大罗-PHP/Python博客

Posted by rollenc

Posted by rollenc

Error-Handing Techniques

• 返回中立值 如：数值返回0
• 换用下一个正确的数据
• 返回与前次相同的数据
• 换用最接近的合法值 如经度设置为(-180,180)之间
• 把警告信息记录到日志文件中 兼用以上的处理，同时记录它。
• 返回一个错误码
• 设置一个状态变量的值（个人不推荐）
• 用状态值作为函数的返回值
• 用语言内建的异常机制跑出一个异常
• 调用错误处理子程序活对象 优点：能把错误处理的职责都集中到一起。代价：错误处理代码与整个程序紧密耦合。
• 当错误发生时显示出错信息 出错信息散布于整个应用程序中。
• 用最妥当的方式在局部处理错误 留给执行设计和实现的程序员来解决，灵活性强，但整体正确性和可靠性无法满足，风险显著
• 关闭程序 用于人身攸关的应用程序

Posted by rollenc

Posted by rollenc

Posted by rollenc

Introduction

While we are not going to go as far as developing a database independent version of SQL, the new features of PHP 5 will be us build a consistent database API with support for stored procedures (where it is not already present). This will then make switching from one database system to another as painless as changing one line of code.

Interfaces Abstract Classes and the Adapter Pattern

The first features new to PHP 5 to be covered in this article are abstract classes and interfaces. These concepts are nothing more than features added to OOP, which help the programmer follow good coding standards.

Abstract Classes

An abstract class is a class that is only partially implemented by the programmer. It may contain one or more abstract methods. An abstract method is simply a function definition that serves to tell the programmer that the method must be implemented in a child class.

To create an abstract class we use the code shown in Listing 1:

Listing 1: An Abstract PHP class

SerialNumber = $SerialNumber;  
    }
    public function getSerialNumber()  
    {  
        return $this->SerialNumber;  
    }  
}  
?>

The abstract class in Listing 1 contains some of the methods required for a weapon. The fire() method however, cannot be implemented because each different weapons use different firing different mechanisms. The method is therefore declared as abstract, meaning it will be implemented in a more specific child class.

Because the class is abstract, an instance of it can never be created (remember, it is only a partial implementation). Instead a child class must be created using inheritance and implement the fire method in itself. Failure to do so will result in a fatal error. Listing 2 shows a child class being created from the Abstract Weapon class.

Listing 2: Extending the Abstract Weapons class

SafetyOff) {  
            return $this->CurrentBullet;  
        }  
    }  
}  

class Cannon extends Weapon  
{  
    public function fire()  
    {  
        $this->NeedsLoading = true;  
        return $this->CurrentCanon;  
    }  
?>

An instance of the Cannon and Gun classes can now be created because they now fully implemented subclasses of weapon.

Interfaces

An interface is similar to an abstract class; indeed interfaces occupy the same namespace as classes and abstract classes. For that reason, you cannot define an interface with the same name as a class. An interface is a fully abstract class; none of its methods are implemented and instead of a class sub-classing from it, it is said to implement that interface.

An interface will be used in the database abstraction layer you create. This ensures that every time you create a class for a particular database, the same API is exposed. When using an interface, you can then rely on the methods defined for the interface to be part of the class because, if they are not, PHP will not parse it.

The MySql functions will be used as an example because they are the most commonly used amongst PHP programmers. The most commonly used functions are:

• mysql_connect()
• mysql_error()
• mysql_errno()
• mysql_query()
• mysql_fetch_array()
• mysql_fetch_row()
• mysql_fetch_assoc()
• mysql_fetch_object()
• mysql_num_rows()
• mysql_close()

If all the database class APIs you create can expose the same methods with the same return types then you can be sure that changing from one database to another, such as from MySql to Postgre SQL, will be painless. As such, the interface in listing 3 can be determined for your API.

Listing 3: An Abstracted Database Interface

interface DB 
{ 
    public function connect(); 
    public function error(); 
    public function errno(); 
    public static function escape_string($string); 
    public function query($query); 
    public function fetch_array($result); 
    public function fetch_row($result); 
    public function fetch_assoc($result); 
    public function fetch_object($result); 
    public function num_rows($result); 
    public function close(); 
} 

Notice how the escape_string() method is included as a static method. This method does not require an active connection to a database and should not require and instance of any object which implements the DB interface. In my opinion, this is the single most important method of any database implementation; a poorly implemented escape string method could make your applications vulnerable SQL injection.

