如何使用Golang实现代理模式控制访问_在访问前后加入处理逻辑

// 定义业务能力接口（例如：用户服务）
type UserService interface {
    GetUser(id int) (*User, error)
    CreateUser(name string) (int, error)
}

type User struct {
    ID   int
    Name string
}

type RealUserService struct{}

func (r *RealUserService) GetUser(id int) (*User, error) {
    // 模拟数据库查询
    if id <= 0 {
        return nil, fmt.Errorf("invalid id")
    }
    return &User{ID: id, Name: "Alice"}, nil
}

func (r *RealUserService) CreateUser(name string) (int, error) {
    // 模拟创建并返回 ID
    return 123, nil
}

type UserServiceProxy struct {
    realService UserService
    logger      *log.Logger
}

func NewUserServiceProxy(real UserService, logger *log.Logger) *UserServiceProxy {
    return &UserServiceProxy{
        realService: real,
        logger:      logger,
    }
}

func (p *UserServiceProxy) GetUser(id int) (*User, error) {
    p.logger.Printf("[BEFORE] GetUser called with id=%d", id)

    user, err := p.realService.GetUser(id) // 调用真实对象

    if err != nil {
        p.logger.Printf("[AFTER-ERROR] GetUser failed: %v", err)
        return nil, err
    }

    p.logger.Printf("[AFTER] GetUser succeeded, found user: %+v", user)
    return user, nil
}

func (p *UserServiceProxy) CreateUser(name string) (int, error) {
    p.logger.Printf("[BEFORE] CreateUser called with name=%q", name)

    id, err := p.realService.CreateUser(name)

    if err == nil {
        p.logger.Printf("[AFTER] CreateUser succeeded, assigned ID=%d", id)
    } else {
        p.logger.Printf("[AFTER-ERROR] CreateUser failed: %v", err)
    }

    return id, err
}

func main() {
    real := &RealUserService{}
    logger := log.New(os.Stdout, "[PROXY] ", log.LstdFlags)

    proxy := NewUserServiceProxy(real, logger)

    // 调用方无感知 —— 仍用 UserService 接口
    user, _ := proxy.GetUser(42)
    fmt.Printf("Retrieved: %+v\n", user)

    id, _ := proxy.CreateUser("Bob")
    fmt.Printf("Created with ID: %d\n", id)
}

[PROXY] [BEFORE] GetUser called with id=42
[PROXY] [AFTER] GetUser succeeded, found user: &{ID:42 Name:Alice}
Retrieved: &{ID:42 Name:Alice}
[PROXY] [BEFORE] CreateUser called with name="Bob"
[PROXY] [AFTER] CreateUser succeeded, assigned ID=123
Created with ID: 123