Saturday, April 12, 2014

Binding JSON to Polymer Attributes

So how do you pass list data into a Polymer element?

I was pretty darn certain that starting with a smaller <x-pizza-toppings> element in the model driven view chapter of Patterns in Polymer would be brilliant. And then I tried using it.

Adding it the page works just fine:

But this was originally meant to be a composable, internal-only element for use in a whole pizza builder element, <x-pizza>. To ensure that all topping elements (first half, second half, whole pizza) used the same list of ingredients, I created the list in <x-pizza>:
class XPizza extends PolymerElement {
  final List ingredients = [
    'green peppers'
  // ...
And then bound that variable to all instances of <x-pizza-toppings> in the corresponding template:
<link rel="import" href="x-pizza-toppings.html">
<polymer-element name="x-pizza">
    <!-- ... -->
    <x-pizza-toppings id="firstHalfToppings"
                      name="First Half Toppings"
    <!-- ... -->
  <script type="application/dart;component=1" src="x_pizza.dart"></script>
But how do you do that from the outside when you don't have a List, Array or Object? How can I bind a list of strings (the ingredients) to the ingredients attribute of <x-pizza-toppings>?

I may have found the answer by accident three nights ago. While fiddling with Polymer.dart JavaScript interoperablity, I found that the only way to make certain things work was to bind attributes as JSON instead of native programming types. Perhaps that will work here?
         ingredients="['pepperoni', 'sausage', 'green peppers']">
And indeed that does work:

Cool! But does it work in the JavaScript version of Polymer as well? Yup:

It makes sense that Polymer would behave this way, but it is good to have confirmation. I cannot find any documentation for this behavior, but binding JSON seems a reasonable thing to do. And, since it behaves like this in both Dart and JavaScript, it seems safe to use this feature.

So, phew! I can finish off the MDV chapter rewrite without any further worries. Well, until I try the next easy thing...

Day #32

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