slim3中的middleware有两种方式添加：

<?php


require 'vendor/autoload.php';

$app = new Slim\App();

// 第一种方式（应用级中间件）
$app->add(new HMiddleWare());

// 第二种方式 （路由中间件）
$app->get('/hello/{name}', function ($request, $response, $args) {
    return $response->getBody()->write("Hello, " . $args['name']);
})->add(function($request, $response, $next) {
    $response->getBody()->write('BEFORE middle1');
    $response = $next($request, $response);
    $response->getBody()->write('AFTER middle1');

    return $response;
});

$app->run();


class HMiddleWare
{
    function __invoke($request, $response, $next)
    {
        $response->getBody()->write('BEFORE middle2');
        $response = $next($request, $response);
        $response->getBody()->write('AFTER middle2');

        return $response;
    }
}\

slim添加中间件时会一层一层的套，先添加的在最里面，后添加的在最外面，这样request请求进来时也是从外层到里层再到外层。应用级中间件先执行，执行到$route->run()时，会继续执行路由级中间件

image.png

所以访问http://127.0.0.1:8000/hello/fdas，结果是

BEFORE middle2
BEFORE middle1
Hello, fdas
AFTER middle1
AFTER middle2

接着我们来瞅瞅源码的实现，首先我们先看第一种方式是如何添加middleware，

public function add($callable)
{
    return $this->addMiddleware(new DeferredCallable($callable, $this->container));
}

protected function addMiddleware(callable $callable)
{
    if ($this->middlewareLock) {
        throw new RuntimeException('Middleware can’t be added once the stack is dequeuing');
    }
    
    // 第一次将app对象赋值给$this->tip
    if (is_null($this->tip)) {
        $this->seedMiddlewareStack();
    }
    $next = $this->tip;
    $this->tip = function (
        ServerRequestInterface $request,
        ResponseInterface $response
    ) use (
        $callable,
        $next
    ) {
        $result = call_user_func($callable, $request, $response, $next);
        if ($result instanceof ResponseInterface === false) {
            throw new UnexpectedValueException(
                'Middleware must return instance of \Psr\Http\Message\ResponseInterface'
            );
        }

        return $result;
    };

    return $this;
}

protected function seedMiddlewareStack(callable $kernel = null)
{
    if (!is_null($this->tip)) {
        throw new RuntimeException('MiddlewareStack can only be seeded once.');
    }
    if ($kernel === null) {
        $kernel = $this;
    }
    $this->tip = $kernel;
}

先判断$this->tip是否为null，即第一次将app对象赋值给$this->tip，然后将$this->tip赋值给$next，然后将一个闭包函数赋值给$this->tip。在这个函数中，call_user_func($callable, $request, $response, $next)就是调用我们middleware中的__invoke方法。

我们结合上面的例子来讲一讲，在我$app->add(new HMiddleWare())时

/**
 *  第一次添加middleware （应用级）
 */  
$callable      =>     HMiddleWare对象
$this->tip     =>     app对象
$next          =>     app对象
$this->tip     =>     function($request, $response) use (HMiddleWare对象,  app对象) {
    $result = call_user_func(HMiddleWare对象, $request, $response,  app对象);
}


/**
 *  第二次添加middleware (路由级)
 */
$callable         =>    function($request, $response, $next) {
    $response->getBody()->write('BEFORE middle1');
    $response = $next($request, $response);
    $response->getBody()->write('AFTER middle1');
    return $response;
}
$this->tip        =>    route对象
$next             =>    route对象
$this->tip        =>    function($request, $response) use ($callable,  route对象) {
    $result = call_user_func(
                          function($request, $response, $next) {
                                     $response->getBody()->write('BEFORE middle1');
                                     $response = $next($request, $response);
                                     $response->getBody()->write('AFTER middle1');
                                    return $response;
                          },
                         $request,
                         $response,  
                         route对象
             }

这样当我们执行request时(可以看我的另一篇文章关于request的)，即执行$this->tip(),

• 1、先走HMiddleWare的__invoke方法，输出brfore后执行next（）
• 2、next()就是$app->__invoke()方法，在里面执行$route->run()
• 3、在$route->run()添加了路由中间后在执行，先执行callable,输出brfore后，然后在执行next()，
• 4、next（）就是route->__invoke(), 执行完后调用callable的after， 在调用HMiddleWare中的after