为什么tp5.1的middleware必须要有handle方法

1，在thinkphp\libary\think\Middleware中的add()方法有关

  /**
     * 注册中间件
     * @access public
     * @param  mixed  $middleware
     * @param  string $type  中间件类型
     */
    public function add($middleware, $type = 'route')
    {
        if (is_null($middleware)) {
            return;
        }

        $middleware = $this->buildMiddleware($middleware, $type);

        if ($middleware) {
            $this->queue[$type][] = $middleware;
        }
    }

 /**
     * 解析中间件
     * @access protected
     * @param  mixed  $middleware
     * @param  string $type  中间件类型
     */
    protected function buildMiddleware($middleware, $type = 'route')
    {
        if (is_array($middleware)) {
            list($middleware, $param) = $middleware;
        }

        if ($middleware instanceof \Closure) {
            return [$middleware, isset($param) ? $param : null];
        }

        if (!is_string($middleware)) {
            throw new InvalidArgumentException('The middleware is invalid');
        }

        if (false === strpos($middleware, '\\')) {
            if (isset($this->config[$middleware])) {
                $middleware = $this->config[$middleware];
            } else {
                $middleware = $this->config['default_namespace'] . $middleware;
            }
        }

        if (is_array($middleware)) {
            return $this->import($middleware, $type);
        }

        if (strpos($middleware, ':')) {
            list($middleware, $param) = explode(':', $middleware, 2);
        }

        return [[$this->app->make($middleware), 'handle'], isset($param) ? $param : null];
    }

2，为何要有$requst,和next（也是在此文件中）

 /**此方法在app的run()中执行
     * 中间件调度
     * @access public
     * @param  Request  $request
     * @param  string   $type  中间件类型
     */
    public function dispatch(Request $request, $type = 'route')
    {
        return call_user_func($this->resolve($type), $request);
    }

    // 被多次调用因为里面的 $middleware = array_shift($this->queue[$type]); call_user_func_array($call, [$request, $this->resolve($type), 
    protected function resolve($type = 'route')
    {
        return function (Request $request) use ($type) {

            $middleware = array_shift($this->queue[$type]);
            if (null === $middleware) {
                throw new InvalidArgumentException('The queue was exhausted, with no response returned');
            }

            list($call, $param) = $middleware;

            try {
                $response = call_user_func_array($call, [$request, $this->resolve($type), $param]);//要传入的参数是$request $next
            } catch (HttpResponseException $exception) {
                $response = $exception->getResponse();
            }

            if (!$response instanceof Response) {
                throw new LogicException('The middleware must return Response instance');
            }

            return $response;
        };
    }