Conditionally wrapping <content> insertion points in Polymer - polymer

Question

I am trying to make a Polymer element that is a simple box or container with 3 major elements: a header, body, and footer. The header and footer would be optional, but if they are there, I want to put stuff around them.
My question is how to put a wrapper element around a <content> insertion point that only appears if the select attribute matches something?
My code is something like this (doesn't work):
<polymer-element name="example-card" attributes="">
<template>
<paper-shadow z="1">
<div vertical layout>
<div class="card card-header" hidden?="{{!$.header}}">
<content id="header" select=".header"></content>
</div>
<div class="card card-body">
<content id="body" select=":not(.footer)"></content>
</div>
<div class="card card-footer" hidden?="{{!$.footer}}">
<content id="footer" select=".footer"></content>
</div>
</div>
</paper-shadow>
</template>
<script>
Polymer({});
</script>
</polymer-element>
Expected use - no footer in this example:
<example-card>
<div class="header">Header...</div>
<div>Body...</div>
</example-card>
I either want to hide the wrapping div if its content is not selected, or put it in a <template if="..."> to get rid of it from the light DOM altogether. The hidden? expressions above aren't working.
I've tried a few different things with no success, including:
hidden?="{{!$.footer.getDistributedNodes().length}}"
Is there any way to conditionally wrap insertion points depending if their selection matched? Perhaps I am going about this all wrong. Thanks!
EDIT: thanks to Fuzzical Logic's help, this is what my revised version looks like, which uses lodash for collection filtering to only select header and footer classes of immediate children:
<polymer-element name="example-card" attributes="">
<template>
<paper-shadow z="1">
<div vertical layout>
<template if="{{showheader}}">
<div class="card card-header">
<content id="header" select=".header"></content>
</div>
</template>
<div class="card card-body">
<content select=":not(.footer)"></content>
</div>
<template if="{{showfooter}}">
<div class="card card-footer">
<content id="footer" select=".footer"></content>
</div>
</template>
</div>
</paper-shadow>
</template>
<script>
Polymer({
domReady: function () {
// Doesn't work in IE11...
// this.showheader = this.querySelector(':scope > .header');
// this.showfooter = this.querySelector(':scope > .footer');
this.showheader = _.filter(this.children, function(e) { return e.className === "header"; }).length;
this.showfooter = _.filter(this.children, function(e) { return e.className === "footer"; }).length;
},
publish: {
showheader: {
value: false,
reflect: true
},
showfooter: {
value: false,
reflect: true
}
}
});
</script>
</polymer-element>

Answer 1

The problem is not that the hidden? are not working. It's that $.header always resolves to the header with the id === "header" (i.e. this.$.header). This means it will always be truthy. The easiest way to fix this is to set attributes in your domReady event that checks the children.
Polymer('example-card', {
... other element code ...
domReady: function() {
this.showheader = this.querySelector('.header');
this.showfooter = this.querySelector('.footer');
},
publish: {
showheader: {
value: false,
reflect: true
},
showfooter: {
value: false,
reflect: true
}
},
... other element code ...
});
In order to make this work correctly, adjust your element as follows:
<div class="card card-header" hidden?="{{!showheader}}">
<content id="header" select=".header"></content>
</div>
<div class="card card-footer" hidden?="{{!showfooter}}">
<content id="footer" select=".footer"></content>
</div>
Important Note
The above domReady code is merely an example and not complete and will find any .header in the tree starting from the content. The correct way to do this would be to check only the children.