DecoratedCommand

API design

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This is a very common pattern and also very simple, it's really just the decorator pattern applied to commands. I've seen it used a lot with CommandOrientedInterfaces. You also hear this referred to as interceptors and as a form of Aspect Oriented Programming.

You start with some command, usually of some form of basic functionality that may need some additional function added to it later. So this might be a domain oriented command such as PayInvoice. These commands will have some kind of execute method.

// psuedo C#
class PayInvoiceCommand : Command ...
void Execute() {
  // do interesting domain logic
}

Let's say we want to do this inside a transaction. We can decorate the command with a suitable transactional decorator.

// pedudo C#
class TransactionalDecorator : CommandDecorator ...
  void Execute() {
    Transaction t = TransactionManager.beginTransaction();
    try {
      Component.Execute();
      t.commit();
    } catch (Exception) {
      t.rollback();
    }
  }
    

We can also do a security check this way

// pseduo C#
class SecurityDecorator : CommandDecorator ...
  void Execute() {
    if (passesSecurityCheck())
      Component.Execute();
  }

With these classes in place we can then easily combine them to get the right kinds of behavior.

//psuedo C#
  // Transaction Invoice Payment
  Command c = new TransactionalDecorator(new PayInvoiceCommand(invoice));
  c.Execute();
  //Transactional and secure payment
  Command c = new SecurityDecorator(
                  new TransactionalDecorator(
                      new PayInvoiceCommand(invoice)));
  c.Execute();

Indeed this ability to add behavior dynamically is one of the big benefits of a CommandOrientedInterface.

A lot of things are doing this kind of thing under the aspect oriented banner these days. At some point I'm going to dig into this more, to see if there's more than this pattern in play.

This is aspectish, but there's more to aspect oriented programming than this. In aspect terms, the decorators provide advice to the domain command's Execute method. However in order to do this, you have to organize everything around the commands, since only the Execute method can be advised. More flexible AOP tools, such as aspectJ allow you to advise any method, and indeed some other things such as field access.

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