | |
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364 | and omit the second call to C<A::foo> (since it would not be distinct |
365 | from the first call to C<A::foo>). |
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366 | |
367 | Note that you can also use: |
368 | |
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369 | $self->NEXT::DISTINCT::ACTUAL::method(); |
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370 | |
371 | or: |
372 | |
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373 | $self->NEXT::ACTUAL::DISTINCT::method(); |
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374 | |
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375 | to get both unique invocation I<and> exception-on-failure. |
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376 | |
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377 | Note that, for historical compatibility, you can also use |
378 | C<NEXT::UNSEEN> instead of C<NEXT::DISTINCT>. |
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379 | |
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380 | |
381 | =head2 Invoking all versions of a method with a single call |
382 | |
383 | Yet another pseudo-class that NEXT.pm provides is C<EVERY>. |
384 | Its behaviour is considerably simpler than that of the C<NEXT> family. |
385 | A call to: |
386 | |
387 | $obj->EVERY::foo(); |
388 | |
389 | calls I<every> method named C<foo> that the object in C<$obj> has inherited. |
390 | That is: |
391 | |
392 | use NEXT; |
393 | |
394 | package A; @ISA = qw(B D X); |
395 | sub foo { print "A::foo " } |
396 | |
397 | package B; @ISA = qw(D X); |
398 | sub foo { print "B::foo " } |
399 | |
400 | package X; @ISA = qw(D); |
401 | sub foo { print "X::foo " } |
402 | |
403 | package D; |
404 | sub foo { print "D::foo " } |
405 | |
406 | package main; |
407 | |
408 | my $obj = bless {}, 'A'; |
409 | $obj->EVERY::foo(); # prints" A::foo B::foo X::foo D::foo |
410 | |
411 | Prefixing a method call with C<EVERY::> causes every method in the |
412 | object's hierarchy with that name to be invoked. As the above example |
413 | illustrates, they are not called in Perl's usual "left-most-depth-first" |
414 | order. Instead, they are called "breadth-first-dependency-wise". |
415 | |
416 | That means that the inheritance tree of the object is traversed breadth-first |
417 | and the resulting order of classes is used as the sequence in which methods |
418 | are called. However, that sequence is modified by imposing a rule that the |
419 | appropritae method of a derived class must be called before the same method of |
420 | any ancestral class. That's why, in the above example, C<X::foo> is called |
421 | before C<D::foo>, even though C<D> comes before C<X> in C<@B::ISA>. |
422 | |
423 | In general, there's no need to worry about the order of calls. They will be |
424 | left-to-right, breadth-first, most-derived-first. This works perfectly for |
425 | most inherited methods (including destructors), but is inappropriate for |
426 | some kinds of methods (such as constructors, cloners, debuggers, and |
427 | initializers) where it's more appropriate that the least-derived methods be |
428 | called first (as more-derived methods may rely on the behaviour of their |
429 | "ancestors"). In that case, instead of using the C<EVERY> pseudo-class: |
430 | |
431 | $obj->EVERY::foo(); # prints" A::foo B::foo X::foo D::foo |
432 | |
433 | you can use the C<EVERY::LAST> pseudo-class: |
434 | |
435 | $obj->EVERY::LAST::foo(); # prints" D::foo X::foo B::foo A::foo |
436 | |
437 | which reverses the order of method call. |
438 | |
439 | Whichever version is used, the actual methods are called in the same |
440 | context (list, scalar, or void) as the original call via C<EVERY>, and return: |
441 | |
442 | =over |
443 | |
444 | =item * |
445 | |
446 | A hash of array references in list context. Each entry of the hash has the |
447 | fully qualified method name as its key and a reference to an array containing |
448 | the method's list-context return values as its value. |
449 | |
450 | =item * |
451 | |
452 | A reference to a hash of scalar values in scalar context. Each entry of the hash has the |
453 | fully qualified method name as its key and the method's scalar-context return values as its value. |
454 | |
455 | =item * |
456 | |
457 | Nothing in void context (obviously). |
458 | |
459 | =back |
460 | |
461 | =head2 Using C<EVERY> methods |
462 | |
463 | The typical way to use an C<EVERY> call is to wrap it in another base |
464 | method, that all classes inherit. For example, to ensure that every |
465 | destructor an object inherits is actually called (as opposed to just the |
466 | left-most-depth-first-est one): |
467 | |
468 | package Base; |
469 | sub DESTROY { $_[0]->EVERY::Destroy } |
470 | |
471 | package Derived1; |
472 | use base 'Base'; |
473 | sub Destroy {...} |
474 | |
475 | package Derived2; |
476 | use base 'Base', 'Derived1'; |
477 | sub Destroy {...} |
478 | |
479 | et cetera. Every derived class than needs its own clean-up |
480 | behaviour simply adds its own C<Destroy> method (I<not> a C<DESTROY> method), |
481 | which the call to C<EVERY::LAST::Destroy> in the inherited destructor |
482 | then correctly picks up. |
483 | |
484 | Likewise, to create a class hierarchy in which every initializer inherited by |
485 | a new object is invoked: |
486 | |
487 | package Base; |
488 | sub new { |
489 | my ($class, %args) = @_; |
490 | my $obj = bless {}, $class; |
491 | $obj->EVERY::LAST::Init(\%args); |
492 | } |
493 | |
494 | package Derived1; |
495 | use base 'Base'; |
496 | sub Init { |
497 | my ($argsref) = @_; |
498 | ... |
499 | } |
500 | |
501 | package Derived2; |
502 | use base 'Base', 'Derived1'; |
503 | sub Init { |
504 | my ($argsref) = @_; |
505 | ... |
506 | } |
507 | |
508 | et cetera. Every derived class than needs some additional initialization |
509 | behaviour simply adds its own C<Init> method (I<not> a C<new> method), |
510 | which the call to C<EVERY::LAST::Init> in the inherited constructor |
511 | then correctly picks up. |
512 | |
513 | |
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514 | =head1 AUTHOR |
515 | |
516 | Damian Conway (damian@conway.org) |
517 | |
518 | =head1 BUGS AND IRRITATIONS |
519 | |
520 | Because it's a module, not an integral part of the interpreter, NEXT.pm |
521 | has to guess where the surrounding call was found in the method |
522 | look-up sequence. In the presence of diamond inheritance patterns |
523 | it occasionally guesses wrong. |
524 | |
525 | It's also too slow (despite caching). |
526 | |
527 | Comment, suggestions, and patches welcome. |
528 | |
529 | =head1 COPYRIGHT |
530 | |
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531 | Copyright (c) 2000-2001, Damian Conway. All Rights Reserved. |
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532 | This module is free software. It may be used, redistributed |
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533 | and/or modified under the same terms as Perl itself. |