WooCommerce : add custom fields to product variations v2.0

gistfile1.php

<?php

//Display Fields
add_action( 'woocommerce_product_after_variable_attributes', 'variable_fields', 10, 3 );
//JS to add fields for new variations
add_action( 'woocommerce_product_after_variable_attributes_js', 'variable_fields_js' );
//Save variation fields
add_action( 'woocommerce_save_product_variation', 'save_variable_fields', 10, 1 );

/**
 * Create new fields for variations
 *
*/
function variable_fields( $loop, $variation_data, $variation ) {
?>
	<tr>
		<td>
			<?php
			// Text Field
			woocommerce_wp_text_input( 
				array( 
					'id'          => '_text_field['.$loop.']', 
					'label'       => __( 'My Text Field', 'woocommerce' ), 
					'placeholder' => 'http://',
					'desc_tip'    => 'true',
					'description' => __( 'Enter the custom value here.', 'woocommerce' ),
					'value'       => get_post_meta($variation->ID, '_text_field'], true)
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Number Field
			woocommerce_wp_text_input( 
				array( 
					'id'          => '_number_field['.$loop.']', 
					'label'       => __( 'My Number Field', 'woocommerce' ), 
					'desc_tip'    => 'true',
					'description' => __( 'Enter the custom number here.', 'woocommerce' ),
					'value'       => get_post_meta($variation->ID, '_number_field'], true),
					'custom_attributes' => array(
									'step' 	=> 'any',
									'min'	=> '0'
								) 
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Textarea
			woocommerce_wp_textarea_input( 
				array( 
					'id'          => '_textarea['.$loop.']', 
					'label'       => __( 'My Textarea', 'woocommerce' ), 
					'placeholder' => '', 
					'description' => __( 'Enter the custom value here.', 'woocommerce' ),
					'value'       => get_post_meta($variation->ID, '_textarea'], true)
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Select
			woocommerce_wp_select( 
			array( 
				'id'          => '_select['.$loop.']', 
				'label'       => __( 'My Select Field', 'woocommerce' ), 
				'description' => __( 'Choose a value.', 'woocommerce' ),
				'value'       => get_post_meta($variation->ID, '_select'], true),
				'options' => array(
					'one'   => __( 'Option 1', 'woocommerce' ),
					'two'   => __( 'Option 2', 'woocommerce' ),
					'three' => __( 'Option 3', 'woocommerce' )
					)
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Checkbox
			woocommerce_wp_checkbox( 
			array( 
				'id'            => '_checkbox['.$loop.']', 
				'label'         => __('My Checkbox Field', 'woocommerce' ), 
				'description'   => __( 'Check me!', 'woocommerce' ),
				'value'         => get_post_meta($variation->ID, '_checkbox'], true)
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Hidden field
			woocommerce_wp_hidden_input(
			array( 
				'id'    => '_hidden_field['.$loop.']', 
				'value' => 'hidden_value'
				)
			);
			?>
		</td>
	</tr>
<?php
}

/**
 * Create new fields for new variations
 *
*/
function variable_fields_js() {
?>
	<tr>
		<td>
			<?php
			// Text Field
			woocommerce_wp_text_input( 
				array( 
					'id'          => '_text_field[ + loop + ]', 
					'label'       => __( 'My Text Field', 'woocommerce' ), 
					'placeholder' => 'http://',
					'desc_tip'    => 'true',
					'description' => __( 'Enter the custom value here.', 'woocommerce' ),
					'value'       => ''
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Number Field
			woocommerce_wp_text_input( 
				array( 
					'id'                => '_number_field[ + loop + ]', 
					'label'             => __( 'My Number Field', 'woocommerce' ), 
					'desc_tip'          => 'true',
					'description'       => __( 'Enter the custom number here.', 'woocommerce' ),
					'value'             => '',
					'custom_attributes' => array(
									'step' 	=> 'any',
									'min'	=> '0'
								) 
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Textarea
			woocommerce_wp_textarea_input( 
				array( 
					'id'          => '_textarea[ + loop + ]', 
					'label'       => __( 'My Textarea', 'woocommerce' ), 
					'placeholder' => '', 
					'description' => __( 'Enter the custom value here.', 'woocommerce' ),
					'value'       => '',
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Select
			woocommerce_wp_select( 
			array( 
				'id'          => '_select[ + loop + ]', 
				'label'       => __( 'My Select Field', 'woocommerce' ), 
				'description' => __( 'Choose a value.', 'woocommerce' ),
				'value'       => '',
				'options' => array(
					'one'   => __( 'Option 1', 'woocommerce' ),
					'two'   => __( 'Option 2', 'woocommerce' ),
					'three' => __( 'Option 3', 'woocommerce' )
					)
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Checkbox
			woocommerce_wp_checkbox( 
			array( 
				'id'            => '_checkbox[ + loop + ]', 
				'label'         => __('My Checkbox Field', 'woocommerce' ), 
				'description'   => __( 'Check me!', 'woocommerce' ),
				'value'         => '', 
				)
			);
			?>
		</td>
	</tr>
	<tr>
		<td>
			<?php
			// Hidden field
			woocommerce_wp_hidden_input(
			array( 
				'id'    => '_hidden_field[ + loop + ]', 
				'value' => 'hidden_value'
				)
			);
			?>
		</td>
	</tr>
<?php
}

