feat(ngScopeAs): directive to save the current scope as an alias

[shahata]

This is useful when we need to access an outer scope in nested ngRepeat and generally in many of the places $parent is used to access an outer scope.

Closes #8398

[shahata]

This can be used to do things like:

<tr ng-repeat="row in rows" ng-scope-as="rowItems">
  <td ng-repeat="col in row.columns" ng-scope-as="colItems">
    {{ rowItems.$index }}.{{ colItems.$index }} - {{ col.value }}
  </td>
</tr>

But there are other cases where ppl use $parent to access an outer scope, which this can make more readable and safe. I've copied the ngMinErr bit from #8440 since this directive is similar in that it also requires the expression to be assignable.

/cc @caitp @petebacondarwin WDYT?

[caitp]

well I mean, $scope already has a built-in alias this, so it seems like you could just ng-init="alias = this" and that would be good enough, wouldn't it?

[caitp]

You would need something like this for that use case once we actually remove assignment expressions, but I wouldn't really count on that ever happening

[shahata]

Yes. In fact the documentation says that the only reason anyone would need to use ng-init is for working with nested ngRepeat. But I'm thinking that since we don't want to encourage ppl to use the assignment operator in expressions, maybe this directive can help deprecate ngInit.

[shahata]

Well, even if the assignment operator is not going away any time soon, I think we should still try and move away from it. The as alias syntax in ngRepeat is also possible to do with the assignment operator (ng-repeat="item in (result = items | filter:x)"), but I think giving the right tool is better.

[caitp]

Personally, I don't think it makes a lot of sense to deprecate assignment expressions, because they're frankly very useful --- but if we do we do. However, if the alternative is inventing a bunch of new tools to do the assignment for us, that doesn't seem that useful to me --- it would create more to maintain rather than less.

[shahata]

Well, that's the whole imperative vs declarative debate I guess. Too late for that, going to sleep :)

[rodyhaddad]

The as alias made sense in ngRepeat because the directive already had its own micro-syntax.

I don't think adding a new ngScopeAs directive is the right solution. ngInit with assignments isn't that bad

[petebacondarwin]

I think I agree that this is not a good addition to core. It is fairly to implement in one's own app if needed. But I suspect that controller as, ngRepeat's alias as and ngInit provide enough by themselves.

[shahata]

okay, closing then