Implementing the Decorator Pattern

[hbarcelos]

I managed to implement the something that resembles the Decorator Pattern using composers:

const MyStamp = stampit({
  name: 'MyStamp',
  init({ foo }, { stamp, args, instance }) {
    const { configuration, deepConfiguration } = stamp.compose;
    this.foo = foo;
  },
  methods: {
    greet({ greeting = 'Hi' } = {}) {
      console.log(`${greeting} from ${this.foo}`)
    },
  },
  props: { foo: null },
});

const MyStampDecorator = stampit.compose(MyStamp, {
  name: 'MyStampDecorator',
  composers({ stamp }) {
    const { greet } = stamp.compose.methods;

    Object.assign(stamp.compose.methods, {
      greet(...args) {
        greet.apply(this, args);
        console.log('this is new!!!');
      }
    })
  }
});

const instance = MyStampDecorator({ foo: 'bar' });
instance.greet({ greeting: 'Hello' });
// yields:
// Hello from bar
// this is new!!!

However, as stated in the docs:

Composers are a very powerful feature. If you can implement something without composers then it's a good idea to avoid them.

It's also possible to achieve the same behavior with init:

const MyStampDecorator = stampit.compose(MyStamp, {
  name: 'MyStampDecorator',
  init(_, { stamp }) {
    const { greet } = stamp.compose.methods;

    Object.assign(stamp.compose.methods, {
      greet(...args) {
        greet.apply(this, args);
        console.log('this is new!!!');
      }
    })
  }
});

I'm new to Stamps, but AFAIK, the difference between the two implementations is that the former will be more memory efficient, since it will modify the [[Prototype]] of the instance at the time of composition, while the later will create a new object with the greet method on each instantiation.

Am I right about this???

Is there any other/recommended way to implement the Decorator Pattern using stamps?

Any ideas on how to generalize decorating stamp methods?

---

Edit:

I found out that my implementation using composers does not work as expected if I do another composition:

const AnotherStamp = stampit.compose(MyStampDecorator);

const instance = AnotherStamp({ foo: 'bar' });
instance.greet({ greeting: 'Hello' });
// yields:
// Hello from bar
// this is new!!!
// this is new!!! <-- this second log shouldn't happen 

If I use init instead, it works just fine.

[koresar]

Hi Henrique.

You did it all right. That's the right way to implement the decorator pattern. To fix the little bug you can just mark your wrapper function with a flag or something.

const GreetDecorator = stampit.composers(({ stamp }) => {
    const { greet } = stamp.compose.methods || {};
    if (!greet || greet.wrapped) return;

    function greetWrapper(...args) {
      greet.apply(this, args);
      console.log('this is new!!!');
    }
    greetWrapper.wrapped = true;
    stamp.compose.methods.greet = greetWrapper;
  }
});

[koresar]

Side tip. You can make the method decorator stamp more universal and reusable via static methods.

const MethodDecorator = stampit.statics({
    decorateMethod(name, func) {
      return this.composers(({ stamp }) => {
        const method = stamp.compose.methods || stamp.compose.methods[name];
        if (!method || method.wrapped) return;

        function methodWrapper(...args) {
          method.apply(this, args);
          func.apply(this, args);
        }
        methodWrapper.wrapped = true;
        stamp.compose.methods.method = methodWrapper;
      }
    })
});

And usages:

MyStamp = MyStamp.compose(MethodDecorator).decorateMethod("greet", function () {
  console.log('this is new!!!');
});

The above code is not tested :)

[koresar]

Closing this issue considering all questions are answered. Feel free to reopen any time.

And thanks for the code. :) It sounds like a new Fun With Stamps episode.

[hbarcelos]

Hi @koresar. Sorry for the long delay, things have been pretty crazy this year.

Your suggestions helped me a lot. I think I managed to pull a implementation that works very well for most use cases.

I was about to start writing some documentation before publishing it. As soon as I finish it, I'll post it here.

Thank you very much.

[koresar]

No worries at all mate! I need nothing from you. Just be happy! 😀

On Mon., 16 Sep. 2019, 09:04 Henrique Barcelos, notifications@github.com wrote:

Hi @koresar https://github.com/koresar. Sorry for the long delay, things have been pretty crazy this year.

Your suggestions helped me a lot. I think I managed to pull a implementation that works very well for most use cases.

I was about to start writing some documentation before publishing it. As soon as I finish it, I'll post it here.

Thank you very much.

— You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub https://github.com/stampit-org/stampit/issues/340?email_source=notifications&email_token=AAMMEL72CQOHUL5UBS4LJ23QJ25RNA5CNFSM4GTDLBQ2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOD6X24PQ#issuecomment-531607102, or mute the thread https://github.com/notifications/unsubscribe-auth/AAMMEL5M36GI3GNBWTHE4HDQJ25RNANCNFSM4GTDLBQQ .