This is a followup to an earlier post.

After pushing that hack out to the rest of my team, I felt some modicum of pride - I had, through understanding little-used parts (at least, publicly) of a poorly-documented framework, managed to reason out a nice solution.

Days passed and the codebase expanded. One day a teammate approached me and said, "Hey, you know that trick you used to pass the ID into the controller?" "Yes," I replied cautiously. "You didn't need it."

Wat.

As a refresher, here's what I had ended up implementing:

<div data-ng-controller="ParentController">
    <div data-ng-repeat="childGuid in childrenGuids" data-ng-controller="ChildController" data-id="{{childGuid}}"/>
</div>

... which had some companion JavaScript in the repeated controller that read the value of data-id when the view was updated. There was no other way to pass data in to the ChildController, right? Wrong. Let's step back for a second.

Scope in Angular is special. While the framework goes out of its way to make it seem like things are tightly bound to the encompassing controller and that the DOM merely exists for presentation purposes, nothing (especially in web development) is ever that cut and dry.

Quoth the Angular documentation:

Scopes are arranged in hierarchical structure which mimic the DOM structure of the application.

and...

The Model is scope properties; scopes are attached to DOM where scope properties are accessed through bindings.

As my coworker reminded me, the scope referenced within the ChildController isn't bound to the controller, it's bound to the DOM. And, in the case of the repeater, the scope on the repeater node includes the childGuid property.

What the code looks like now...

<div data-ng-controller="ParentController">
    <div data-ng-repeat="childGuid in childrenGuids" data-ng-controller="ChildController"/>
</div>

var ChildController = function() {
    $scope.id = $scope.childGuid;

    var init = function() {
        // do initialization
    };

    init();
};

As you can see, on the second line of the controller, I'm directly referencing the repeated element. You don't need to re-assign it like I'm doing in this example, by the way, calling it directly won't/shouldn't present any issues.

An important caveat: if you think the controller might get used elsewhere, it would be beneficial to guard against instantiation without the required arguments...

var ChildController = function($exceptionHandler) {
    $scope.id = '';

    var init = function() {
        if (typeof $scope.childGuid === "undefined") {
            $exceptionHandler("The ChildController must be initialized with a childGuid in scope");
        }
        $scope.id = $scope.childGuid;
        // continue iniitialization
    };

    init();
};

And with that my hack was dead. This was a good thing as hacks are, by nature, smells that should be minimized.

The moral of the story? Write self-congratulatory blog posts about slightly-hacky solutions. Someone will submit a patch.

Don't use this code, instead, read this post and then read the follow-up.

Since my last post I've switched teams at work after completing a massive redesign of all of my old team's webapps. The overhaul saw the unification of the look and feel of the product's core webapps, with a significant amount of instrumentation and modularity to enable the less aesthetically-inclined people within the team to contribute without reducing the overall coherence of the application.

It just so happens that I joined my new team just as they were planning a massive redesign of their frontend! After assessing our users' needs and desires, we set out to redesign our stack. I'm not going to go into the full details of the transition in this post, but we settled on a Python stack running AngularJS, Flask, uWSGI and nginx (a huge improvement, development-wise, from our current JQueryUI, TurboGears, mod_wsgi and Apache system).

Prior to this redesign effort, my only experiences with client side MVC frameworks were with KnockoutJS, a great JavaScript library for manging databinding. However, when you scaled it up to include things like validation, complex objects, RESTful behavior against remote resources, and DOM manipulation, it really started to show its seams. Part of what attracted me to Angular was its similar approach to data binding, but with a stricter separation of concerns between the controller and view layers.

I could continue to wax poetic about our decision, but I'll save that for another post. I just wanted to provide some backstory for this post.

Just the other day I found myself writing a controller that instantiates new child controllers via a repeater...

<div data-ng-controller="ParentController">
    <div data-ng-repeat="childGuid in childrenGuids" data-ng-controller="ChildController" />
</div>

This worked fine. Pushing a new item to ParentController's $scope.children array would generate a new ChildController. However, I soon discovered that what I was attempting to engineer would require each ChildController to have a unique ID, generated by the ParentController prior to the child's instantiation. I attempted to use templating to my advantage in various ways, for example:

<div data-ng-controller="ParentController">
    <div data-ng-repeat="childGuid in childrenGuids" data-ng-controller="ChildController">
        <input type="hidden" value="{{childGuid}}" />
    </div>
</div>

No dice. Why not try broadcasting from the parent scope after I push something to the stack? I feared race conditions. Why not a directive? It felt like too much code for something that should be simple. After reading the relevant portions of the Angular docs several times over, I threw together a nice little hack that got the job done.

<div data-ng-controller="ParentController">
    <div data-ng-repeat="childGuid in childrenGuids" data-ng-controller="ChildController" data-id="{{childGuid}}"/>
</div>

var ParentController = function() {
    $scope.childrenGuids = [];

    $scope.addChild = function() {
        var guid = newGuid();
        $scope.childrenGuids.push(guid);
    };
};

var ChildController = function() {
    $scope.id = '';

    var init = function() {
        //finish initialization
    };

    $attrs.$observe('id', function(value) {
        if (!$scope.id) { //defensive sanity check
            $scope.id = value;
            init();
        }
    });
};

Essentially, what I'm doing is passing the ID in via the controller declaration. However, because of the order in which Angular digests the markup, interpolation of templated attributes happens after the construction of the controller. Because of that, I had to instruct the framework to do some extra processing when the interpolation event fired and the data-id attribute was updated. After that the controller is free to finish its initialization phase.

Happy with the outcome, I pushed my changes out to my team and made a note to revisit this block when I get around to refactoring this section (it'll happen, don't worry).

EDIT: It happened.