以送货方式init - Woocommerce获取所有送货课程

Question

I've created a custom shipping method following Woocommerce Shipping Method API. In the init method of my shipping method class, I'm trying to get all shipping classes using WC()->shipping->get_shipping_classes().

This call fails as a PHP fatal error:
Fatal error: Uncaught Error: Call to a member function get_shipping_classes() on null...
This suggests that WC()->shipping is null which is basically an instance of WC_Shipping class.

I'm doing similar to Flat rate shipping method of Woocommerce core. The similar code works in the Woocommerce as seen here.
Here's my shipping method class:

class WCS_City_Shipping_Method extends WC_Shipping_Flat_Rate {

    /**
     * Cities applicable on
     * 
     * @var array
     */
    public $cities = array();

    /**
     * Constructor.
     *
     * @since 1.0.0
     */
    public function __construct( $instance_id = 0 ) {

        $this->id                 = 'city_shipping';
        $this->instance_id        = absint( $instance_id );
        $this->method_title       = __( 'Flat Rate City Shipping', 'woocommerce-city-shipping' );
        $this->method_description = __( 'Applies only when shipping city matches one of provided.', 'woocommerce-city-shipping' );
        $this->supports           = array( 'shipping-zones', 'instance-settings', );

        $this->init();

        // Save settings
        add_action( 'woocommerce_update_options_shipping_' . $this->id, array( $this, 'process_admin_options' ) );
    }


    /**
     * Init.
     *
     * Initialize user set variables.
     *
     * @since 1.0.0
     */
    public function init() {
        $this->instance_form_fields = include( 'settings-city-shipping.php' );
        $this->title                = $this->get_option( 'title' );
        $this->tax_status           = $this->get_option( 'tax_status' );
        $this->cities               = $this->get_option( 'cities' );
        $this->cost                 = $this->get_option( 'cost' );
        $this->type                 = $this->get_option( 'type', 'class' );
    }

    /**
     * ... Rest of code
     * 
     */
}

And here's settings-city-shipping.php which was included in init method.

<?php

if ( ! defined( 'ABSPATH' ) ) {
    exit;
}

$shipping_classes = WC()->shipping->get_shipping_classes(); // Fatal error here 

The shipping method is added using the filter as:

// Add shipping method
add_filter( 'woocommerce_shipping_methods', array( $this, 'add_shipping_method_class' ) );
public function add_shipping_method_class( $methods ) {

    if ( class_exists( 'WCS_City_Shipping_Method' ) ) {
        $methods['city_shipping'] = 'WCS_City_Shipping_Method';
    }
    return $methods;

}

Please help to find what is causing fatal error and how to get all shipping classes.

Answer 1

It was a silly mistake. I had initialized the WCS_City_Shipping_Method class in the initialization code of the plugin class for this custom shipping method. The code was running before Woocommerce was ready and so causing fatal error.

I had missed this important line from Woocommerce Shipping API documentation:

To ensure the classes you need to extend exist, you should wrap your class in a function which is called after all plugins are loaded.

Anyways, I solved issue by not initializing the class and wrapping it into a method which runs on plugins_loaded action hook. Here are the changes:

public function plugins_loaded_action() {
    // Load shipping method class
    add_action( 'woocommerce_shipping_init', array( $this, 'wcs_shipping_method' ) );
}

// Placed in plugin class init method
add_action( 'plugins_loaded', array( $this, 'plugins_loaded_action' ) );

public function wcs_shipping_method() {
    require_once plugin_dir_path( __FILE__ ) . 'includes/class-wcs-method.php';
}