如何重载Moose构造函数？

Question

很抱歉使用Java术语，但是我如何重载Moose构造函数呢？

假设我代表的是一个片段。我可以取一个起点和一个点，或者一个起点和长度，或者终点和长度。

我怎样才能考虑到这种替代的构造方法呢？

Answer 1

您不需要重写new。You can supply your own BUILD

#!/usr/bin/perl

package My::Segment;

use Moose;
use namespace::autoclean;
use Carp qw( confess );

has 'start' => (is => 'ro', isa => 'Num',
    predicate => 'has_start', writer => '_set_start',
);

has 'end' => (is => 'ro', isa => 'Num',
    predicate => 'has_end', writer => '_set_end',
);

has 'length' => (is => 'ro', isa => 'Num',
    predicate => 'has_length', writer => '_set_length',
);

sub BUILD {
    my $self = shift;

    $self->has_start and $self->has_end and $self->length and do {
        return if $self->length == $self->end - $self->start;
        confess "Inconsistent start, end and length";
    };

    $self->has_start and $self->has_end and do {
        $self->_set_length($self->end - $self->start);
        return;
    };
    $self->has_start and $self->has_length and do {
        $self->_set_end($self->start + $self->length);
        return;
    };
    $self->has_end and $self->has_length and do {
        $self->_set_start($self->end - $self->length);
        return;
    };
    confess "At least two of start, end or length must be supplied";
}

__PACKAGE__->meta->make_immutable;

package main;
use YAML;

my $x = My::Segment->new(start => 0, length => 3);
my $y = My::Segment->new(start => 1, end => 4);
my $z = My::Segment->new(end => 5, length => 3);

print Dump($_) for $x, $y, $z;

my $w = My::Segment->new(start => 0, end => 0, length => 1);

Answer 2

思南的BUILD回答可能是最明智、最直接的解决方案。正如戴夫提到的那样，使用BUILDARGS也是一个合理的解决方案。

我觉得值得一提的是，我们也可以使用类型强制。给定一个类：

class LineSegment { 
  has [qw(startX startY endX endY)] => ( 
         isa => 'Num', 
         is  => 'ro', 
         required => 1 
  );
}

您可以使用一组强制，如下所示：

class_type 'LineSegment';

subtype StartLength 
     => as Hashref 
     => where { exists $_->{startX} && $_->{startY} && $_->{length} };

subtype EndLength 
     => as Hashref 
     => where { exists $_->{endX} && $_->{endY} && $_->{length} };    

coerce LineSegment 
     => from StartLength 
     => via { my ($endX, $endY) = calc_end($_); 
              LineSegment->new(
                   startX => $_->{startX}, 
                   startY => $_->{startY},
                   endX => $endX,
                   endY => $endY,
            )};
coerce LineSegment 
     => from EndLength 
     => via { my ($startX, $startY) = calc_start($_); 
              LineSegment->new(
                   startX => $startX, 
                   startY => $startY,
                   endX => $_->{endX},
                   endY => $_->{endY},
            )};               

然后在你的代码中：

 use Moose::Util::TypeConstraints;
 find_type_constraint('LineSegment')->coerce({
       startX => $x, 
       startY => $y, 
       length => $length
 });

虽然对于这个例子来说可能有些夸大其词，但在某些情况下，强制是一种优雅的解决方案。例如，如果您有一个预先存在的LineSegment类，但您不希望向其中添加length属性(尽管BUILDARGS在那里也可以很好地工作)