Listing 4: Implementing the database interface

class MySqlDB implements DB 
{ 
        private  $link; 
         
        public function connect($server='', $username='', $password='', $new_link=true, $client_flags=0) 
        { 
            $this->link = mysql_connect($server, $username, $password, $new_link, $client_flags); 
        } 
     
        public function errno() 
        { 
            return mysql_errno($this->link); 
        } 

        public function error() 
        { 
            return mysql_error($this->link); 
        } 

        public static function escape_string($string) 
        { 
            return mysql_real_escape_string($string); 
        } 

        public function query($query) 
        { 
            return mysql_query($query, $this->link); 
        } 
         
        public function fetch_array($result, $array_type = MYSQL_BOTH) 
        { 
            return mysql_fetch_array($result, $array_type); 
        } 

        public function fetch_row($result) 
        { 
            return mysql_fetch_row($result); 
        } 
         
        public function fetch_assoc($result) 
        { 
            return mysql_fetch_assoc($result); 
        } 
         
        public function fetch_object($result) 
        { 
            return mysql_fetch_object($result); 
        } 
         
        public function num_rows($result) 
        { 
            return mysql_num_rows($result); 
        } 
         
        public function close() 
        { 
            return mysql_close($this->link); 
        } 
}
?>

You'll notice that there are many more mysql functions than methods that are adapted in the interface in listing 3. However, this small subset of functions is sufficient to meet the needs of most applications requiring trivial data storage and retrieval. The additional functions can be implemented and I have done this in the attached example file, you may also choose to add additional functionality to the class and the interface.

Listing 5: Creating a database class

    $db = new MySqlDb; 
    $db->connect('host', 'username', 'password'); 
    $db->query('use users'); // we could also use $db->select_db here but it is not consistent with the interface 

    $result = $db->query("SELECT username FROM users"); 
         
    while($row = $db->fetch_assoc($reuslt)) 
    { 
        echo($row['username']); 
    } 

As shown in listing 5, you can now create a class for each database you want and as long as it implements the DB interface, switching from one to another is as easy as changing one line of code:

$db = new MsSqlDb; 

Conclusion

In this first article, you've seen how to create an abstraction layer to access a database. Using this, you saw how creating an interface helps you to isolate your application so that you can easily switch from one database to anther without rewriting your own applications.

Downloads

• db.php
• mysql_db.php

About the Author

rollenc总结

Posted by rollenc

Posted by rollenc

Posted by rollenc

Now, how is the products.html page supposed to call these functions? We want to be able to let the user select an item, and then load the values based on that selection. Using a "Search" or "Go" button is a little archaic, so let's use the onChange method, which we can use to call javascript functions whenever the user changes the selected item. Change the <select> tag in products.html to look like this:

那现在，products.html如何来调用这些函数呢 ?我们需要在用户选择条目后，加载与这个条目相关的信息。使用“搜索”或“go”显得有点老套，所以我们使用了onChange方法（事件），当用户改变选择的条目时，我们通过JAVASCRIPT来调用。将products.html文件中的<select>标签修改成这样：

Whenever a user changes his selection in the select box, getProducts() will be called. So, now that we've got a script that sends some data to a PHP file and receives data returned from a PHP file, we need to program the PHP file.


一旦用户改变了选择框中的选项，getProducts()就会被调用。这样我们已经编写好了发送请求和接受数据的脚本，我们接下来需要的是编写PHP文件。




Programming the PHP back-end


编写PHP后台




In our javascript function getProducts(), we made a call to the following:


在Javascript函数getProducts()，我们做了这样的调用：




    http.open('get', 'internal_request.php?action=get_products&id=' + document.getElementById('product_categories').selectedIndex);






So we obviously need a PHP script that handles the action and id arguments from a GET request.





                Any output to the browser will be
                retrieved in the XMLHttpRequest object's
                responseText property */
            echo '
                

                    
CDex
                    
CoolEdit
                    
Winamp
                    
XMMS
                
';
            break;
        case 1: //Games
            echo '
                

                    
Blackjack
                    
Calculatron
                    
Hold\'em
                    
Minesweeper
                    
Tetris
                
';
            break;
        case 2: //Internet
            echo '
                

                    
Epiphany
                    
Internet Explorer
                    
Mozilla
                    
Netscape
                    
Opera
                    
Safari
                
';
            break;
        default:
            echo 'You didn\'t select an item from above!';
            break;     
    }    
}
?>




Save the above in a file called internal_request.php.

保存以上文件为internal_request.php.



Browse to products.html using your favorite browser, and select one of the category options. You'll see the product_cage div change according to which items you selected. It may not seem like much with all the work that you put into it, but giving the user the ability to search and receive results while on the same page is great. Imagine posting to a forum without leaving the thread, or even something similar to the above for a large corporation with thousands of products. Bandwidth saved is money saved, and your company will no doubt appreciate that.

在你喜欢的浏览器中打开products.html，选择一个选项，你会发现，名称为product_cage的DIV内容根据你的选择改变了。这并不是所有的好处。想象一下，如果在论坛上恢复一篇文章而不需要重新刷新本页，或者当网站拥有成千上万的产品时，毫无疑问，贵公司会因为流量的节省而高兴的。

There are many resources available to learn more about AJAX and the XMLHttpRequest object, and now that you know the basics, you shouldn't have any problem expanding your knowledge to fit your application. Good luck!

有许多可以利用和学习的Ajax，XMLHttpRequest的知识，到现在你只是了解到了一些基础知识而已。希望你不会因为进一步学习和应用而遇到困惑。祝你好运！

Source for Included File: products.html

products.html的源代码：


    
        
        
    
    
        

            
            


                
                    Audio
                    Games

                    Internet