Teach you how to manually create a PHP DI container

Regarding dependency injection, I believe everyone should be exposed to it frequently or at least have heard about it. The relatively well-known frameworks all support dependency injection, such as Java's Spring, PHP's Laravel, Symfony, etc. Now let me start to implement a simple DI container manually.

Start by driving

Let’s drive a car first and give you an example:

class Driver{    public function drive()
    {
        $car = new Car();        echo '老司机正在驾驶', $car->getCar(), PHP_EOL;
    }
}class Car{    protected $name = '普通汽车';    public function getCar()
    {        return $this->name;
    }
}

There are two categories, Driver and Car, for experienced drivers Driver has a method driver. When calling, first get the entire car $car, and then start the car. Most students have written this or similar code. There is nothing wrong with this code and it is quite normal. However, if I want to change my car, I won’t be able to attract girls with a regular car.

class Benz extends Car{    protected $name = '奔驰';
}

At this time, you need to do a rather disgusting operation, and you have to change the old driver's code. (Old driver: What did I do wrong? If I change a car, I have to relearn my driver’s license...). Therefore, we need to inject Car into the outside world and decouple Driver and Car, so that experienced drivers no longer have to build their own cars when they drive. So we have the following result

class Driver{    protected $car;    public function __construct(Car $car)
    {        $this->car = $car;
    }    public function drive()
    {        echo '老司机正在驾驶', $this->car->getCar(), PHP_EOL;
    }
}

At this time, the Driver and Car classes have been decoupled, and the dependencies of these two classes are managed by the upper-layer code. At this time, the experienced driver will "drive" like this:

$car = new Car();
$driver = new Driver($car);
$driver->drive();

At this time, we create an instance of the Driver dependency and inject it. In the above example, we implemented dependency injection, but it was manual, and it still felt uncomfortable to write. How can such a heavy work be done manually? We have to let the program do it by itself. Ever since, the DI container was born.

Dependency Injection Container

Dependency injection is similar to the IoC model and the factory model. It is a model that solves the dependency coupling relationship between the caller and the callee. It solves the dependency relationship between objects, making the objects only rely on the IoC/DI container, no longer directly dependent on each other, achieving loose coupling, and then when the object is created, the IoC/DI container injects its dependent (Dependency) objects ( Inject), this can achieve maximum loose coupling. To put it bluntly, dependency injection means that the container injects instances of other classes that a certain class depends on into instances of this class.

This paragraph may be a bit abstract. Let’s go back to the example just now. I just completed the dependency injection manually, which is quite troublesome. If it is done in a large project, it will definitely be very cumbersome and not elegant enough. Therefore, we need a steward to do this for us, and this steward is the container. All dependency management of classes is left to the container. Therefore, generally speaking, a container is a global object that is shared by everyone.

Make your own DI container

To write a function, we first need to analyze the problem, so we must first understand what functions are needed for a simple DI container, which is directly related to our code of preparation. For a simple container, at least the following points need to be met:

• Create instances of the required classes

• Complete dependency management (DI)

• You can get the instance of the singleton

• Globally unique

In summary, our container class It looks like this:

class Container{    /**
     * 单例
     * @var Container
     */
    protected static $instance;    /**
     * 容器所管理的实例
     * @var array
     */
    protected $instances = [];    private function __construct(){}  
    private function __clone(){}    /**
     * 获取单例的实例
     * @param string $class
     * @param array ...$params
     * @return object
     */
    public function singleton($class, ...$params)
    {}    /**
     * 获取实例（每次都会创建一个新的）
     * @param string $class
     * @param array ...$params
     * @return object
     */
    public function get($class, ...$params)
    {}    /**
     * 工厂方法，创建实例，并完成依赖注入
     * @param string $class
     * @param array $params
     * @return object
     */
    protected function make($class, $params = [])
    {}    /**
     * @return Container
     */
    public static function getInstance()
    {        if (null === static::$instance) {            static::$instance = new static();
        }        return static::$instance;
    }
}

