[PerlChina] CN Advent Day 3: End Scope

http://perlchina.org/advent/2009/EndScope.html

=for advent_year 2009

=for advent_day 3

=for advent_title End Scope

=for advent_author Fayland Lam

CPAN 上有很多支持在 scope 结束后运行某些代码，其中最常用的是
M<Scope::Guard> 和 M<B::Hooks::EndOfScope>

M<Scope::Guard> 采用最常见的 B<DESTROY> 方法。Scope::Guard 可以通过
dismiss 来取消。

=begin codeNNN

use Net::FTP;
use Scope::Guard;

{
my $err;
my $sg = Scope::Guard->new(
sub { email_error($err); die $err; }
);
my $ftp = Net::FTP->new($host, Debug => 0)
or do { return $err = "Cannot connect to $host: $@"; };
$ftp->login($user, $pass)
or do { return $err = "Cannot login " . $ftp->message; };

$sg->dismiss();
}

=end codeNNN

M<B::Hooks::EndOfScope> 使用 M<Variable::Magic> 中的魔术 B<$^H>.
EndOfScope 的强悍之处在于支持 Moose 的方法修改，并且支持多个
on_scope_end 加载。

=begin codeNNN

# Make sure that the application class becomes immutable at this point,
# which ensures that it gets an inlined constructor. This means that it
# works even if the user has added a plugin which contains a new method.
# Note however that we have to do the work on scope end, so that method
# modifiers work correctly in MyApp (as you have to call setup _before_
# applying modifiers).
B::Hooks::EndOfScope::on_scope_end {
return if $@;
my $meta = Class::MOP::get_metaclass_by_name($class);
if ( $meta->is_immutable && ! { $meta->immutable_options
}->{inline_constructor} ) {
warn "You made your application class ($class) immutable, "
. "but did not inline the constructor.\n"
. "This will break catalyst, please pass "
. "(replace_constructor => 1) when making your class immutable.\n";
}
$meta->make_immutable(replace_constructor => 1) unless $meta->is_immutable;
};

$class->setup_finalize;

=end codeNNN

上述代码来自 A<Catalyst>

我们一般在多处 return, next 或复杂子程序调用的时候，为了使代码更简洁而采
用上述模块。

谢谢。

===================
我们还在寻求更多的文章支持，谢谢。

--
Fayland Lam // http://www.fayland.org/

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