摘要：學習筆記之已經聊過使用了來設計，看源碼發現其巧妙用了和的一些數組函數來設計。開發環境內置函數和看源碼之前，先看下這幾個內置函數的使用。學習筆記之實例化源碼解析已經聊過的實例化，得到中的變量，即的實例化對象。后面再學習下的源碼，到時見。

說明：本文主要學習Laravel的Middleware的源碼設計思想，并將學習心得分享出來，希望對別人有所幫助。Laravel學習筆記之Decorator Pattern已經聊過Laravel使用了Decorator Pattern來設計Middleware，看Laravel源碼發現其巧妙用了Closure和PHP的一些數組函數來設計Middleware。

開發環境：Laravel5.3 + PHP7 + OS X 10.11

看Laravel源碼之前，先看下這幾個PHP內置函數的使用。首先array_reverse()函數比較簡單，倒置數組，看測試代碼：

$pipes = [
    "Pipe1",
    "Pipe2",
    "Pipe3",
    "Pipe4",
    "Pipe5",
    "Pipe6",
];

$pipes = array_reverse($pipes);

var_dump($pipes);

// output
array(6) {
  [0] =>
  string(5) "Pipe6"
  [1] =>
  string(5) "Pipe5"
  [2] =>
  string(5) "Pipe4"
  [3] =>
  string(5) "Pipe3"
  [4] =>
  string(5) "Pipe2"
  [5] =>
  string(5) "Pipe1"
}

array_reduce內置函數主要是用回調函數去迭代數組中每一個值，并且每一次回調得到的結果值作為下一次回調的初始值，最后返回最終迭代的值：

/**
 * @link http://php.net/manual/zh/function.array-reduce.php
 * @param int $v
 * @param int $w
 *
 * @return int
 */
function rsum($v, $w)
{
    $v += $w;
    return $v;
}

$a = [1, 2, 3, 4, 5];
// 10為初始值
$b = array_reduce($a, "rsum", 10);
// 最后輸出 (((((10 + 1) + 2) + 3) + 4) + 5) = 25
echo $b . PHP_EOL; 

call_user_func()是執行回調函數，并可輸入參數作為回調函數的參數，看測試代碼：

class TestCallUserFunc
{
    public function index($request)
    {
        echo $request . PHP_EOL;
    }
}   

/**
 * @param $test
 */
function testCallUserFunc($test)
{
    echo $test . PHP_EOL;
}

// [$class, $method]
call_user_func(["TestCallUserFunc", "index"], "pipes"); // 輸出"pipes"

// Closure
call_user_func(function ($passable) {
    echo $passable . PHP_EOL;
}, "pipes"); // 輸出"pipes"

// function
call_user_func("testCallUserFunc" , "pipes"); // 輸出"pipes"

call_user_func_array與call_user_func基本一樣，只不過傳入的參數是數組：

class TestCallUserFuncArray
{
    public function index($request)
    {
        echo $request . PHP_EOL;
    }
}

/**
 * @param $test
 */
function testCallUserFuncArray($test)
{
    echo $test . PHP_EOL;
}

// [$class, $method]
call_user_func_array(["TestCallUserFuncArray", "index"], ["pipes"]); // 輸出"pipes"

// Closure
call_user_func_array(function ($passable) {
    echo $passable . PHP_EOL;
}, ["pipes"]); // 輸出"pipes"

// function
call_user_func_array("testCallUserFuncArray" , ["pipes"]); // 輸出"pipes"

了解了幾個PHP內置函數后再去看下Middleware源碼就比較簡單了。Laravel學習筆記之IoC Container實例化源碼解析已經聊過Application的實例化，得到index.php中的$app變量，即IlluminateFoundationApplication的實例化對象。然后繼續看下index.php的源碼：

/**
 * @var AppHttpKernel $kernel
 */
$kernel = $app->make(IlluminateContractsHttpKernel::class);

$response = $kernel->handle(
    $request = IlluminateHttpRequest::capture()
);

$response->send();

$kernel->terminate($request, $response);

首先從容器中解析出Kernel對象，對于AppHttpKernel對象的依賴:IlluminateFoundationApplication和IlluminateRoutingRouter，容器會自動解析。看下Kernel的構造函數：

