Simple Reducer property behavior

[justinbmeyer]

I'd like something like the following to work:

DefineMap.extend({
  name: "string"
  nameChangeCount: {
    default: 0,
    reduce: {
        "name": (last, nameEvent)=> last + 1; 
    }
  }
})

Essentially, the reducer would specify events to bind to and a corresponding function that gets the last value, event emitted and returns the new value. Here's how fullName could be built without a getter (and https://github.com/canjs/can-define/pull/434 merged) :

DefineMap.extend({
  first: "string",
  last: "string",
  nameParts: {
    default: [],
    reduce: {
        "first": ( [first, last], {newValue} ) => [newValue, last],
        "last": ( [first, last], {newValue} ) => [first, newValue],
    }
  },
  fullName: {
    default: "",
    reduce: {
        nameParts: (last, {newValue} )=> newValue.join(" ")
    }
  }
})

Notes / Questions

• The functions should be called with the last value and all arguments used to dispatch the event. This makes it possible to have a funciton like (last, event, newValue, oldValue) => {}.

• Should "deep" values be supported? For example reduce: { "task.length": ()=>{} }.

• How should the set values be treated? Perhaps a reserved name?

  fullName: {
     default: "",
     reduce: {
        $lastSet: (last, newValue) => newValue,
        nameParts: (last, {newValue} )=> newValue.join(" ")
     }
  }

• Could a get be used as a "final" stage and avoid intermediate steps?

  DefineMap.extend({
   first: "string",
   last: "string",
   fullName: {
     default: [],
     reduce: {
      "first": ( [first, last], {newValue} ) => [newValue, last],
      "last": ( [first, last], {newValue} ) => [first, newValue],
     },
     get: (reducedValue) => reducedValue.join(" ")
   }
  })

• Can these "reducers" be shared and tested somehow? Or would we need can-reducer for this?

How to build this

We can add a reducer observable that would work like:

new ReducerObservable({
  "first": ( [first, last], {newValue} ) => [newValue, last],
   "last": ( [first, last], {newValue} ) => [first, newValue],
}, this, 0 )

This can be mixed in similar to how ResolverObservable is here: https://github.com/canjs/can-define/blob/10d906817e238286acfcf3a1d5120232232f0983/can-define.js#L455

ReducerObservable would be a constructor function very similar in API to ResolverObservable, with the right symbols:

Reducer.prototype = {
    [getValue]()
    [setValue]()
    [onValue]() 
    [offValue]()
}