NAME<\/a> Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD \u2212 Using BUILDARGS and BUILD to hook into object construction<\/p>\n version 2.2014<\/p>\n package Person; This recipe demonstrates the use of “BUILDARGS” and “BUILD”. By defining these methods, we can hook into the object construction process without overriding “new”.<\/p>\n The “BUILDARGS” method is called before<\/i> an object has been created. It is called as a class method, and receives all of the parameters passed to the “new” method. It is expected to do something with these arguments and return a hash reference. The keys of the hash must be attribute “init_arg”s.<\/p>\n The primary purpose of “BUILDARGS” is to allow a class to accept something other than named arguments. In the case of our “Person” class, we are allowing it to be called with a single argument, a social security number:<\/p>\n my $person = Person\u2212>new(‘123\u221245\u22126789’);<\/p>\n The key part of our “BUILDARGS” is this conditional:<\/p>\n if ( @_ == 1 && ! ref $_[0] ) { By default, Moose constructors accept a list of key-value pairs, or a hash reference. We need to make sure that $_[0] is not a reference before assuming it is a social security number.<\/p>\n We call the original “BUILDARGS” method to handle all the other cases. You should always do this in your own “BUILDARGS” methods, since Moose::Object provides its own “BUILDARGS” method that handles hash references and a list of key-value pairs.<\/p>\n The “BUILD” method is called after<\/i> the object is constructed, but before it is returned to the caller. The “BUILD” method provides an opportunity to check the object state as a whole. This is a good place to put logic that cannot be expressed as a type constraint on a single attribute.<\/p>\n In the “Person” class, we need to check the relationship between two attributes, “ssn” and “country_of_residence”. We throw an exception if the object is not logically consistent.<\/p>\n This recipe is made significantly simpler because all of the attributes are read-only. If the “country_of_residence” attribute were settable, we would need to check that a Person had an “ssn” if the new country was “usa”. This could be done with a “before” modifier.<\/p>\n We have repeatedly discouraged overriding “new” in Moose classes. This recipe shows how you can use “BUILDARGS” and “BUILD” to hook into object construction without overriding “new”.<\/p>\n The “BUILDARGS” method lets us expand on Moose\u2019s built-in parameter handling for constructors. The “BUILD” method lets us implement logical constraints across the whole object after it is created.<\/p>\n
VERSION<\/a>
SYNOPSIS<\/a>
DESCRIPTION<\/a>
MORE CONSIDERATIONS<\/a>
CONCLUSION<\/a>
AUTHORS<\/a>
COPYRIGHT AND LICENSE<\/a> <\/p>\n
\nNAME <\/a> <\/h2>\n
VERSION <\/a> <\/h2>\n
SYNOPSIS <\/a> <\/h2>\n
has ‘ssn’ => (
is => ‘ro’,
isa => ‘Str’,
predicate => ‘has_ssn’,
);
has ‘country_of_residence’ => (
is => ‘ro’,
isa => ‘Str’,
default => ‘usa’
);
has ‘first_name’ => (
is => ‘ro’,
isa => ‘Str’,
);
has ‘last_name’ => (
is => ‘ro’,
isa => ‘Str’,
);
around BUILDARGS => sub {
my $orig = shift;
my $class = shift;
if ( @_ == 1 && ! ref $_[0] ) {
return $class\u2212>$orig(ssn => $_[0]);
}
else {
return $class\u2212>$orig(@_);
}
};
sub BUILD {
my $self = shift;
if ( $self\u2212>country_of_residence eq ‘usa’ ) {
die ‘Cannot create a Person who lives in the USA without an ssn.’
unless $self\u2212>has_ssn;
}
}<\/p>\nDESCRIPTION <\/a> <\/h2>\n
return $class\u2212>$orig(ssn => $_[0]);
}<\/p>\nMORE CONSIDERATIONS <\/a> <\/h2>\n
CONCLUSION <\/a> <\/h2>\n
AUTHORS <\/a> <\/h2>\n
| <\/td>\n | \n \u2022<\/p>\n<\/td>\n | <\/td>\n | \n Stevan Little \u2022<\/p>\n<\/td>\n Dave Rolsky \u2022<\/p>\n<\/td>\n Jesse Luehrs \u2022<\/p>\n<\/td>\n Shawn M Moore \u2022<\/p>\n<\/td>\n \u00d7\u00d7\u00d7\u00d7 \u00d7\u00a7\u00d7\u00d7\u2019\u00d7\u00d7 (Yuval Kogman) \u2022<\/p>\n<\/td>\n Karen Etheridge \u2022<\/p>\n<\/td>\n Florian Ragwitz \u2022<\/p>\n<\/td>\n Hans Dieter Pearcey \u2022<\/p>\n<\/td>\n |