perl Moose::Cookbook::Basics::Recipe2

A simple BankAccount example

 package BankAccount;
  use Moose;
  
  has 'balance' => (isa => 'Int', is => 'rw', default => 0);
  
  sub deposit {
      my ($self, $amount) = @_;
      $self->balance($self->balance + $amount);
  }
  
  sub withdraw {
      my ($self, $amount) = @_;
      my $current_balance = $self->balance();
      ($current_balance >= $amount)
          || confess "Account overdrawn";
      $self->balance($current_balance - $amount);
  }
  
  package CheckingAccount;
  use Moose;
  
  extends 'BankAccount';
  
  has 'overdraft_account' => (isa => 'BankAccount', is => 'rw');        
  
  before 'withdraw' => sub {
      my ($self, $amount) = @_;
      my $overdraft_amount = $amount - $self->balance();
      if ($self->overdraft_account && $overdraft_amount > 0) {
          $self->overdraft_account->withdraw($overdraft_amount);
          $self->deposit($overdraft_amount);
      }
  };

知识点：
属性的类型可以是另外一个类的名字
新modifier before，他在父类函数之前调用定义的sub
相当于

  sub withdraw {
      my ($self, $amount) = @_;
      my $overdraft_amount = $amount - $self->balance();
      if ($self->overdraft_account && $overdraft_amount > 0) {
          $self->overdraft_account->withdraw($overdraft_amount);
          $self->deposit($overdraft_amount);
      }
      $self->SUPER::withdraw($amount);
  }