在 PHP 中动态填充静态变量

Question

我有两个静态值：“type”和“typeID”。类型是人类可读的且恒定的，并且需要根据类型的值从数据库中查找类型ID。我需要在首次加载类定义时进行一次查找。

为了说明这一点，这里有一些代码不起作用，因为您无法在声明空间中调用函数。

MyClass extends BaseClass {
  protected static $type = "communities";
  protected static $typeID = MyClass::lookupTypeID(self::$type);
}

是否有一个魔术方法在加载类定义时只调用一次？如果有什么明显的东西我会错过它。

Answer 1

无耻地从php手册的静态关键字注释中提取：

Because php does not have a static constructor and you may want to initialize static class vars, there is one easy way, just call your own function directly after the class definition.

for example.

<?php
function Demonstration()
{
    return 'This is the result of demonstration()';
}

class MyStaticClass
{
    //public static $MyStaticVar = Demonstration(); //!!! FAILS: syntax error
    public static $MyStaticVar = null;

    public static function MyStaticInit()
    {
        //this is the static constructor
        //because in a function, everything is allowed, including initializing using other functions

        self::$MyStaticVar = Demonstration();
    }
} MyStaticClass::MyStaticInit(); //Call the static constructor

echo MyStaticClass::$MyStaticVar;
//This is the result of demonstration()
?>

Answer 2

简单且不需要魔法，不要忘记您始终可以将变量定义为 null 并测试它是否为 null（然后才进行 db 调用）。那么，如果您希望在构造或包含类时发生这种情况（include_once 等...）

MyClass extends BaseClass {
    protected static $type = "communities";
    protected static $typeID = null;

    public function __construct(){
        if(is_null(self::$typeID)){
            self::lookupTypeID(self::$type);
        }
    }

    public static lookupTypeID($type){
        self::$typeID = //result of database query
    }
}

，或者

MyClass::lookupTypeID(); //call static function when class file is included (global space)

MyClass extends BaseClass {
    protected static $type = "communities";
    protected static $typeID = null;

    public function __construct(){

    }

    public static lookupTypeID($type=null){
        if(is_null($type)){
            $type = self::$type;
        }
        self::$typeID = //result of database query (SELECT somefield FROM sometable WHERE type=$type) etc..
    }
}

静态构造函数更像是工厂方法，那么这只是一个问题

if(!function_exists(build_myclass)){
    function build_myclass(){
        return MyClass::build();
    }
}

MyClass extends BaseClass {
    protected static $type = "communities";
    protected static $typeID = null;

    public function __construct(){

    }

    public static function build(){
        return new self(); //goes to __construct();
    }

 }

$class = new MyClass(); //or
$class = MyClass::build(); //or
$class = build_myclass();

Answer 3

这样的东西通常被称为“静态构造函数”，但 PHP 缺少这样的东西。您可能需要考虑 PHP 手册注释中建议的解决方法之一，例如 http://www.php.net/manual/en/language.oop5.static.php#95217