Provided by: libur-perl_0.450-1_all 
NAME
UR::Role - Roles in UR, an alternative to inheritance
SYNOPSIS
package My::Role;
role My::Role {
id_by => [
role_id_property => { is => 'String' },
],
has => [
role_property => { is => 'String' },
another_prop => { is => 'Integer' },
},
requires => ['class_method'],
excludes => ['Bad::Role'],
};
sub role_method { ... }
package My::Class;
class My::Class {
has => [
class_property => { is => 'Integer ' },
],
roles => ['My::Role'],
};
sub class_method { ... }
my $obj = My::Class->new();
$obj->does('My::Role'); # true
DESCRIPTION
Roles are used to encapsulate a piece of behavior to be used in other classes. They have properties and
methods that get melded into any class that composes them. A Role can require any composing class to
implement a list of methods or properties.
Roles are not classes. They can not be instantiated or inherited from. They are composed into a class
by listing their names in the "roles" attribute of a class definition.
Defining a Role
Roles are defined with the "role" keyword. Their definition looks very similar to a class definition as
described in UR::Object::Type::Initializer. In particular, Roles have a "has" section to define
properties, and accept many class-meta attributes such as 'id_generator', 'valid_signals', and 'doc'.
Roles may implement operator overloading via the 'use overload' mechanism.
Roles also have unique atributes to declare restrictions on their use.
requires
A listref of property and method names that must appear in any class composing the Role. Properties
and methods defined in other roles or parent classes can satisfy a requirement.
excludes
A listref of Role names that may not be composed together with this Role. This is useful to declare
incompatibilities between roles.
Composing a Role
Compose one or more Roles into a class using the 'roles' attribute in a class definition.
class My::Class {
roles => ['My::Role', 'Other::Role'],
is => ['Parent::Class'],
has => ['prop_a','prop_b'],
};
Properties and meta-attributes from the Roles get copied into the composing class. Subroutines defined
in the Roles' namespaces are imported into the class's namespace. Operator overloads defined in the
Roles are applied to the class.
Property and meta-attribute conflicts
An exception is thrown if multiple Roles are composed together that define the same property, even if the
composing class defines the same property in an attempt to override them.
A class may declare a property with the same name that a role also declares. The definition in the class
overrides whatever appears in the role. An exception is thrown if a role declares an ID property in the
'id_by' section and the consuming class redeclares it in the 'has' section as a normal property.
Method conflicts
An exception is thrown if multiple Roles are composed together that define the same subroutine, or if the
composing class (or any of its parent classes) defines the same subroutine as any of the roles.
If the class wants to override a subroutine defined in one of its roles, the override must be declared
with the "Overrides" attribute.
sub overridden_method : Overrides(My::Role, Other::Role) { ... }
All the conflicting role names must be listed in the override, separated by commas. The class will
probably implement whatever behavior is required, maybe by calling one role's method or the other, both
methods, neither, or anything else.
To call a function in a role, the function's fully qualified name, including the role's package, must be
used.
Overload conflicts
Like with method conflicts, an exception is thrown if multiple Roles are composed together that overload
the same operator unless the composing class also overloads that same operator.
An exception is also thrown if composed roles define incompatible 'fallback' behavior. If a role does
not specify 'fallback', or explicitly sets it to "undef", it is compatible with other values. A Role
that sets its 'fallback' value to true or false is only compatible with other roles' values of undef or
the same true or false value.
__import__
Each time a Role is composed into a class, its "__import__()" method is called. "__import__()" is passed
two arguments:
• The name of the role
• The class metadata object composing the role.
This happens after the class is completely constructed.
Parameterized Roles
Scalar variables with the "RoleParam" attribute are designated as role params. Values can be supplied
when a role composes the role as a means to provide more flexibility and genericity for a role.
package ObjectDisplayer;
use ProjectNamespace;
our $target_type : RoleParam(target_type);
role ObjectDisplayer {
has => [
target_object => { is => $target_type },
],
};
package ShowCars;
class ShowCars {
roles => [ ObjectDisplayer->create(target_type => 'Car' ],
};
When the role is composed, the call to "create()" in the class definition creates a UR::Role::Instance to
represent the ObjectDisplayer role being composed into the ShowCars class with the params "{ target_type
=" 'car' }>. Values for the role param values in the role definition are swapped out with the provided
values as the role's properties are composed into the class.
At run-time, these role param variables are tied with the UR::Role::Param class. Its "FETCH" method
searches the call stack for the first function whose invocant composes the role where the variable's
value is being fetched from. The proper param value is returned.
An exception is thrown if a class composes a role and either provides unknown role params or omits values
for existing params.
Method Modifiers
Roles can hook into methods defined in consuming classes by using the "before", "after" and "around"
method modifiers.
use UR;
package RoleWithModifiers;
use UR::Role qw(before after around);
role RoleWithModifiers { };
before 'do_something' => sub {
my($self, @params) = @_;
print "Calling do_something with params ",join(',',@params),"\n";
};
after 'do_something' => sub {
my($rv, $self, @params) = @_;
print "Result from do_something: $rv\n";
};
around 'do_something' => sub {
my($orig, $self, @params) = @_;
print "Wrapped call to do_something params ",join(',',@params),"\n";
my $rv = $self->$orig(@params);
print "The wrapped call to do_something returned $rv\n";
return 123;
};
package ClassUsingRole;
class ClassUsingRole { roles => 'RoleWithModifiers' };
sub do_something {
print "In original do_something\n";
return 'abc';
}
my $rv = ClassUsingRole->create()->do_something();
print "The call to do_something returned $rv\n";
Running this code will generate the following output:
Wrapped call to do_something params
Calling do_something with params
In original do_something
Result from do_something: abc
The wrapped call to do_something returned abc
The call to do_something returned 123
Method modifiers are applied in the order they appear in the role's implementation.
before(@params)
A "before" modifier runs before the named method. It receives all the arguments and "wantarray"
context as the original method call. It cannot affect the parameters to the original method call,
and its return value is ignored.
after($rv, @params)
The first argument to an "after" modifier is the return value of the original method call, the
remaining arguments and "wantarray" context are the same as the original method call. If the
original method was called in list context, then $rv will be an arrayref containing the list of
return values. This modifier's return value is ignored.
around($orig, @params)
An "around" modifier is run in place of the original method, and receives a coderef of the original
method as its first argument. Around modifiers can munge arguments and return values, and control
when and whether the original method is called.
SEE ALSO
UR, UR::Object::Type::Initializer, UR::Role::Instance, UR::Role::Param