WooCommerce更新订单状态时会发送邮件，系统默认的邮件有自己的触发条件，当触发条件满足时，邮件就会自动发送。本文大体描述了这个过程。

发送过程

订单状态由pending变为processing并发送admin new order的过程：

1. 调用$order->update_status( 'processing' )更新状态，触发提示状态变化的action，如下所示

// abstract-wc-order.php
public function update_status( $new_status, $note = '', $manual = false ) {
    ...
    do_action( 'woocommerce_order_status_' . $new_status, $this->id );
    do_action( 'woocommerce_order_status_' . $old_status . '_to_' . $new_status, $this->id );
    do_action( 'woocommerce_order_status_changed', $this->id, $old_status, $new_status );
    ...
}

这时会触发woocommerce_order_status_pending_to_processing这个action

2. 触发send_transactional_email()函数

// class-wc-emails.php
public static function init_transactional_emails() {
    $email_actions = apply_filters( 'woocommerce_email_actions', array(
        ...
        'woocommerce_order_status_pending_to_processing',
        ...
    ) );

    foreach ( $email_actions as $action ) {
        add_action( $action, array( __CLASS__, 'send_transactional_email' ), 10, 10 );
    }
}

代码执行了add_action( 'woocommerce_order_status_pending_to_processing', array( __CLASS__, 'send_transactional_email' ), 10, 10 );
进而触发send_transactional_email()函数。

3. 触发发送邮件的action

// class-wc-emails.php
public static function send_transactional_email() {
    self::instance();
    $args = func_get_args();
    do_action_ref_array( current_filter() . '_notification', $args );
}

相当于执行了do_action( 'woocommerce_order_status_pending_to_processing_notification' );

4. 发送邮件

WooCommerce会加载所有注册过的email classes，但只有满足触发条件的才能执行。

// class-wc-email-new-order.php
add_action( 'woocommerce_order_status_pending_to_processing_notification', array( $this, 'trigger' ) );
add_action( 'woocommerce_order_status_pending_to_completed_notification', array( $this, 'trigger' ) );
add_action( 'woocommerce_order_status_pending_to_on-hold_notification', array( $this, 'trigger' ) );
add_action( 'woocommerce_order_status_failed_to_processing_notification', array( $this, 'trigger' ) );
add_action( 'woocommerce_order_status_failed_to_completed_notification', array( $this, 'trigger' ) );
add_action( 'woocommerce_order_status_failed_to_on-hold_notification', array( $this, 'trigger' ) );

admin new order执行了add_action( 'woocommerce_order_status_pending_to_processing_notification', array( $this, 'trigger' ) )，trigger()函数负责发送邮件，当运行do_action( 'woocommerce_order_status_pending_to_processing_notification' )时，该函数会被执行，进而邮件被发给管理员。

综上所述，更新订单状态时，如果状态变化符合某个邮件的触发条件，该邮件就会被发送。

实际执行顺序

1.  init_transactional_emails
2.  update_status
3.  send_transactional_email
4.  trigger

发送邮件的函数

function wc_mail( $to, $subject, $message, $headers = "Content-Type: text/html\r\n", $attachments = "" ) {
	$mailer = WC()->mailer();

	$mailer->send( $to, $subject, $message, $headers, $attachments );
}