如何在没有任何插件的情况下在WordPress的侧边栏中显示选定类别的图书？

class Custom_Book_Widget extends WP_Widget {
    // 构造函数
    public function __construct() {
        parent::__construct(
            'custom_book_widget', // 小工具的ID
            '选定类别的图书', // 小工具的名称
            array( 'description' => '显示选定类别的图书' ) // 小工具的描述
        );
    }

    // 前端显示
    public function widget( $args, $instance ) {
        $category_id = $instance['category_id']; // 获取选定的类别ID
        $books = get_posts( array(
            'category' => $category_id, // 设置类别ID
            'post_type' => 'book', // 设置自定义文章类型为'book'
            'posts_per_page' => 5 // 设置显示的图书数量
        ) );

        echo $args['before_widget'];
        echo $args['before_title'] . '选定类别的图书' . $args['after_title'];
        echo '<ul>';
        foreach ( $books as $book ) {
            echo '<li><a href="' . get_permalink( $book->ID ) . '">' . $book->post_title . '</a></li>';
        }
        echo '</ul>';
        echo $args['after_widget'];
    }

    // 后台设置
    public function form( $instance ) {
        $category_id = ! empty( $instance['category_id'] ) ? $instance['category_id'] : '';
        ?>
        <p>
            <label for="<?php echo $this->get_field_id( 'category_id' ); ?>">选择类别:</label>
            <select class="widefat" id="<?php echo $this->get_field_id( 'category_id' ); ?>" name="<?php echo $this->get_field_name( 'category_id' ); ?>">
                <?php
                $categories = get_categories();
                foreach ( $categories as $category ) {
                    echo '<option value="' . $category->term_id . '" ' . selected( $category_id, $category->term_id, false ) . '>' . $category->name . '</option>';
                }
                ?>
            </select>
        </p>
        <?php
    }

    // 更新设置
    public function update( $new_instance, $old_instance ) {
        $instance = array();
        $instance['category_id'] = ( ! empty( $new_instance['category_id'] ) ) ? sanitize_text_field( $new_instance['category_id'] ) : '';
        return $instance;
    }
}

// 注册自定义小工具
function register_custom_book_widget() {
    register_widget( 'Custom_Book_Widget' );
}
add_action( 'widgets_init', 'register_custom_book_widget' );