On Mon, Jun 2, 2014 at 11:13 AM, Scott Miles <[email protected]> wrote:
> When two properties are bound together we must merge two sources-of-truth
> into one, which entails inherent conflicts. The rule has always been that
> if one of the two properties is _valued_ (not null or undefined), then it
> becomes the new shared value, regardless of direction.
>
I understand why undefined is considered not valued in JavaScript, but how
come null is not? Null seems to be a completely valid value. I know that
bindings are two-way by default, but they aren't really symmetric and I
expect the data flow to be biased to go from model to element, so in this
case I would expect the element property to be initialized to null.
Another issue is that not all expressions are assignable, and this behavior
results in initialization trying to assign to non-assignable expressions
and limits our ability to warn users.
Consider a binding like <x-foo a="{{ x + 1 }}">. If x is null, the
expression is null, and initialization will try and fail to make the
assignment x + 1 = a. We like to warn when assigning to a non-assignable
expression, because if you write <input value="{{ x + 1 }}"> this is likely
a mistake. But since any binding may now attempt to assign to an expression
that warning becomes console spam. Now I think we are forced to remove that
warning.
> Recently we decided to also prevent transitions between undefined and null
> from being reported as `*Changed`. It's true this is a lossy change, but it
> prevents a lot of unwanted side-effects at construction time where various
> properties tend to be in either state, but are almost always indicative of
> an _uninitialized_ state.
>
>
>
> On Mon, Jun 2, 2014 at 10:32 AM, 'Justin Fagnani' via Polymer <
> [email protected]> wrote:
>
>> I came across the following code in Polymer the other day:
>>
>> // capture A's value if B's value is null or undefined,
>> // otherwise use B's value
>> function resolveBindingValue(oldValue, value) {
>> if (value === undefined && oldValue === null) {
>> return value;
>> }
>> return (value === null || value === undefined) ? oldValue : value;
>> }
>>
>>
>> https://github.com/Polymer/polymer-dev/blob/master/src/instance/properties.js#L167
>>
>> The last line is what interests me. From where this is called in
>> bindProperty(), it means that if a model property being bound to is null,
>> it's set to the element property. Otherwise the flow is from model to
>> element by default, if the binding is not two way.
>>
>> What's the reasoning behind this special initialization path? The
>> behavior is a bit surprising to me. When my model's property that I'm
>> binding to is null, I don't expect any special initialization behavior.
>> Especially since null might be a valid value for the model property. This
>> also makes all bindings potentially two way, even when you think you know
>> that the element property you're binding to doesn't ever update.
>>
>> Thanks,
>> Justin
>>
>> Follow Polymer on Google+: plus.google.com/107187849809354688692
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>
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