什么是容器

在开发过程中，经常会用到的一个概率就是依赖注入。我们借助依懒注入来解耦代码，选择性的按需加载服务，而这些通常都是借助容器来实现。

容器实现对类的统一管理，并且确保对象实例的唯一性

常用的容器网上有很多，如PHP-DI 、 YII-DI 等各种实现，通常他们要么大而全，要么高度适配特定业务，与实际需要存在冲突。

出于需要，我们自己造一个轻量级的轮子，为了保持规范，我们基于PSR-11 来实现。

PSR-11

PSR 是 php-fig 提供的标准建议，虽然不是官方组织，但是得到广泛认可。PSR-11 提供了容器接口。他包含 ContainerInterface 和 两个异常接口，提供使用建议。

/**
 * Describes the interface of a container that exposes methods to read its entries.
 */
interface ContainerInterface
{
  /**
   * Finds an entry of the container by its identifier and returns it.
   *
   * @param string $id Identifier of the entry to look for.
   *
   * @throws NotFoundExceptionInterface No entry was found for **this** identifier.
   * @throws ContainerExceptionInterface Error while retrieving the entry.
   *
   * @return mixed Entry.
   */
  public function get($id);

  /**
   * Returns true if the container can return an entry for the given identifier.
   * Returns false otherwise.
   *
   * `has($id)` returning true does not mean that `get($id)` will not throw an exception.
   * It does however mean that `get($id)` will not throw a `NotFoundExceptionInterface`.
   *
   * @param string $id Identifier of the entry to look for.
   *
   * @return bool
   */
  public function has($id);
}

实现示例

我们先来实现接口中要求的两个方法

abstract class AbstractContainer implements ContainerInterface
{

  protected $resolvedEntries = [];

  /**
   * @var array
   */
  protected $definitions = [];

  public function __construct($definitions = [])
  {
    foreach ($definitions as $id => $definition) {
      $this->injection($id, $definition);
    }
  }

  public function get($id)
  {

    if (!$this->has($id)) {
      throw new NotFoundException("No entry or class found for {$id}");
    }

    $instance = $this->make($id);

    return $instance;
  }

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

实际我们容器中注入的对象是多种多样的，所以我们单独抽出实例化方法。

public function make($name)
  {
    if (!is_string($name)) {
      throw new \InvalidArgumentException(sprintf(
        'The name parameter must be of type string, %s given',
        is_object($name) ? get_class($name) : gettype($name)
      ));
    }

    if (isset($this->resolvedEntries[$name])) {
      return $this->resolvedEntries[$name];
    }

    if (!$this->has($name)) {
      throw new NotFoundException("No entry or class found for {$name}");
    }

    $definition = $this->definitions[$name];
    $params = [];
    if (is_array($definition) && isset($definition['class'])) {
      $params = $definition;
      $definition = $definition['class'];
      unset($params['class']);
    }

    $object = $this->reflector($definition, $params);

    return $this->resolvedEntries[$name] = $object;
  }

  public function reflector($concrete, array $params = [])
  {
    if ($concrete instanceof \Closure) {
      return $concrete($params);
    } elseif (is_string($concrete)) {
      $reflection = new \ReflectionClass($concrete);
      $dependencies = $this->getDependencies($reflection);
      foreach ($params as $index => $value) {
        $dependencies[$index] = $value;
      }
      return $reflection->newInstanceArgs($dependencies);
    } elseif (is_object($concrete)) {
      return $concrete;
    }
  }

  /**
   * @param \ReflectionClass $reflection
   * @return array
   */
  private function getDependencies($reflection)
  {
    $dependencies = [];
    $constructor = $reflection->getConstructor();
    if ($constructor !== null) {
      $parameters = $constructor->getParameters();
      $dependencies = $this->getParametersByDependencies($parameters);
    }

    return $dependencies;
  }

  /**
   *
   * 获取构造类相关参数的依赖
   * @param array $dependencies
   * @return array $parameters
   * */
  private function getParametersByDependencies(array $dependencies)
  {
    $parameters = [];
    foreach ($dependencies as $param) {
      if ($param->getClass()) {
        $paramName = $param->getClass()->name;
        $paramObject = $this->reflector($paramName);
        $parameters[] = $paramObject;
      } elseif ($param->isArray()) {
        if ($param->isDefaultValueAvailable()) {
          $parameters[] = $param->getDefaultValue();
        } else {
          $parameters[] = [];
        }
      } elseif ($param->isCallable()) {
        if ($param->isDefaultValueAvailable()) {
          $parameters[] = $param->getDefaultValue();
        } else {
          $parameters[] = function ($arg) {
          };
        }
      } else {
        if ($param->isDefaultValueAvailable()) {
          $parameters[] = $param->getDefaultValue();
        } else {
          if ($param->allowsNull()) {
            $parameters[] = null;
          } else {
            $parameters[] = false;
          }
        }
      }
    }
    return $parameters;
  }

如你所见，到目前为止我们只实现了从容器中取出实例，从哪里去提供实例定义呢，所以我们还需要提供一个方水法

/**
   * @param string $id
   * @param string | array | callable $concrete
   * @throws ContainerException
   */
  public function injection($id, $concrete)
  {
    if (is_array($concrete) && !isset($concrete['class'])) {
      throw new ContainerException('数组必须包含类定义');
    }

    $this->definitions[$id] = $concrete;
  }

只有这样吗？对的，有了这些操作我们已经有一个完整的容器了，插箱即用。

不过为了使用方便，我们可以再提供一些便捷的方法，比如数组式访问。

class Container extends AbstractContainer implements \ArrayAccess
{

  public function offsetExists($offset)
  {
    return $this->has($offset);
  }

  public function offsetGet($offset)
  {
    return $this->get($offset);
  }

  public function offsetSet($offset, $value)
  {
    return $this->injection($offset, $value);
  }

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

这样我们就拥有了一个功能丰富，使用方便的轻量级容器了，赶快整合到你的项目中去吧。