Add declarative way to set up observers and event listeners

[PaulMaly]

I think we really need to have a declarative way to set up observers and event listeners. It could be implemented just like Ractive does:

observe: {
    show ( value, old ) {

    },
    users: {
      handler ( value, old ) {

      },
      init: false,
      strict: true
    }
  },
  on: {
    create () {

    },
    somethingHappened: {
      handler ( ctx ) {
      },
      once: true
    }
  }
}

[PaulBGD]

I think adding event listeners is super useful, as most event listeners I use in a component immediately call a method in methods anyways, so I really like the idea of just telling it which event listener to use (and also being able to pass options is important!)

[PaulMaly]

@Rich-Harris Any updates here? I think this solution gives us the way to keep our oncreate hook is clean.

[jacwright]

If we implemented on would we move oncreate and ondestroy into it if they are still needed?

export default {
  on: {
    create() {
      // setup
    },
    destroy() {
      // setup
    }
  }
};

I like the observe. Not sure about on. And I'm getting along fine without either so far and not feeling my code is too messy.

[PaulMaly]

@jacwright In Ractive all life-cycle hooks available like regular custom events. And able to be listened from the parent:

<Widget on:init="..." /> 

[Rich-Harris]

Closing as this isn't applicable to Svelte 3