Repeat light dom element - polymer

Question

In my component I would like to repeat a list of item with template provided by the light dom of the component. For example:
<template is="dom-repeat" items="{{items}}">
<content select="#itemTemplate"></content>
</template>
However, it seems that Polymer only inserts the light dom element #itemTemplate exactly one time instead of multiple times. Is there other way to repeat a light dom element?

Answer 1

I have created a simple prototype, that lets you specify the number of repetitions of the light DOM template.
Because the content is in the light DOM, you can style it from the outside as you would usually do. And data binding inside the template also works, since I have implemented the _forwardParentProp, and _forwardParentPath methods from the Templatizer.
Be aware, that I have not implemented the instance specific properties, which would allow per row specific variables, such as index and item. This can, of course, be done, but would need a bit more work.
See the prototype in action: JSBin.
OK, let's go into details:
The usage of the test-element along with data-binding to both input elements is fairly straightforward:
<template is="dom-bind">
Number of repeats: <input type="text" value="{{repeats::input}}" /> <br />
Custom message: <input type="text" value="{{customMessage::input}}" />
<test-element repeats="{{repeats}}">
<template>
<h1>Title!</h1>
<p>
Custom message: <em>[[customMessage]]</em>
</p>
</template>
</test-element>
</template>
Notice the dom-bind, which is needed to create a data-binding scope.
As for the test-element, the whole source code looks like this:
<dom-module id="test-element">
<template>
<style>
:host {
display: block;
}
</style>
<content></content>
</template>
<script>
Polymer({
is: 'test-element',
behaviors: [
Polymer.Templatizer,
],
properties: {
repeats: {
type: Number,
value: 3,
notify: true,
},
},
observers: [
'_repeatsChanged(repeats)',
],
_repeatsChanged: function(repeats) {
// First time only: initialize template
if (this.template === undefined) {
this.template = Polymer.dom(this).querySelector('template');
this.templatize(this.template);
}
// Remove previously stamped children
while (Polymer.dom(this).firstChild) {
Polymer.dom(this).removeChild(Polymer.dom(this).firstChild);
}
// Stamp new ones
this.stamped = new Array(repeats);
var inst;
for (var i = 0; i < repeats; i++) {
inst = this.stamp(null);
this.stamped[i] = inst.root.querySelector('*');
Polymer.dom(this).appendChild(inst.root);
}
},
// Copied from iron-list
_forwardParentProp: function(prop, value) {
if (this.stamped) {
this.stamped.forEach(function(item) {
item._templateInstance[prop] = value;
}, this);
}
},
// Copied from iron-list
_forwardParentPath: function(path, value) {
if (this.stamped) {
this.stamped.forEach(function(item) {
item._templateInstance.notifyPath(path, value, true);
}, this);
}
},
});
</script>
</dom-module>
There is only one property, repeats, which specifies the number of stamped instances. Default value is 3. To accomodate changes of said property's value, a observer has been created. This is also the place where the stamping takes place:
_repeatsChanged: function(repeats) {
// First time only: initialize template
if (this.template === undefined) {
this.template = Polymer.dom(this).querySelector('template');
this.templatize(this.template);
}
// Remove previously stamped children
while (Polymer.dom(this).firstChild) {
Polymer.dom(this).removeChild(Polymer.dom(this).firstChild);
}
// Stamp new ones
this.stamped = new Array(repeats);
var inst;
for (var i = 0; i < repeats; i++) {
inst = this.stamp(null);
this.stamped[i] = inst.root.querySelector('*');
Polymer.dom(this).appendChild(inst.root);
}
},
Firstly (and only once), the template is read from the light DOM and
the templatize method is called. This method initializes the
Templatize behavior.
Secondly, all previously stamped children are removed (so that the
elements don't just build up infinitely).
Thirdly, new children are stamped, according to the current value of
repeats. All stamped instances are saved to this.stamped, which
is needed for the data-binding from the outside to work.
Last but not least, the Templatizer behavior is implemented via two methods (and two are left unimplemented):
// Copied from iron-list
_forwardParentProp: function(prop, value) {
if (this.stamped) {
this.stamped.forEach(function(item) {
item._templateInstance[prop] = value;
}, this);
}
},
// Copied from iron-list
_forwardParentPath: function(path, value) {
if (this.stamped) {
this.stamped.forEach(function(item) {
item._templateInstance.notifyPath(path, value, true);
}, this);
}
},
Both methods are taken from the iron-list. They iterate through the stamped children and propagate property changes and path notifications.

Answer 2

You can include your content in a separate element and use it.
<template is="dom-repeat" items={{items}}">
<child-element item=[[item]]></child-element>
</template>