Add $asyncComputed status property

[michaelzangl]

This patch is an improved version of #29 and adds a status property to each Vue instance that contains information about it's asyncComputed properties.

If you access $asyncComputed.myAsyncPropName on the instance, you will get an object like this:

{
  // Can be one of updating, success, error
  state: 'updating',
  // A boolean that is true while the property is updating.
  updating: true,
  // The property finished updating wihtout errors (the promise was resolved) and the current value is available.
  success: false,
  // The promise was rejected.
  error: false,
  // The raw error/exception with which the promise was rejected.
  exception: null
  // A function that, when called, forces the property to be re-calculated
  update: function() {...}
}

The boolean properties are redundant, since there already is the state property. But they make implementing loading spinners and error indicators easier.

Use like this:

<div v-if="$asyncComputed.myProperty.updating">
  Please wait, data is loading
</div>
<div v-else-if="$asyncComputed.myProperty.error">
  Error while computing the data: {{ $asyncComputed.myProperty.exception }}
  <button @click="$asyncComputed.myProperty.update()">Retry</button>
</div>
<div v-else>
  Here is your data: {{ myProperty }}
</div>

@gavinhungry wanted to be mentioned.

To test, use:

"dependencies": {
  "vue-async-computed": "fumiX/vue-async-computed#dist",
}

[foxbenjaminfox]

Thanks for the PR, this is a good and worthwhile feature.

Everything looks good, so I've merged the PR and published a new minor version of vue-async-computed that includes it, v3.5.0.