Magento - get filterable attributes by category - attributes

Question

I have created a custom navigation module specifically for a website, but I really want to be able to list filterable attributes by a specific category. So for instance my main navigation is:
Category 1
Category 2
Category 3 etc.
I then that when a user mouses over a category, they are then presented with an expanded menu with a few filterable options e.g.:
Category 1
View by manufacturer:
Manufacturer 1
Manufacturer 2
Manufacturer 3 etc.
I am able to get all filterable attributes for the store, but I want this list to pull in only the filterable attributes per category, as for instance Category 1 may have different manufacturers to Category 2. I then need to cache these results as this will not change often.

Answer 1

The answer that Joe gave was a good starting point, but the attributes didn't returned any options yet. After a lot of frustrations I solved the problem with the following code. Hope it helps all of you out.
$layer = Mage::getModel("catalog/layer");
foreach($categories as $categoryid) {
$category = Mage::getModel("catalog/category")->load($categoryid);
$layer->setCurrentCategory($category);
$attributes = $layer->getFilterableAttributes();
foreach ($attributes as $attribute) {
if ($attribute->getAttributeCode() == 'price') {
$filterBlockName = 'catalog/layer_filter_price';
} elseif ($attribute->getBackendType() == 'decimal') {
$filterBlockName = 'catalog/layer_filter_decimal';
} else {
$filterBlockName = 'catalog/layer_filter_attribute';
}
$result = $this->getLayout()->createBlock($filterBlockName)->setLayer($layer)->setAttributeModel($attribute)->init();
foreach($result->getItems() as $option) {
echo $option->getLabel().'<br/>';
echo $option->getValue();
}
}
The only thing you'll need to do yourself is create the correct link using the getValue() functions.
This code has been tested in Magento 1.5

Answer 2

Magento uses the model Catalog_Model_Layer to accomplish this, so I'm guessing this may be your best bet. Caveat emptor, I have not tested this code yet:
$layer = Mage::getModel("catalog/layer");
foreach($categories as $categoryid) {
$category = Mage::getModel("catalog/category")->load($categoryid);
$layer->setCurrentCategory($category);
$attributes = $layer->getFilterableAttributes();
// do something with your attributes
}
Each iteration here will give you an object of the class Mage_Catalog_Model_Resource_Eav_Mysql4_Attribute_Collection, which you should be able to iterate over in a foreach loop to get your desired output.
For caching, try enabling block caching on your site and give the block a cache tag like the following. Magento will cache the HTML output and all will be right with the world:
protected function _construct() {
$this->addData(array(
'cache_lifetime' => 3600,
'cache_tags' => array(Mage_Catalog_Model_Product::CACHE_TAG),
'cache_key' => $someUniqueIdentifierYouCreate,
));
}
The cache will only be valid for the key you pass, so make sure that, if the menu is to change (w/o flushing the cache, for instance), that the cache key is different.
Hope that helps!
Thanks,
Joe