详解如何实现Laravel的服务容器的方法示例(7)

class Container
{
  // 已绑定的服务
  protected $instances = [];
  // 已绑定的回调函数
  protected $bindings = [];
  // 服务别名
  protected $aliases = [];
  // 存放扩展器的数组
  protected $extenders = [];
  // 依赖上下文
  protected $context = [];

  // 绑定服务实例
  public function instance($name, $instance)
  {
    $this->instances[$name] = $instance;
  }

  // 绑定服务
  public function bind($name, $instance, $shared = false)
  {
    if ($instance instanceof Closure) {
      // 如果$instance是一个回调函数，就绑定到bindings。
      $this->bindings[$name] = [
        'callback' => $instance,
        // 标记是否单例
        'shared' => $shared
      ];
    } else {
      // 调用make方法，创建实例
      $this->instances[$name] = $this->make($name);
    }
  }

  // 绑定一个单例
  public function singleton($name, $instance)
  {
    $this->bind($name, $instance, true);
  }

  // 给服务绑定一个别名
  public function alias($alias, $name)
  {
    $this->aliases[$alias] = $name;
  }

  // 给服务绑定扩展器
  public function extend($name, $extender)
  {
    if (isset($this->instances[$name])) {
      // 已经实例化的服务，直接调用扩展器
      $this->instances[$name] = $extender($this->instances[$name]);
    } else {
      $this->extenders[$name][] = $extender;
    }
  }

  // 获取服务
  public function make($name, array $parameters = [])
  {
    // 先用别名查找真实服务名
    $name = isset($this->aliases[$name]) ? $this->aliases[$name] : $name;

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

    if (isset($this->bindings[$name])) {
      // 执行回调函数并返回
      $instance = call_user_func($this->bindings[$name]['callback']);

      if ($this->bindings[$name]['shared']) {
        // 标记为单例时，存储到服务中
        $this->instances[$name] = $instance;
      }
    } else {
      // 使用build方法构建此类
      $instance = $this->build($name, $parameters);
    }

    if (isset($this->extenders[$name])) {
      // 调用扩展器
      foreach ($this->extenders[$name] as $extender) {
        $instance = $extender($instance);
      }
    }

    return $instance;
  }

  // 构建一个类，并自动注入服务
  public function build($class, array $parameters = [])
  {
    $reflector = new ReflectionClass($class);

    $constructor = $reflector->getConstructor();

    if (is_null($constructor)) {
      // 没有构造函数，直接new
      return new $class();
    }

    $dependencies = [];

    // 获取构造函数所需的参数
    foreach ($constructor->getParameters() as $dependency) {

      if (isset($this->context[$class]) && isset($this->context[$class][$dependency->getName()])) {
        // 先从上下文中查找
        $dependencies[] = $this->context[$class][$dependency->getName()];
        continue;
      }

      if (isset($parameters[$dependency->getName()])) {
        // 从自定义参数中查找
        $dependencies[] = $parameters[$dependency->getName()];
        continue;
      }

      if (is_null($dependency->getClass())) {
        // 参数类型不是类或接口时，无法从容器中获取依赖
        if ($dependency->isDefaultValueAvailable()) {
          // 查找默认值，如果有就使用默认值
          $dependencies[] = $dependency->getDefaultValue();
        } else {
          // 无法提供类所依赖的参数
          throw new Exception('找不到依赖参数：' . $dependency->getName());
        }
      } else {
        // 参数类型是一个类时，就用make方法构建该类
        $dependencies[] = $this->make($dependency->getClass()->name);
      }
    }

    return $reflector->newInstanceArgs($dependencies);
  }

  // 绑定上下文
  public function addContextualBinding($when, $needs, $give)
  {
    $this->context[$when][$needs] = $give;
  }

  // 支持链式方式绑定上下文
  public function when($when)
  {
    return new Context($when, $this);
  }
}

class Context
{
  protected $when;

  protected $needs;

  protected $container;

  public function __construct($when, Container $container)
  {
    $this->when = $when;
    $this->container = $container;
  }

  public function needs($needs)
  {
    $this->needs = $needs;

    return $this;
  }

  public function give($give)
  {
    // 调用容器绑定依赖上下文
    $this->container->addContextualBinding($this->when, $this->needs, $give);
  }
}