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// Add Variation Settings
add_action( 'woocommerce_product_after_variable_attributes', 'variation_settings_fields', 10, 3 );
// Save Variation Settings
add_action( 'woocommerce_save_product_variation', 'save_variation_settings_fields', 10, 2 );
/**
 * Create new fields for variations
 *
*/
function variation_settings_fields( $loop, $variation_data, $variation ) {
    // Text Field
    woocommerce_wp_text_input( 
        array( 
            'id'          => '_text_field[' . $variation->ID . ']', 
            '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 )
        )
    );
    // Number Field
    woocommerce_wp_text_input( 
        array( 
            'id'          => '_number_field[' . $variation->ID . ']', 
            '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'
                        ) 
        )
    );
    // Textarea
    woocommerce_wp_textarea_input( 
        array( 
            'id'          => '_textarea[' . $variation->ID . ']', 
            'label'       => __( 'My Textarea', 'woocommerce' ), 
            'placeholder' => '', 
            'description' => __( 'Enter the custom value here.', 'woocommerce' ),
            'value'       => get_post_meta( $variation->ID, '_textarea', true ),
        )
    );
    // Select
    woocommerce_wp_select( 
    array( 
        'id'          => '_select[' . $variation->ID . ']', 
        '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' )
            )
        )
    );
    // Checkbox
    woocommerce_wp_checkbox( 
    array( 
        'id'            => '_checkbox[' . $variation->ID . ']', 
        'label'         => __('My Checkbox Field', 'woocommerce' ), 
        'description'   => __( 'Check me!', 'woocommerce' ),
        'value'         => get_post_meta( $variation->ID, '_checkbox', true ), 
        )
    );
    // Hidden field
    woocommerce_wp_hidden_input(
    array( 
        'id'    => '_hidden_field[' . $variation->ID . ']', 
        'value' => 'hidden_value'
        )
    );
}
/**
 * Save new fields for variations
 *
*/
function save_variation_settings_fields( $post_id ) {
    // Text Field
    $text_field = $_POST['_text_field'][ $post_id ];
    if( ! empty( $text_field ) ) {
        update_post_meta( $post_id, '_text_field', esc_attr( $text_field ) );
    }
    
    // Number Field
    $number_field = $_POST['_number_field'][ $post_id ];
    if( ! empty( $number_field ) ) {
        update_post_meta( $post_id, '_number_field', esc_attr( $number_field ) );
    }
    // Textarea
    $textarea = $_POST['_textarea'][ $post_id ];
    if( ! empty( $textarea ) ) {
        update_post_meta( $post_id, '_textarea', esc_attr( $textarea ) );
    }
    
    // Select
    $select = $_POST['_select'][ $post_id ];
    if( ! empty( $select ) ) {
        update_post_meta( $post_id, '_select', esc_attr( $select ) );
    }
    
    // Checkbox
    $checkbox = isset( $_POST['_checkbox'][ $post_id ] ) ? 'yes' : 'no';
    update_post_meta( $post_id, '_checkbox', $checkbox );
    
    // Hidden field
    $hidden = $_POST['_hidden_field'][ $post_id ];
    if( ! empty( $hidden ) ) {
        update_post_meta( $post_id, '_hidden_field', esc_attr( $hidden ) );
    }
}