    /**
     * Create a new HTTP kernel instance.
     *
     * @param  IlluminateContractsFoundationApplication  $app
     * @param  IlluminateRoutingRouter  $router
     */
    public function __construct(Application $app, Router $router)
    {
        $this->app    = $app;
        $this->router = $router;

        foreach ($this->middlewareGroups as $key => $middleware) {
            $router->middlewareGroup($key, $middleware);
        }

        foreach ($this->routeMiddleware as $key => $middleware) {
            $router->middleware($key, $middleware);
        }
    }
    
    // IlluminateRoutingRouter內的方法
    public function middlewareGroup($name, array $middleware)
    {
        $this->middlewareGroups[$name] = $middleware;

        return $this;
    }
    
    public function middleware($name, $class)
    {
        $this->middleware[$name] = $class;

        return $this;
    }

構造函數初始化了幾個中間件數組，$middleware[ ], $middlewareGroups[ ]和$routeMiddleware[ ]，Laravel5.0的時候記得中間件數組還沒有分的這么細。然后就是Request的實例化：

$request = IlluminateHttpRequest::capture()

這個過程以后再聊吧，不管咋樣，得到了IlluminateHttpRequest對象，然后傳入Kernel中：

    /**
     * Handle an incoming HTTP request.
     *
     * @param  IlluminateHttpRequest  $request
     * @return IlluminateHttpResponse
     */
    public function handle($request)
    {
        try {
            $request->enableHttpMethodParameterOverride();

            $response = $this->sendRequestThroughRouter($request);
        } catch (Exception $e) {
            $this->reportException($e);

            $response = $this->renderException($request, $e);
        } catch (Throwable $e) {
            $this->reportException($e = new FatalThrowableError($e));

            $response = $this->renderException($request, $e);
        }

        $this->app["events"]->fire("kernel.handled", [$request, $response]);

        return $response;
    }

主要是sendRequestThroughRouter($request)函數執行了轉換操作：把IlluminateHttpRequest對象轉換成了IlluminateHttpResponse，然后通過Kernel的send()方法發送給客戶端。同時，順便觸發了kernel.handled內核已處理請求事件。OK，重點關注下sendRequestThroughRouter($request)方法：

    /**
     * Send the given request through the middleware / router.
     *
     * @param  IlluminateHttpRequest  $request
     * @return IlluminateHttpResponse
     */
    protected function sendRequestThroughRouter($request)
    {
        $this->app->instance("request", $request);

        Facade::clearResolvedInstance("request");

        /* 依次執行$bootstrappers中每一個bootstrapper的bootstrap()函數,做了幾件準備事情:
        1. 環境檢測
        2. 配置加載
        3. 日志配置
        4. 異常處理
        5. 注冊Facades
        6. 注冊Providers
        7. 啟動服務
         protected $bootstrappers = [
            "IlluminateFoundationBootstrapDetectEnvironment",
            "IlluminateFoundationBootstrapLoadConfiguration",
            "IlluminateFoundationBootstrapConfigureLogging",
            "IlluminateFoundationBootstrapHandleExceptions",
            "IlluminateFoundationBootstrapRegisterFacades",
            "IlluminateFoundationBootstrapRegisterProviders",
            "IlluminateFoundationBootstrapBootProviders",
        ];*/
        $this->bootstrap();

        return (new Pipeline($this->app))
                    ->send($request)
                    ->through($this->app->shouldSkipMiddleware() ? [] : $this->middleware)
                    ->then($this->dispatchToRouter());
    }

