# Filters and hooks - Checkoutline 1.x documentation

> Read answers in code, change a field in either checkout, check answers, and the other hooks add-ons use.

Checkoutline's hooks are WordPress filters and actions. Put the examples in a small custom plugin or your child theme's `functions.php`.

## Reading an answer

Answers are WooCommerce additional checkout field values, so read them with WooCommerce's own API, or Checkoutline's shortcut:

```php
use Extendiment\Checkoutline\Checkout;

// A contact or order field.
$gift = Checkout::value( $order, 'gift_message' );

// An address field, from the shipping address.
$gate = Checkout::value( $order, 'gate_code', 'shipping' );
```

Checkboxes come back as `true` or `false`, everything else as the saved string (`''` when there's no answer). `Extendiment\Checkoutline\Display::format( $field, $value )` turns a value into what customers read (an option's label, Yes/No), with `Extendiment\Checkoutline\Fields::get( 'gift_message' )` as the field.

## checkoutline_block_field

The arguments a field is registered with for the Checkout block, through WooCommerce's `woocommerce_register_additional_checkout_field()`. Gets the arguments and the field.

```php
// Let the browser fill in a customer number.
add_filter( 'checkoutline_block_field', function ( $args, $field ) {
	if ( 'customer_number' === $field['key'] ) {
		$args['attributes'] = array( 'autocomplete' => 'on', 'maxLength' => 12 );
	}
	return $args;
}, 10, 2 );
```

WooCommerce only accepts certain attributes on text fields; see its documentation for additional checkout fields.

## checkoutline_classic_field

The same field in the classic checkout, as `woocommerce_form_field()` arguments. Gets the arguments, the field, and the group it's in (`billing`, `shipping` or `order`).

```php
// Half-width, next to each other.
add_filter( 'checkoutline_classic_field', function ( $args, $field, $group ) {
	if ( in_array( $field['key'], array( 'floor', 'door' ), true ) ) {
		$args['class'] = array( 'door' === $field['key'] ? 'form-row-last' : 'form-row-first', 'checkoutline-field' );
	}
	return $args;
}, 10, 3 );
```

## checkoutline_value_problem

Checks a customer's answer before the order is placed, in both checkouts. Return a message to refuse it, or `''` to accept it. Gets the problem so far, the field, the answer (trimmed, never empty: empty answers are WooCommerce's required check) and which checkout asks (`block` or `classic`).

```php
// Gate codes are four digits.
add_filter( 'checkoutline_value_problem', function ( $problem, $field, $value, $checkout ) {
	if ( '' === $problem && 'gate_code' === $field['key'] && ! preg_match( '/^\d{4}$/', $value ) ) {
		return __( 'The gate code is four digits.', 'my-plugin' );
	}
	return $problem;
}, 10, 4 );
```

## checkoutline_display_value

An answer as it's shown on the order screen, the order confirmation, My Account and in emails, as text. Gets the text, the field and the saved value.

```php
add_filter( 'checkoutline_display_value', function ( $text, $field, $value ) {
	return 'vat_number' === $field['key'] ? strtoupper( $text ) : $text;
}, 10, 3 );
```

## checkoutline_loaded

An action that fires once Checkoutline has loaded (on `plugins_loaded`, and only when WooCommerce is active). Hook an add-on's setup to it, so it only runs when Checkoutline is there. `CHECKOUTLINE_VERSION` is defined from then on, for version checks.

```php
add_action( 'checkoutline_loaded', function () {
	// Checkoutline's classes, such as Extendiment\Checkoutline\Fields, are available here.
} );
```

## Hooks for add-ons

These are what Checkoutline Pro is built on, and what you'd use to add a field type or settings of your own:

| Hook | What it's for |
|---|---|
| `checkoutline_field_types` | Register a field type. See **Adding a field type**. |
| `checkoutline_clean_field` | Clean and keep a setting of your own on a field (`$clean, $raw`). Checkoutline's own settings in the result are ignored. |
| `checkoutline_handles_conditions` | Return `true` if your code checks the `conditions` setting on fields. Without it, Checkoutline leaves fields with conditions out of checkout. |
| `checkoutline_classic_shows` | Whether a field with conditions shows while a classic checkout order is placed (`$shows, $field, $group, $posted`). |
| `checkoutline_field_rows` | Print rows in a field's settings table (`$value`). A row with class `checkoutline-for` and `data-types="text textarea"` only shows for those types. |
| `checkoutline_field_sections` | Print a section after the field's settings table, inside its form. |
| `checkoutline_posted_field` | Add your settings from `$_POST` when a field is saved (the nonce and capability are already checked). |
| `checkoutline_field_error` | Return a message code to refuse a field's settings; add its text with `checkoutline_admin_messages`. |
| `checkoutline_field_summary` | Lines of plain text under a field's name in the list. |

The code documents each hook's arguments where it's applied.

Source: https://extendiment.com/plugins/checkoutline/docs/1.x/filters-and-hooks