hyperion-hook

Hyperion-Hook is one of the fundumental concepts in hyperion. It provide a very fast and JIT friendly mechanism to maintain a list of functions to be called sequenctially.

This mechansim can be used to implement callbacks, transformers, ...

All interceptors in hyperion-core rely on the Hook

The Hook class has a type parameter that determines the type of the function it represents.

import { Hook } from "@hyperion/hyperion-hook";

const h1 = new Hook<(i: number)=>void>();
h1.add(()=> console.log('Called'));
h1.add(i => console.log('with argument', i));
h1.call(42);

When a Hook has 0 or 1 callbacks, it is easy to understand what it does. However, for more than 1 callback, we might need to create special logic. This is specially true if the function return a value and act as serries of transformers.

The Hook as a protected createMultiCallbackCall method that you can overload and add your own special behavior.

import { Hook } from "@hyperion/hyperion-hook";

type CallbackType = (i: number)=> number;
class MyHook extends Hook<CallbackType>{
   protected createMultiCallbackCall(callbacks: CallbackType[][): CallbackType {
    return function (this, args) {
      let result = args;
      for (let i = 0, len = callbacks.length; i < len; ++i) {
        result = callbacks[i].call(this, result);
      }
      return result;
    }
  }
}

const h1 = new MyHook();
h1.add(i => i + 1);
h1.add(i => i * 2);
const result = h1.call(20);
console.log(result); // (20 + 1) * 2 = 42

Hook vs Event

Hook enables message bassing mechanism for event driven system. However, we call it a hook and not an event because:

• Usually events are asynchrounous, but hooks are synchroneously called
• Hook can reutnr a value and impact the execution of the system, while events by definition are now supposed to have impact on the normal flow of execution

This distinction is commonly accepted in other frameworks as well.