针对条件的访问者模式？

发布于 2024-08-01 12:42:39 字数 641 浏览 6 评论 0原文

我似乎没有在访问者模式的使用场景中找到这一点（或者也许我不明白）。它也没有等级之分。

让我们使用一个身份验证示例。 UserAuthenticator 验证用户提供的凭据。它返回一个结果对象。结果对象包含身份验证的结果：身份验证成功、由于未找到用户名而未成功、由于使用了非法字符而未成功等。客户端代码可以诉诸条件来处理此问题。用伪代码来说：

AuthResult = Userauthenticator.authenticate(Username, Password)
if AuthResult.isAuthenticated: do something
else if AuthResult.AuthFailedBecauseUsernameNotFound: do something else
else if etc...

访客模式适合这里吗？：

Authresult.acceptVisitor(AuthVisitor)

Authresult 然后根据结果调用 AuthVisitor 上的方法：

AuthVisitor.handleNotAuthenticatedBecauseUsernameNotFound

原文

I don't seem to find this in usage scenarios for the visitor pattern (or maybe I don't get it). It's also not hierarchical.

Let's use an authentication example. A UserAuthenticator authenticates credentials given by a user. It returns a result object. The result object contains the result of the authentication: authentication succeeded, not succeeded because username was not found, not succeeded because illegal characters were used etc. Client code may resort to conditionals to handle this.
In pseudocode:

AuthResult = Userauthenticator.authenticate(Username, Password)
if AuthResult.isAuthenticated: do something
else if AuthResult.AuthFailedBecauseUsernameNotFound: do something else
else if etc...

Would a visitor pattern fit here? :

Authresult.acceptVisitor(AuthVisitor)

Authresult then calls a method on AuthVisitor depending on the result :

AuthVisitor.handleNotAuthenticatedBecauseUsernameNotFound

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冷月断魂刀 2024-08-08 12:42:39

当您的数据不会随着您的行为而快速变化时，访问者是一个有价值的设计。一个典型的例子是解析树：

你的类层次结构（你的数据）被冻结
你的行为变化太大，你不想破坏你的类添加另一个虚拟方法

我不认为访问者在这里是一个有价值的解决方案，因为每次添加 AuthResult 的子类都会破坏访问者。

Visitor是关于双重调度的交易封装。

您可以尝试类似的方法：

interface Handler {

    void onUsernameNotFound();

    void onWrongPassword();

    void authOk();
}

interface Authenticator {
    void authenticate(String username, String password, Handler handler);  
}   

class SimpleAuthenticator implements Authetnciator {

    void authenticate(String username, String password, Handler handler) {
        if (username.equals("dfa")) {
            if (password.equals("I'm1337")) {
                handler.authOk();
            } else {
                handler.onWrongPassword();
            }
        } else {
            handler.onUsernameNotFound();
        }
    }
}

一些处理程序策略：

class FatalHandler implements Handler {

    void onUsernameNotFound() {
        throw new AuthError("auth failed");
    }

    void onWrongPassword() {
        throw new AuthError("auth failed");
    }

    void authOk() {
        /* do something */
    }   
}

并且：

class DebugHandler implements Handler {

    void onUsernameNotFound() {
        System.out.println("wrong username");
    }

    void onWrongPassword() {
        System.out.println("wrong password");
    }

    void authOk() {
        System.out.println("ok");
    }   
}

现在您可以将错误处理和操作封装在您的处理程序中，这比访问者的代码少得多，因为您实际上不需要双重分派。

Visitor is a valuable design when your data doesn't change fast as your behaviour. A typical example is with a parse tree:

your class hierarchy (your data) is frozen
your behaviour varies too much, you don't want to break your classes adding another virtual method

I don't think that a Visitor is a valuable solution here, since each time you add a subclass of AuthResult you break your visitor.

Visitor is about trading encapsulation with double dispatch.

You can try a similar approach:

interface Handler {

    void onUsernameNotFound();

    void onWrongPassword();

    void authOk();
}

interface Authenticator {
    void authenticate(String username, String password, Handler handler);  
}   

class SimpleAuthenticator implements Authetnciator {

    void authenticate(String username, String password, Handler handler) {
        if (username.equals("dfa")) {
            if (password.equals("I'm1337")) {
                handler.authOk();
            } else {
                handler.onWrongPassword();
            }
        } else {
            handler.onUsernameNotFound();
        }
    }
}

some Handler stategies:

class FatalHandler implements Handler {

    void onUsernameNotFound() {
        throw new AuthError("auth failed");
    }

    void onWrongPassword() {
        throw new AuthError("auth failed");
    }

    void authOk() {
        /* do something */
    }   
}

and:

class DebugHandler implements Handler {

    void onUsernameNotFound() {
        System.out.println("wrong username");
    }

    void onWrongPassword() {
        System.out.println("wrong password");
    }

    void authOk() {
        System.out.println("ok");
    }   
}

now you can encapsulate error handling and operatorion in your Handlers that is much less code than Visitor since you don't really need double dispatch here.

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