$this->bootstrap()主要是做了程序初始化工作，以后再聊具體細節。然后是Pipeline來傳輸Request，Laravel中把Pipeline管道多帶帶拿出來作為一個service(可看Illuminate/Pipeline文件夾)，說明Pipeline做的事情還是很重要的：主要就是作為Request的傳輸管道，依次通過$middlewares[ ], 或middlewareGroups[ ], 或$routeMiddleware[ ]這些中間件的前置操作，和控制器的某個action或者直接閉包處理得到Response，然后又帶著Reponse依次通過$middlewares[ ], 或middlewareGroups[ ], 或$routeMiddleware[ ]這些中間件的后置操作得到準備就緒的Response，然后通過send()發送給客戶端。
這個過程有點像汽車工廠的生產一樣，Pipeline是傳送帶，起初Request可能就是個汽車空殼子，經過傳送帶旁邊的一個個機械手middleware@before的過濾和操作(如檢查零件剛度是不是合格，殼子尺寸是不是符合要求，給殼子噴個漆或抹個油啥的)，然后進入中央控制區加個發動機(Controller@action ，或Closure)，然后又繼續經過檢查和附加操作middleware@after(如添加個擋風鏡啥的)，然后通過門外等著的火車直接運送到消費者手里send()。在每一步裝配過程中，都需要Service來支持，Service是通過Container來解析{make()}提供的，并且Service是通過ServiceProvider注冊綁定{bind(),singleton(),instance()}到Container中的。

看下Pipeline的send()和through()源碼：

    public function send($passable)
    {
        $this->passable = $passable;

        return $this;
    }
    
    public function through($pipes)
    {
        $this->pipes = is_array($pipes) ? $pipes : func_get_args();

        return $this;
    }

send()傳送的對象是Request，through()所要通過的對象是$middleware[ ]，OK，再看下dispatchToRouter()的源碼直接返回一個Closure：

    protected function dispatchToRouter()
    {
        return function ($request) {
            $this->app->instance("request", $request);

            return $this->router->dispatch($request);
        };
    }

然后重點看下then()函數源碼：

    public function then(Closure $destination)
    {
        $firstSlice = $this->getInitialSlice($destination);

        $pipes = array_reverse($this->pipes);

        // $this->passable = Request對象
        return call_user_func(
            array_reduce($pipes, $this->getSlice(), $firstSlice), $this->passable
        );
    }
    
    protected function getInitialSlice(Closure $destination)
    {
        return function ($passable) use ($destination) {
            return call_user_func($destination, $passable);
        };
    }

這里假設$middlewares為(盡管源碼中$middlewares只有一個CheckForMaintenanceMode::class)：

$middlewares = [
    CheckForMaintenanceMode::class,
    AddQueuedCookiesToResponse::class,
    StartSession::class,
    ShareErrorsFromSession::class,
    VerifyCsrfToken::class,
];

先獲得第一個slice(這里作者是比作"洋蔥"，一層層的穿過，從一側穿過到另一側，比喻倒也形象)并作為array_reduce()的初始值，就像上文中array_reduce()測試例子中的10這個初始值，這個初始值現在是個閉包：

$destination = function ($request) {
    $this->app->instance("request", $request);
    return $this->router->dispatch($request);
};

$firstSlice = function ($passable) use ($destination) {
    return call_user_func($destination, $passable);
};

OK,然后要對$middlewares[ ]進行翻轉，為啥要翻轉呢？
看過這篇Laravel學習筆記之Decorator Pattern文章就會發現，在Client類利用Decorator Pattern進行依次裝飾的時候，是按照$middlewares[ ]數組中值倒著new的：

    public function wrapDecorator(IMiddleware $decorator)
    {
        $decorator = new VerifyCsrfToken($decorator);
        $decorator = new ShareErrorsFromSession($decorator);
        $decorator = new StartSession($decorator);
        $decorator = new AddQueuedCookiesToResponse($decorator);
        $response  = new CheckForMaintenanceMode($decorator);

        return $response;
    }

這樣才能得到一個符合$middlewares[ ]順序的$response對象：

$response = new CheckForMaintenanceMode(
                new AddQueuedCookiesToResponse(
                    new StartSession(
                        new ShareErrorsFromSession(
                            new VerifyCsrfToken(
                                new Request()
                        )
                    )
                )
            )
        );

