Binding undefined object sub-properties may result in multi-prop observers not running

[kevinpschaaf]

Binding structured data with undefined sub-properties into an element results in undefined being set to the property in question; typically we only expect undefined values transiently at startup, since they should either become valued by attributes in markup, bindings from above, or default values, and Polymer avoids calling multi-property observers until all dependencies become valued.

However, in the case of bindings to structured data, if a new object is set with undefined properties, this results in the element receiving undefined at runtime and may not properly react to this change in state.

See http://jsbin.com/cejusi/edit?html,output for repro, where dom-if receives undefined but since its observer is multi-prop, it waits until it becomes defined.

One idea is to explicitly trap the case that undefined is being set to a Polymer element property and convert that to the element's default value for the property, if it exists.

[robrez]

I reported some problems w/ a multi-property attribute binding earlier, #1874, it sounds like what kevin's describing:

Polymer avoids calling multi-property observers until all dependencies become valued

[kevinpschaaf]

Issue for proposed fix here: #1946 with PR in #1941

[sorvell]

Alternatively object sub-properties could be treated as null. This would indicate a value has been set causing observers to fire without requiring a default value to be set.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed after being marked stale. If you're still facing this problem with the above solution, please comment and we'll reopen!