《编写可读代码的艺术》第4章 审美

1. 为什么审美这么重要？

    让人愉悦的代码更易读，因此更容易使用。

2. 重新安排换行来保持一致和紧凑

    下面的代码有着优雅一致的风格，并且很容易从头看到尾快速浏览。

    但是缺憾在于用了更多的纵向空间，并且注释重复了三遍。

public class PerformanceTester {
    public static final TcpConnectionSimulator wifi =
        new TcpConnectionSimulator(
            500,   /* Kbps */
            80,    /* millisecs latency */
            200,   /* jitter */
            1      /* packet loss % */);
            
    public static final TcpConnectionSimulator t3_fiber =
        new TcpConnectionSimulator(
            45000, /* Kbps */
            10,    /* millisecs latency */
            0,     /* jitter */
            0      /* packet loss % */);
            
    public static final TcpConnectionSimulator cell =
        new TcpConnectionSimulator(
            100,   /* Kbps */
            400,   /* millisecs latency */
            250,   /* jitter */
            5      /* packet loss % */);
}

    更紧凑的书写方式如下：

public class PerformanceTester {
    // TcpConnectionSimulator(throughput, latency, jitter, packet_loss)
    //                        [Kbps]        [ms]    [ms]    [percent]
    public static final TcpConnectionSimulator wifi =
        new TcpConnectionSimulator(500, 80, 200, 1);
    public static final TcpConnectionSimulator t3_fiber =
        new TcpConnectionSimulator(45000, 10, 0, 0);
    public static final TcpConnectionSimulator cell =
        new TcpConnectionSimulator(100, 400, 250, 5);
}

3. 用方法（函数）整理不规则的东西

    下面这段代码没有什么美感可言。

DatabaseConnection database_connection;
string error;
assert(ExpandFullName(database_connection, "Doug Adams", &error)
    == "Mr. Douglas Adams");
assert(error == "");
assert(ExpandFullName(database_connection, " Jake Brown ", &error)
    == "Mr. Jacob Brown III");
assert(error == "");
assert(ExpandFullName(database_connection, "No Such Guy", &error) == "");
assert(error == "no match found");
assert(ExpandFullName(database_connection, "John", &error) == "");
assert(error == "more than one result");

    需要一个辅助函数来改进。

void CheckFullName(string partial_name,
                   string expected_full_name,
                   string expected_error) {
    // database_connection is now a class member
    string error;
    string full_name = ExpandFullName(database_connection, partial_name, &error);
    assert(error == expected_error);
    assert(full_name == expected_full_name);
}

CheckFullName("Doug Adams", "Mr. Douglas Adams", "");
CheckFullName(" Jake Brown ", "Mr. Jake Brown III", "");
CheckFullName("No Such Guy", "", "no match found");
CheckFullName("John", "", "more than one result");

    尽管目的仅仅是让代码更有美感，但是还产生了几个附带的效果

    （1）消除了代码中的重复，使之变得更紧凑

    （2）每个测试用例重要的部分都变得更直白

    （3）添加新的测试更加简单

 4. 在需要时使用列对齐

    对于上面的例子，可以使用列对齐。这样能使得错误更加明显，阅读时很容易从一列跳到另一列。

CheckFullName("Doug Adams"  , "Mr. Douglas Adams" , "");
CheckFullName(" Jake Brown ", "Mr. Jake Brown III", "");
CheckFullName("No Such Guy" , ""                  , "no match found");
CheckFullName("John"        , ""                  , "more than one result");

commands[] = {
    ...
    { "timeout",       NULL,               cmd_spec_timeout },
    { "timestamping",  &opt.timestamping,  cmd_boolean },
    { "tries",         &opt.ntry,          cmd_number_inf },
    { "useproxy",      &opt.use_proxy,     cmd_boolean },
    { "useragent",     NULL,               cmd_spec_useragent },
    ...
};

    有些人不喜欢列对齐，因为维护工作量大。实际可能不会有太大的工作量，不妨试试。

5. 把声明按块（逻辑）组织起来

class FrontendServer {
    public:
        FrontendServer();
        ~FrontendServer();
        
        // Handlers
        void ViewProfile(HttpRequest* request);
        void SaveProfile(HttpRequest* request);
        void FindFriends(HttpRequest* request);
        
        // Request/Reply Utilities
        string ExtractQueryParam(HttpRequest* request, string param);
        void ReplyOK(HttpRequest* request, string html);
        void ReplyNotFound(HttpRequest* request, string error);
        
        // Database Helpers
        void OpenDatabase(string location, string user);
        void CloseDatabase(string location);
};

6. 把代码分成段落，同时为段落加上总结性注释

def suggest_new_friends(user, email_password):
    # Get the user's friends' email addresses.
    friends = user.friends()
    friend_emails = set(f.email for f in friends)
    
    # Import all email addresses from this user's email account.
    contacts = import_contacts(user.email, email_password)
    contact_emails = set(c.email for c in contacts)
    
    # Find matching users that they aren't already friends with.
    non_friend_emails = contact_emails - friend_emails
    suggested_friends = User.objects.select(email__in=non_friend_emails)

    # Display these lists on the page.
    display['user'] = user
    display['friends'] = friends
    display['suggested_friends'] = suggested_friends
    return render("suggested_friends.html", display)

7. 个人风格与一致性

    一致的风格比“正确”的风格更重要

class Logger {
    //...
};

class Logger
{
    //...
};