看下array_reduce()中的迭代回調函數getSlice(){這個迭代回調函數比作剝洋蔥時獲取每一層洋蔥slice，初始值是$firstSlice}：

    protected function getSlice()
    {
        return function ($stack, $pipe) {
            return function ($passable) use ($stack, $pipe) {
                if ($pipe instanceof Closure) {
                    return call_user_func($pipe, $passable, $stack);
                } elseif (! is_object($pipe)) {
                    list($name, $parameters) = $this->parsePipeString($pipe);
                    $pipe = $this->container->make($name);
                    $parameters = array_merge([$passable, $stack], $parameters);
                } else{
                    $parameters = [$passable, $stack];
                }

                return call_user_func_array([$pipe, $this->method], $parameters);
            };
        };
    }

返回的是個閉包，仔細看下第二層閉包里的邏輯，這里$middlewares[ ]傳入的是每一個中間件的名字，然后通過容器解析出每一個中間件對象：

$pipe = $this->container->make($name);

并最后用call_user_func_array([$class, $method], array $parameters)來調用這個$class里的$method方法，參數是$parameters。

接下來寫個demo看下整個流程。
先簡化下getSlice()函數，這里就默認$pipe傳入的是類名稱(整個demo中所有class都在同一個文件內)：

// PipelineTest.php

// Get the slice in every step.
function getSlice()
{
    return function ($stack, $pipe) {
        return function ($passable) use ($stack, $pipe) {
            /**
             * @var Middleware $pipe
             */
            return call_user_func_array([$pipe, "handle"], [$passable, $stack]);
        };
    };
}

再把$middlewares[ ]中五個中間件類寫上，對于前置操作和后置操作做個簡化，直接echo字符串：

// PipelineTest.php


給上完整的一個Pipeline類，這里的Pipeline對Laravel中的Pipeline做了稍微簡化，只選了幾個重要的函數：
// PipelineTest.php

class Pipeline 
{
    /**
     * @var array
     */
    protected $middlewares = [];

    /**
     * @var int
     */
    protected $request;

    // Get the initial slice
    function getInitialSlice(Closure $destination)
    {
        return function ($passable) use ($destination) {
            return call_user_func($destination, $passable);
        };
    }
    
    // Get the slice in every step.
    function getSlice()
    {
        return function ($stack, $pipe) {
            return function ($passable) use ($stack, $pipe) {
                /**
                 * @var Middleware $pipe
                 */
                return call_user_func_array([$pipe, "handle"], [$passable, $stack]);
            };
        };
    }
    
    // When process the Closure, send it as parameters. Here, input an int number.
    function send(int $request)
    {
        $this->request = $request;
        return $this;
    }

    // Get the middlewares array.
    function through(array $middlewares)
    {
        $this->middlewares = $middlewares;
        return $this;
    }
    
    // Run the Filters.
    function then(Closure $destination)
    {
        $firstSlice = $this->getInitialSlice($destination);
    
        $pipes = array_reverse($this->middlewares);
        
        $run = array_reduce($pipes, $this->getSlice(), $firstSlice);
    
        return call_user_func($run, $this->request);
    }
}
OK,現在開始傳入Request,這里簡化為一個整數而不是Request對象了：
// PipelineTest.php

/**
 * @return Closure
 */
function dispatchToRouter()
{
    return function ($request) {
        echo $request . ": Send Request to the Kernel, and Return Response." . PHP_EOL;
    };
}

$request = 10;

$middlewares = [
    CheckForMaintenanceMode::class,
    AddQueuedCookiesToResponse::class,
    StartSession::class,
    ShareErrorsFromSession::class,
    VerifyCsrfToken::class,
];

(new Pipeline())->send($request)->through($middlewares)->then(dispatchToRouter());
執行php PipelineTest.php得到Response：
10: Check if the application is in the maintenance status.
10: Start session of this request.
10: Verify csrf token when post request.
10: Send Request to the Kernel, and Return Response.
10: Share the errors variable from response to the views.
10: Close session of this response.
10: Add queued cookies to the response.

