API to track dependencies

[andres-kovalev]

At the moment we have createAtom() API to build custom observables. It would be nice to have something similar to track dependencies to build custom computeds/reactions. There is a computed() method to create computed from an expression (function), but it only requires to track dependencies inside a certain function call when sometimes it's essential to track dependencies (and react) across several execution contexts. For instance, when the total computation is divided in several separate independent steps for optimisation (e.g. fibers) or the flow consist of several consecutive steps (e.g. async flow).

It would be nice to have an abstraction to collect dependencies and react on updates:

// super raw API example
interface ExecutionContext {
  // start tracking dependencies
  start(): void;
  // pause tracking dependencies for a while
  pause(): void;
  // resume tracking dependencies
  resume(): void;
  // stop tracking dependencies
  stop(): void;
  // reset dependencies list and unobeseve current dependencies
  release(): void;
}

// example computed() implementation
function computed(fn) {
  let isObsolete = true;
  const onInvalidate = () => isObsolete = true;
  // flag indicates whether we create a reaction (with reactive context) or not
  const context = createExecutionContext('...', false, onInvalidate);

  let isObserved = false;

  const onBO = () => isObserved = true;
  const onBUO = () => { isObserved = false; context.release(); }
  const atom = createAtom('...', onBO, onBUO);

  let cache;

  return () => {
    atom.reportObserved();
    if (isObserved && !isObsolete) return cache;
    context.start();
    const value = fn();
    context.stop();
    if (isObserved) cache = value;
    return value;
  }
}

WDYT?