/**
 * Save new fields for variations
 *
*/
function save_variable_fields( $post_id ) {
	if (isset( $_POST['variable_sku'] ) ) :

		$variable_sku          = $_POST['variable_sku'];
		$variable_post_id      = $_POST['variable_post_id'];
		
		// Text Field
		$_text_field = $_POST['_text_field'];
		for ( $i = 0; $i < sizeof( $variable_sku ); $i++ ) :
			$variation_id = (int) $variable_post_id[$i];
			if ( isset( $_text_field[$i] ) ) {
				update_post_meta( $variation_id, '_text_field', stripslashes( $_text_field[$i] ) );
			}
		endfor;
		
		// Number Field
		$_number_field = $_POST['_number_field'];
		for ( $i = 0; $i < sizeof( $variable_sku ); $i++ ) :
			$variation_id = (int) $variable_post_id[$i];
			if ( isset( $_text_field[$i] ) ) {
				update_post_meta( $variation_id, '_number_field', stripslashes( $_number_field[$i] ) );
			}
		endfor;
		
		// Textarea
		$_textarea = $_POST['_textarea'];
		for ( $i = 0; $i < sizeof( $variable_sku ); $i++ ) :
			$variation_id = (int) $variable_post_id[$i];
			if ( isset( $_textarea[$i] ) ) {
				update_post_meta( $variation_id, '_textarea', stripslashes( $_textarea[$i] ) );
			}
		endfor;
		
		// Select
		$_select = $_POST['_select'];
		for ( $i = 0; $i < sizeof( $variable_sku ); $i++ ) :
			$variation_id = (int) $variable_post_id[$i];
			if ( isset( $_select[$i] ) ) {
				update_post_meta( $variation_id, '_select', stripslashes( $_select[$i] ) );
			}
		endfor;
		
		// Checkbox
		$_checkbox = $_POST['_checkbox'];
		for ( $i = 0; $i < sizeof( $variable_sku ); $i++ ) :
			$variation_id = (int) $variable_post_id[$i];
			if ( isset( $_checkbox[$i] ) ) {
				update_post_meta( $variation_id, '_checkbox', stripslashes( $_checkbox[$i] ) );
			}
		endfor;
		
		// Hidden field
		$_hidden_field = $_POST['_hidden_field'];
		for ( $i = 0; $i < sizeof( $variable_sku ); $i++ ) :
			$variation_id = (int) $variable_post_id[$i];
			if ( isset( $_hidden_field[$i] ) ) {
				update_post_meta( $variation_id, '_hidden_field', stripslashes( $_hidden_field[$i] ) );
			}
		endfor;
		
	endif;
}

@roltex @whoaloic
I use

get_post_meta($variation_id, '_class_duration', true);

to display it in the frontend, where _class_duration is the name of my variable. As I saved just a number I want it to be returned not as an array but the value which is why I use the true.
See https://developer.wordpress.org/reference/functions/get_post_meta/ for further details.

Full code looks like this:

$all_orders = get_posts( array(
'numberposts' => -1,
'meta_key' => '_customer_user',
'meta_value' => get_current_user_id(),
'post_type' => wc_get_order_types(),
'post_status' => array_keys( wc_get_order_statuses() ),
) );

foreach($all_orders as $order){
$order_id = $order->ID;
$order = wc_get_order($order_id);

foreach ($order->get_items() as $item_id => $item_obj) {
	$variation_id = $item_obj->get_variation_id();
	$months = get_post_meta($variation_id, _class_duration, true);
	echo $months;
}

}

(Sorry, seems like the code is not correctly recognised as one by syntax highlighting. Not sure why)

Hope it helps anyway.
Best
Felix