Polymer: reverting/ordering items in repeat without touching array order - polymer

Question

This seems a trivial thing but I'm unable to find it:
What if I want to reverse the order of my items in a repeat, without actually touching the order of the array, like in:
<template repeat="{{layer in layers}}">
<div>{{layer.name}}</div>
</template>
where layers is an array of objects.
I've tried applying a filter and then working with a copy of the array, like in:
<template repeat="{{layer in layers | reverse}}">
<div>{{layer.name}}</div>
</template>
...
reverse: function(arr){
return _(arr).reverse();
}
but that results in some observers failing since they're looking at the copy instead of the original objects. I don't want to apply a sort to my original array since other parts of the code depend on that order.
Anyone knows of an option where just the order of display in the DOM is affected?

Answer 1

I think you need to do something like this
<template repeat="{{layer in temp_array}}">
<div>{{layer.name}}</div>
</template>
<script>
Polymer('el-name',{
ready: function(){
this.temp_array =[];
this.temp_array = layers.reverse();
}
}
);
</script>
if your layers is empty when ready called, use change listener
<script>
Polymer('el-name',{
ready: function(){
this.temp_array =[];
},
layersChanged: function(oldValue, newValue){
if(newValue.length != 0)
this.temp_array = newValue.reverse();
}
}
);
</script>
Hope it help for you

Answer 2

If it is possible to put the repeated elements in a vertical/horizontal layout, then reverse might do the trick (see layout documentation):
<div vertical layout reverse?="{{ isReversed }}">
<template repeat="{{ layer in layers }}">
<div>{{ layer.name }}</div>
</template>
</div>

Answer 3

I would like to offer a safier and more clear way to revert an array for repeat binding:
<polymer-element name="my-element" attributes="layers layersReversed">
<template>
<template repeat="{{layer in layers}}">
<div>{{layer.name}}</div>
</template>
</template>
<script>
Polymer({
layersReversedChanged: function() {
var layers = this.layersReversed.slice();
layers.reverse();
this.layers = layers;
}
});
</script>
</polymer-element>
<my-element layers="{{layers}}"><!-- direct order --></my-element>
<my-element layersReversed="{{layers}}"><!-- reverse order --></my-element>
Direct or reverse order is defined by used attribute: layers or layersReversed.
There are no value changing in corresponding -Changed event by itself (which may cause falling to endless loop).
The .reverse() method changes the original array, so it should be applied on its copy.

Answer 4

There is another funny and extravagant way to do the same via an intermediate web-component:
<polymer-element name="reverse-order" attributes="in out">
<template></template>
<script>
Polymer({
inChanged: function() {
var out = this.in.slice();
out.reverse();
this.out = out;
}
});
</script>
</polymer-element>
It can be used to bind some elements with different order. I.e., array is populated by .push() method, while preferred array presentation is in reverse order:
<my-element layers="{{layersReversed}}"></my-element>
<reverse-order in="{{layers}}" out="{{layersReversed}}"></reverse-order>
<core-localstorage name="layers" value="{{layers}}"></core-localstorage>