Create for(key in map) helper for stache

[mikemitchel]

It would helpful to be able to iterate through keys of a Map in stache, and have stache do the DOM diffing if a key gets added, removed, or the key's value changes. The order of the keys inserted to the underlying Map(s) would need to be preserved to work in this use case, a Message Log.

For example, I have traceLog Map --> pageName Map--> message Map --> fragments List

data:

traceLog: { // map
  pageName: { // map
    messageKey: { // map
      fragments: [ // list
        {format: 'var', msg: 'first name'},
        {msg: ' = '},
        {format: 'val', msg: 'Max'}
      ]
    }
  }
}

stache:

{{# for(page in traceLog) }} -> map iteration
    render page stuff
  {{# for(message in page) }} -> map iteration
       render message stuff
     {{# for(fragment of message.fragments) }} -> list iteration
        render fragment stuff
      {{/for}}
  {{/for}}
{{/for}}

this would likely involve some changes in the way can-view-live handles patches as per discussion with @justinbmeyer

[matthewp]

What's the problem with for/of exactly?

[mikemitchel]

I'm looking for a few things here @matthewp does for/of do all these?:

• underlying Map retains order of inserted keys so iteration order can be assured in stache (this is likely ok already, but not guaranteed like an ES6 Map)
• if a key is added/removed from the Map, or the value for that key is updated, stache should only re-render that diff
• access to the current key for that Map, so for (page in traceLog) above would allow me to do {{page}} in the stache. I guess you would have to do {{let value = traceLog[page]}} then to use the value, unless the helper worked like {{for (pageKey, pageValue of traceLog)}} to capture both the key and value for use in the stache.