Learn Design patterns with me (-), learn design patterns _php Tutorials

Learn Design patterns with me (-), learn design patterns

Today we will study the singleton mode:

To learn a pattern, we always have to know the application scene. Otherwise, it is like learning a martial arts moves, if you do not know under what circumstances to use, it is not wasted Kung fu ah.

For Singleton mode, one of the most common applications is database connectivity. If you open a Web page, build a link, and that kind of sales is a big waste. So we need to use a singleton pattern to make sure that the system only creates new connections when necessary.

If you are using pseudo-code to describe a singleton pattern, you can write:

if (connection exists) {

Return to this link

}else{

Create a new link

}

However, this implementation is obviously problematic, how to save the link, can be accessed externally, but also can not be destroyed by the outside. This is easy if it is implemented in PHP. Let's look at a code together:

// Get Data class class DB {    private$_db;     Private Static $_instance ;     Private function __construct () {        $DSN = '.. /.. /content/xxtebook.db ';         $this New PDO (' SQLite: '.  $DSN);    }     // initializing a database connection     Public Static function Initdb () {        if(! (Self::$_instance  instanceof Self)) {            self::$_instancenew self ();        }         return self::$_instance;      }        // Other methods       }

Why do you want to privatize the constructor? This question is very good, and you are welcome to ask such questions.

First, as a singleton, it is to avoid accidental creation of new instances by outsiders. This is ensured by an internal approach to the creation of the instance. and internal methods access internal variables, with congenital advantages.

In fact, we can save the instance's node to the inside of the class so that the next time the value is checked, it can be removed and used without having to recreate it.

Of course, I saw some books that made an empty __clone () {} method to prevent cloning, which is good. The icing on the cake, I left it to the specific project to do.

Finally, let's look at how to use this single example:

$db = db::initdb (); // $db->sommethod ();

Isn't it very simple.

In addition, for which methods are static, which methods are not, my idea is that if the method inside to access the $this variable, then you should not use static methods, and vice versa can be used static. In other words, static methods do not need to be used by example real objects.

Next time we'll learn the factory model together.

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