How to access parent scope from within a custom directive *with own scope* in AngularJS?

Question

I'm looking for any manner of accessing the "parent" scope within a directive. Any combination of scope, transclude, require, passing in variables (or the scope itself) from above, etc. I'm totally willing to bend over backwards, but I want to avoid something totally hacky or unmaintainable. For example, I know I could do it right now by taking the $scope from the preLink parameters and iterating over it's $sibling scopes to find the conceptual "parent".

What I really want is to be able to $watch an expression in the parent scope. If I can do that, then I can accomplish what I'm trying to do over here: AngularJS - How to render a partial with variables?

An important note is that the directive must be re-usable within the same parent scope. Therefore the default behavior (scope: false) doesn't work for me. I need an individual scope per instance of the directive, and then I need to $watch a variable that lives in the parent scope.

A code sample is worth 1000 words, so:

app.directive('watchingMyParentScope', function() {
    return {
        require: /* ? */,
        scope: /* ? */,
        transclude: /* ? */,
        controller: /* ? */,
        compile: function(el,attr,trans) {
            // Can I get the $parent from the transclusion function somehow?
            return {
                pre: function($s, $e, $a, parentControl) {
                    // Can I get the $parent from the parent controller?
                    // By setting this.$scope = $scope from within that controller?

                    // Can I get the $parent from the current $scope?

                    // Can I pass the $parent scope in as an attribute and define
                    // it as part of this directive's scope definition?

                    // What don't I understand about how directives work and
                    // how their scope is related to their parent?
                },
                post: function($s, $e, $a, parentControl) {
                    // Has my situation improved by the time the postLink is called?
                }
            }
        }
    };
});

Answer 1

See What are the nuances of scope prototypal / prototypical inheritance in AngularJS?

To summarize: the way a directive accesses its parent ($parent) scope depends on the type of scope the directive creates:

1. default (scope: false) - the directive does not create a new scope, so there is no inheritance here. The directive's scope is the same scope as the parent/container. In the link function, use the first parameter (typically scope).

2. scope: true - the directive creates a new child scope that prototypically inherits from the parent scope. Properties that are defined on the parent scope are available to the directive scope (because of prototypal inheritance). Just beware of writing to a primitive scope property -- that will create a new property on the directive scope (that hides/shadows the parent scope property of the same name).

3. scope: { ... } - the directive creates a new isolate/isolated scope. It does not prototypically inherit the parent scope. You can still access the parent scope using $parent, but this is not normally recommended. Instead, you should specify which parent scope properties (and/or function) the directive needs via additional attributes on the same element where the directive is used, using the =, @, and & notation.

4. transclude: true - the directive creates a new "transcluded" child scope, which prototypically inherits from the parent scope. If the directive also creates an isolate scope, the transcluded and the isolate scopes are siblings. The $parent property of each scope references the same parent scope.
   Angular v1.3 update: If the directive also creates an isolate scope, the transcluded scope is now a child of the isolate scope. The transcluded and isolate scopes are no longer siblings. The $parent property of the transcluded scope now references the isolate scope.

The above link has examples and pictures of all 4 types.

You cannot access the scope in the directive's compile function (as mentioned here: https://github.com/angular/angular.js/wiki/Dev-Guide:-Understanding-Directives). You can access the directive's scope in the link function.

Watching:

For 1. and 2. above: normally you specify which parent property the directive needs via an attribute, then $watch it:

<div my-dir attr1="prop1"></div>

scope.$watch(attrs.attr1, function() { ... });

If you are watching an object property, you'll need to use $parse:

<div my-dir attr2="obj.prop2"></div>

var model = $parse(attrs.attr2);
scope.$watch(model, function() { ... });

For 3. above (isolate scope), watch the name you give the directive property using the @ or = notation:

<div my-dir attr3="{{prop3}}" attr4="obj.prop4"></div>

scope: {
  localName3: '@attr3',
  attr4:      '='  // here, using the same name as the attribute
},
link: function(scope, element, attrs) {
   scope.$watch('localName3', function() { ... });
   scope.$watch('attr4',      function() { ... });