I have a Polymer element (cw-app) property declared like this:
test: {
type: String,
notify: true,
value: 'default test value'
}
As you can see it has a declared default value of default test value.
Initially, when the element is added to the page like this: <cw-app></cw-app> a corresponding test-changed event is being fired.
However, if I add the element and set a value for the property like this: <cw-app test="new value"></cw-app>, then no test-changed event is being fired initially.
Is this a desired behavior and has it been documented anywhere?
I would say it's the expected behaviour:
In the first case the value of test is changed from null (no test attribute set) to "default test value" so the changed event is fired.
In the second case the value is set to "new value" at creation time. It's not changed so there's no changed event.
See a discussion on Polymer issue page on GitHub:
If you declare
<x-foo id="foo" value="foo"></x-foo>the value foo will be assumed to be coming 'from above', t.i. from a user of the element. During config phase Polymer will not echo such data changes upwards. (Assuming the producer of the value/user of the element already has knowledge of this change.)On the other side, if you
<x-foo id="bar"></x-foo>the default value will propagate upwards. For the same reasons, x-foo produced this value, and the user of x-foo has otherwise no knowledge about this. T.i. the default value is not coming from above(, but from x-foo).
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