What is _underscoreCache in Magento

Question

I've been running through some code with xDebug and am constantly noticing _underscoreCache as an array of k v pairs :

object
collection
is_default
default_group_id
front
store_id
action
session_hosts
controller_action
request
secure
visitor_data
website
quote
items_collection
parent_item_id
quote_id
item_id
product_id
code
attributes
website_id
attribute_set_id
additional_attribute_table
attribute_codes
is_global
skip_confirmation_if_email
confirmation
visibility
is_salable
stock_item
udropship_vendor
custom_vars_combined
password_enc
vendor_name
use_local_stock
is_recurring
customer_group_id
date
product_collection
product_name
product_type_id
product_status_changed
is_changed_websites
product_changed_websites
license_key
license_key
license_status
license_expire
server_restriction
products
module_name
use_config_manage_stock
is_in_stock
stock_status
product_type
sku
name
weight
tax_class_id
cost
base_cost
is_qty_decimal
quote_item
product
message
item
is_super_mode
qty_to_add
is_child_item
has_error
item_is_qty_decimal
has_qty_option_update
item_qty
orig_qty
use_config_min_sale_qty
use_config_max_sale_qty
suppress_check_qty_increments
use_config_enable_qty_increments
use_config_min_qty
item_use_old_qty
item_backorders
ignore_old_qty
use_old_qty
skip_check_required_option
ud_skip_quote_load_after_event
order
has_children
vendor_sku
address_type
address
postcode
limit_zipcode
country_id
allowed_countries
stockcheck_method
true_stock
udropship_stock_levels
stock_result
quote_currency_code
currency_code
customer_id
prefix
customer_prefix
firstname
customer_firstname
middlename
customer_middlename
lastname
customer_lastname
suffix
customer_suffix
email
customer_email
dob
customer_dob
taxvat
customer_taxvat
gender
customer_gender
customer_tax_class_id
remote_ip
x_forwarded_for
items_count
checkout_method
checkout_state
collect_shipping_rates
totals_collected_flag
extra_tax_amount
base_extra_tax_amount
subtotal
base_subtotal
subtotal_with_discount
base_subtotal_with_discount
grand_total
base_grand_total
quote_address
is_multi_shipping
total_qty
base_virtual_amount
virtual_amount
subtotal_incl_tax
base_subtotal_incl_tax
applied_rule_ids
shipping_tax_amount
base_shipping_tax_amount
applied_taxes_reset
free_shipping
free_method_weight
region_id
customer_class_id
store
parent_id
product_class_id
shipping_amount
base_shipping_amount
shipping_incl_tax
base_shipping_incl_tax
shipping_taxable
base_shipping_taxable
is_shipping_incl_tax
base_cod_fee
cod_fee
cod_tax_amount
base_cod_tax_amount
method
prev_quote_customer_group_id
discount_amount
base_discount_amount
base_calculation_price
calculation_price
base_original_price
original_custom_price
rate
row_total
base_row_total
coupon_code
no_discount
coupon_type
uses_per_customer
is_primary
uses_per_coupon
use_auto_generation
usage_limit
operator_option
operator_by_input_type
value_option
operator_options
type
aggregator
value
actions
aggregator_option
aggregator_options
rule
conditions_serialized
attribute_option
attribute
operator
is_value_parsed
value_parsed
actions_serialized
simple_free_shipping
stop_rules_processing
tax_percent
custom_price
base_price
price_incl_tax
base_price_incl_tax
row_total_incl_tax
base_row_total_incl_tax
taxable_amount
base_taxable_amount
is_price_incl_tax
rounding_deltas
weee_tax_applied
base_weee_tax_disposition
weee_tax_disposition
base_weee_tax_row_disposition
weee_tax_row_disposition
base_weee_tax_applied_amount
base_weee_tax_applied_row_amount
weee_tax_applied_amount
weee_tax_applied_row_amount
row_weight
all_items
dest_country_id
dest_region_id
dest_region_code
dest_street
city
dest_city
dest_postcode
package_value
package_value_with_discount
package_weight
package_qty
package_physical_value
base_currency
package_currency
limit_carrier
orig
path
cipher
mode
handler
init_vector
active_flag
error
weight_type
full_row_weight
carrier_code
zip
ups_pickup
ups_container
ups_dest_type
udropship_calculate_rates
calculate_rates_by_group_flag
requests
website_ids
system_methods

What is this? I was thinking it would be a window into js underscore similar to mage registry? How can it be used? How is it populated?

Answer 1

Short Version: If you don't know what this is for, you don't need to use it.

Long Version follows.

This has nothing at all to do with underscoe.js. Before it was a javascript framework, the underscore was a simple ASCII character _.

In Magento's system, the majority of objects inherit from the base

Varien_Object 

class. The object gives Magento's object special functionality. For example, in a Magento object, you don't need to define setters and getters. You can just do things like this

$object = new SomeObject; //which inherits form Varien_Object

$object->setSomeValue('Our Value');

echo $object->getSomeValue('Our Value');
echo $object->getData('our_value');

$data = $object->getData();
echo $data['our_value'];

In the above example, there is no concrete method named setSomeValue. Magento magically knows that we just want to set a data property. This is implemented in the PHP magic method __call

#File: lib/Varien/Object.php
public function __call($method, $args)
{
    ...
}

When you call setSomeValue, Magento sets a key on the object's data array named some_value. That is, it needs to convert the camel cased SomeValue into a non-camel cased some_value. You can see that in the magic __call implementation here

#File: lib/Varien/Object.php
public function __call($method, $args)
{
    case 'set' :
        //Varien_Profiler::start('SETTER: '.get_class($this).'::'.$method);
        $key = $this->_underscore(substr($method,3));
        $result = $this->setData($key, isset($args[0]) ? $args[0] : null);
        //Varien_Profiler::stop('SETTER: '.get_class($this).'::'.$method);
        return $result;
}

The _underscore method takes the string SomeValue, and converts it to some_value with the following line

#File: lib/Varien/Object.php
$result = strtolower(preg_replace('/(.)([A-Z])/', "$1_$2", $name));

At some point, either through profiling or intuition, A Magento developer realized calling strToLower and preg_replace every time data was gotten or set on an object meant a performance bottle-neck. To fix this, they introduced the _underscoeCache. This is an array, and a static property on the Varien_Object class

#File: lib/Varien/Object.php
/**
 * Setter/Getter underscore transformation cache
 *
 * @var array
 */
protected static $_underscoreCache = array();

If you look at the entire _underscore method, you can see how it's used

protected function _underscore($name)
{
    if (isset(self::$_underscoreCache[$name])) {
        return self::$_underscoreCache[$name];
    }
    #Varien_Profiler::start('underscore');
    $result = strtolower(preg_replace('/(.)([A-Z])/', "$1_$2", $name));
    #Varien_Profiler::stop('underscore');
    self::$_underscoreCache[$name] = $result;
    return $result;
}

That is, Magento will still perform the strToLower and preg_replace transformation, but it will only do it once per unique string. After it's done the key/value (SomeValue and some_value) are placed in the _underscoreCache

self::$_underscoreCache[$name] = $result;

That way, the next time the method is called, a value from the cache is returned instead

if (isset(self::$_underscoreCache[$name])) {
    return self::$_underscoreCache[$name];
}

This avoids the calls to strToLower and preg_replace for commonly referenced variables.