一步一步分析下執行過程：
1.首先獲取$firstSlice
$destination = function ($request) {
    echo $request . ": Send Request to the Kernel, and Return Response." . PHP_EOL;
};
$firstSlice = function ($passable) use ($destination) {
    return call_user_func($destination, $passable);
};
這時經過初始化后：
$this->request = 10;
$pipes = [
    VerifyCsrfToken::class,
    ShareErrorsFromSession::class,
    StartSession::class,
    AddQueuedCookiesToResponse::class,
    CheckForMaintenanceMode::class,
];
2.執行第一次getSlice()后的結果作為新的$stack，其值為：
$stack   = $firstSlice;
$pipe    = VerifyCsrfToken::class;
$stack_1 = function ($passable) use ($stack, $pipe) {
        /**
        * @var Middleware $pipe
        */            
    return call_user_func_array([$pipe, "handle"], [$passable, $stack]);
};
3.執行第二次getSlice()后的結果作為新的$stack，其值為：
$stack   = $stack_1;
$pipe    = ShareErrorsFromSession::class;
$stack_2 = function ($passable) use ($stack, $pipe) {
        /**
        * @var Middleware $pipe
        */            
    return call_user_func_array([$pipe, "handle"], [$passable, $stack]);
};
4.執行第三次getSlice()后的結果作為新的$stack，其值為：
$stack   = $stack_2;
$pipe    = StartSession::class;
$stack_3 = function ($passable) use ($stack, $pipe) {
        /**
        * @var Middleware $pipe
        */            
    return call_user_func_array([$pipe, "handle"], [$passable, $stack]);
};
5.執行第四次getSlice()后的結果作為新的$stack，其值為：
$stack   = $stack_3;
$pipe    = AddQueuedCookiesToResponse::class;
$stack_4 = function ($passable) use ($stack, $pipe) {
        /**
        * @var Middleware $pipe
        */            
    return call_user_func_array([$pipe, "handle"], [$passable, $stack]);
};
6.執行第五次getSlice()后的結果作為新的$stack，其值為：
$stack   = $stack_4;
$pipe    = CheckForMaintenanceMode::class;
$stack_5 = function ($passable) use ($stack, $pipe) {
        /**
        * @var Middleware $pipe
        */            
    return call_user_func_array([$pipe, "handle"], [$passable, $stack]);
};
這時，$stack_5也就是then()里的$run,然后執行call_user_func($run, 10),看執行過程:
1.$stack_5(10) = CheckForMaintenanceMode::handle(10, $stack_4)
echo "10: Check if the application is in the maintenance status." . PHP_EOL;
stack_4(10);
2.$stack_4(10) = AddQueuedCookiesToResponse::handle(10, $stack_3)
$stack_3(10);
echo "10: Add queued cookies to the response." . PHP_EOL;
3.$stack_3(10) = StartSession::handle(10, $stack_2)
echo "10: Start session of this request." . PHP_EOL;
$stack_2(10);
echo "10: Close session of this response." . PHP_EOL;
4.$stack_2(10) = ShareErrorsFromSession::handle(10, $stack_1)
$stack_1(10);
echo "10: Share the errors variable from response to the views." . PHP_EOL;
5.$stack_1(10) = VerifyCsrfToken::handle(10, $firstSlice)
echo "10: Verify csrf token when post request." . PHP_EOL;
$firstSlice(10);
6.$firstSlice(10) =
$firstSlice(10) = call_user_func($destination, 10) = echo "10: Send Request to the Kernel, and Return Response." . PHP_EOL;
OK,再把上面執行順序整理一下：
1. echo "10: Check if the application is in the maintenance status." . PHP_EOL; // 第一個step

3_1. echo "10: Start session of this request." . PHP_EOL; // 第三個step

5. echo "10: Verify csrf token when post request." . PHP_EOL; // 第五個step

6.echo "10: Send Request to the Kernel, and Return Response." . PHP_EOL; //第六個step

4. echo "10: Share the errors variable from response to the views." . PHP_EOL; // 第四個step

3_2. echo "10: Close session of this response." . PHP_EOL; // 第三個step

2. echo "10: Add queued cookies to the response." . PHP_EOL; // 第二個step

經過上面的一步步分析，就能很清楚Laravel源碼中Middleware的執行步驟了。再復雜的步驟只要一步步拆解，就很清晰每一步的邏輯，然后把步驟組裝，就能知道全貌了。
總結：本文主要學習了Laravel的Middleware的源碼，學習完后就知道沒有什么神秘之處，只需要動手一步步拆解就行。后面再學習下Container的源碼，到時見。
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