The general skeleton has been determined, and then we enter the core make method:

protected function make($class, $params = []){  //如果不是反射类根据类名创建
  $class = is_string($class) ? new ReflectionClass($class) : $class;  //如果传的入参不为空，则根据入参创建实例
  if (!empty($params)) {    return $class->newInstanceArgs($params);
  }  //获取构造方法
  $constructor = $class->getConstructor();  //获取构造方法参数
  $parameterClasses = $constructor ? $constructor->getParameters() : [];  if (empty($parameterClasses)) {    //如果构造方法没有入参，直接创建
    return $class->newInstance();
  } else {    //如果构造方法有入参，迭代并递归创建依赖类实例
    foreach ($parameterClasses as $parameterClass) {
      $paramClass = $parameterClass->getClass();
      $params[] = $this->make($paramClass);
    }    //最后根据创建的参数创建实例，完成依赖的注入
    return $class->newInstanceArgs($params);
  }
}

In order to make the container easier to use, I have made some improvements:

• Implement the ArrayAccess interface so that the singleton instance can be obtained directly through the array. If the instance does not exist, create it

• Override the __get method, which is more convenient Get the final version of

:

class Container implements ArrayAccess{    /**
     * 单例
     * @var Container
     */
    protected static $instance;    /**
     * 容器所管理的实例
     * @var array
     */
    protected $instances = [];    private function __construct(){}    private function __clone(){}    /**
     * 获取单例的实例
     * @param string $class
     * @param array  ...$params
     * @return object
     */
    public function singleton($class, ...$params)
    {        if (isset($this->instances[$class])) {            return $this->instances[$class];
        } else {            $this->instances[$class] = $this->make($class, $params);
        }        return $this->instances[$class];
    }    /**
     * 获取实例（每次都会创建一个新的）
     * @param string $class
     * @param array  ...$params
     * @return object
     */
    public function get($class, ...$params)
    {        return $this->make($class, $params);
    }    /**
     * 工厂方法，创建实例，并完成依赖注入
     * @param string $class
     * @param array  $params
     * @return object
     */
    protected function make($class, $params = [])
    {        //如果不是反射类根据类名创建
        $class = is_string($class) ? new ReflectionClass($class) : $class;        //如果传的入参不为空，则根据入参创建实例
        if (!empty($params)) {            return $class->newInstanceArgs($params);
        }        //获取构造方法
        $constructor = $class->getConstructor();        //获取构造方法参数
        $parameterClasses = $constructor ? $constructor->getParameters() : [];        if (empty($parameterClasses)) {            //如果构造方法没有入参，直接创建
            return $class->newInstance();
        } else {            //如果构造方法有入参，迭代并递归创建依赖类实例
            foreach ($parameterClasses as $parameterClass) {
                $paramClass = $parameterClass->getClass();
                $params[] = $this->make($paramClass);
            }            //最后根据创建的参数创建实例，完成依赖的注入
            return $class->newInstanceArgs($params);
        }
    }    /**
     * @return Container
     */
    public static function getInstance()
    {        if (null === static::$instance) {            static::$instance = new static();
        }        return static::$instance;
    }    public function __get($class)
    {        if (!isset($this->instances[$class])) {            $this->instances[$class] = $this->make($class);
        }        return $this->instances[$class];
    }    public function offsetExists($offset)
    {        return isset($this->instances[$offset]);
    }    public function offsetGet($offset)
    {        if (!isset($this->instances[$offset])) {            $this->instances[$offset] = $this->make($offset);
        }        return $this->instances[$offset];
    }    public function offsetSet($offset, $value)
    {
    }    public function offsetUnset($offset) {        unset($this->instances[$offset]);
    }
}

Now with the help of the container, let’s write the above code:

$driver = $app->get(Driver::class);
$driver->drive();//output:老司机正在驾驶普通汽车复制代码

It’s that simple, experienced drivers can start the train. The default injection here is the instance of Car. If you need to drive a Mercedes-Benz, you only need to do this:

$benz = $app->get(Benz::class);
$driver = $app->get(Driver::class, $benz);
$driver->drive();//output:老司机正在驾驶奔驰复制代码

According to the requirements of PSR-11, the dependency injection container needs to implement the Psr\Container\ContainerInterface interface. This is just a demonstration. To implement it, because it requires the introduction of the Psr dependency library, which is more troublesome, but it is actually very simple. It just has a few more methods. Those who are interested can learn about the requirements of PSR-11 (portal) by themselves.

This is just a very simple DI container. In practice, there is a lot to consider, and the container function here is still very simple. There are still some pitfalls that have not been dealt with, such as how to deal with circular dependencies and the mechanism of delayed loading...

Here are just some records of my weekend practice. If you are interested, you can read the following about Laravel or Symfony containers. Source code, or learn about Spring's container. I will continue to improve it when I have time. [Recommended learning: "PHP Video Tutorial"]

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