add requries feature into Voson::Plugin

lib/Voson/Plugin.pm

@@ -6,6 +6,7 @@ use Carp;
 sub new {
     my ($class, %opts) = @_;
     $class->_check_needs($opts{app});
+    $class->_check_requires($opts{app});
     return bless {%opts}, $class;
 }
 
@@ -21,6 +22,8 @@ sub exports {
 
 sub needs { return () }
 
+sub requires { return () }
+
 sub _check_needs {
     my ($class, $app) = @_;
     local $Carp::CarpLevel = $Carp::CarpLevel + 1;
@@ -30,6 +33,14 @@ sub _check_needs {
     }
 }
 
+sub _check_requires {
+    my ($class, $app) = @_;
+    local $Carp::CarpLevel = $Carp::CarpLevel + 1;
+    for my $requires ($class->requires) {
+        croak "$class requires $requires DSL, you have to load some plugin that provides $requires DSL" unless $app->{DSL}{$requires};
+    }
+}
+
 1;
 
 __END__
@@ -72,7 +83,13 @@ You have to override it if you want to export some DSL.
 
 Specifier for needs plugins.
 
-You have to override it if you want to export some DSL.
+=head2 requires
+
+    sub requires {
+        return qw/dsl_a dsl_b/;
+    }
+
+Specifier for required DSLs.
 
 =head1 AUTHOR
 

t/model/010_plugin.t

@@ -23,6 +23,14 @@ use Voson::Core;
         }; 
     };
 }
+{
+    package Voson::Plugin::TestSeta;
+    use parent 'Voson::Plugin';
+    sub requires { qw/one/ };
+    sub two {
+        sub ($) { one() + 1 };
+    }
+}
 
 subtest basal => sub {
     my $x = Voson::Plugin->new;
@@ -45,4 +53,11 @@ subtest needs_ok => sub {
     is $v->dsl('incr')->(2), 3;
 };
 
+subtest requires_failure => sub {
+    throws_ok(
+        sub { Voson::Core->new(plugins => [qw/TestSeta/]) },
+        qr/Voson::Plugin::TestSeta requires one DSL, you have to load some plugin that provides one DSL/
+    );
+};
+
 done_testing;