【PHP高級特性】ArrayAccess（數組式訪問）接口

php提供了6個常用的預定義接口，實現某些特定的能力。其中最最常用的就是 ArrayAccess 了，像 Laravel 這種流行的框架都用到了它。

ArrayAccess 是啥

如官方文檔所述，它“提供像訪問數組一樣訪問對象的能力的接口”。

它提供了4個接口

/**
 * Interface to provide accessing objects as arrays.
 * @link http://php.net/manual/en/class.arrayaccess.php
 */
interface ArrayAccess {

    /**
     * Whether a offset exists
     * @link http://php.net/manual/en/arrayaccess.offsetexists.php
     * @param mixed $offset <p>
     * An offset to check for.
     * </p>
     * @return boolean true on success or false on failure.
     * </p>
     * <p>
     * The return value will be casted to boolean if non-boolean was returned.
     * @since 5.0.0
     */
    public function offsetExists($offset);

    /**
     * Offset to retrieve
     * @link http://php.net/manual/en/arrayaccess.offsetget.php
     * @param mixed $offset <p>
     * The offset to retrieve.
     * </p>
     * @return mixed Can return all value types.
     * @since 5.0.0
     */
    public function offsetGet($offset);

    /**
     * Offset to set
     * @link http://php.net/manual/en/arrayaccess.offsetset.php
     * @param mixed $offset <p>
     * The offset to assign the value to.
     * </p>
     * @param mixed $value <p>
     * The value to set.
     * </p>
     * @return void
     * @since 5.0.0
     */
    public function offsetSet($offset, $value);

    /**
     * Offset to unset
     * @link http://php.net/manual/en/arrayaccess.offsetunset.php
     * @param mixed $offset <p>
     * The offset to unset.
     * </p>
     * @return void
     * @since 5.0.0
     */
    public function offsetUnset($offset);
}

我們實現這4個接口，依次對應數組的isset,讀取，設置，unset操作。

有什麼用

定義說的很明白啦，提供像訪問數組一樣訪問對象的能力。用上了它，可以讓一個類即可以支持對象引用，也支持數組引用。

代碼實現示例

class Container implements ArrayAccess
{

    /**
     * @var array 單例對象索引
     */
    private $instances = [];

    /**
     * @var array 可實例化對象定義索引
     */
    private $definitions = [];

    public function offsetExists($offset)
    {
        return isset($this->definitions[$offset]);
    }

    public function offsetGet($offset)
    {
        if (isset($this->instances[$offset])) {
            return $this->instances[$offset];
        } elseif (isset($this->definitions[$offset])) {
            return $this->make($offset);
        }

        throw new \Exception('未提供對象定義');
    }

    public function offsetSet($offset, $value)
    {
        // ... 省略一些較驗判斷
        $this->definitions[$offset] = $value;
    }

    public function offsetUnset($offset)
    {
        unset($this->definitions[$offset]);
        unset($this->instances[$offset]);
    }

    private function make($offset)
    {
        $definition = $this->definitions[$offset];

        if ($definition instanceof \Closure) {
            return $this->instances[$offset] = $definition();
        }

        if (is_object($definition)) {
            return $this->instances[$offset] = $definition;
        }

        if (is_array($definition)) {
            $class = $definition['class'];
            $reflection = new \ReflectionClass($class);

            $dependencies = [];
            // ... 省略反射的實現代碼
            $object = $reflection->newInstanceArgs($dependencies);
            return $this->instances[$offset] = $object;
        }

        throw new \Exception('對象定義不合法');
    }
}

使用示例

$container = new Container();

$container['test'] = function () {
  return 'this is a test';
};

var_dump(isset($container['test']));

echo $container['test'];

unset($container['test']);

參考

預定義接口