Welcome to Mw’s documentation!¶

Before/After Middleware¶

Middleware can either be a before or after or both middleware. A before middleware runs before delegating to the $next middleware. An after middleware will runs after delegating to the $next middleware.

Before Style

<?php

function($param, $next) {
    // code goes here
    // you can also modify the $param and pass the modified version to the next middleware
    return $next($param);
}

After Style

<?php

function($param, $next) {
    $result = $next($param);

    // code goes here
    // you can also modify the $result and return the modified version to the previous handler

    return $result;
}

Stack¶

The library also comes with a MwStack that allows you to easily build a set of middleware.

<?php

use Krak\Mw;

$stack = mw\stack('Stack Name');
$stack->push(function($a, $next) {
    return $next($a . 'b');
})
->push(function($a, $next) {
    return $next($a) . 'z';
}, 0, 'c')
// replace the c middleware
->on('c', function($a, $next) {
    return $next($a) . 'c';
})
->before('c', function($a, $next) {
    return $next($a) . 'x';
})
->after('c', function($a, $next) {
    return $next($a) . 'y';
})
// this goes on first
->unshift(function($a, $next) {
    return $a;
});

$handler = $stack->compose();
$res = $handler('a');
assert($res == 'abxcy');

Context¶

Each middleware is invoked with a Mw\Context instance. This is responsible for holding additional data to be used internally within the mw system and to provide additional features/usage for users. The context is available via the Mw\Link object of the middleware.

<?php

use Krak\Mw;

$handle = mw\compose([
    function($v, Mw\Link $next) {
        $ctx = $next->getContext();
        return 1;
    }
], new Krak\Mw\Context\StdContext());

You can configure or pass in any context as long as it implements the Mw\Context interface. Currently, the context provides an invoker via the getInvoke method. This allows custom invocation of the middleware as shown in the Cookbook.

Custom Invocation¶

You can provide custom invocation of the middleware via the context. An invoker is any function that shares the signature of call_user_func. Its sole purpose is to invoke functions with their parameters. With custom invocation, you can do cool things like have middleware as pimple identifiers.

Link¶

The final argument to each middleware is an instance of Mw\Link. The link is represents the link/chain between middlewares. Technically speaking, it’s a singly-linked list of middleware that once executed will invoke the entire chain of middleware.

The link is responsible for building a set of middleware via the chain.

<?php

use Krak\Mw;

$link = new Mw\Link(function($i) {
    return $i * 2;
}, new Mw\Context\StdContext());
$link = $link->chain(function($i, $next) {
    return $next($i) + 1;
});
assert($link(2) == 5);

chain takes a middleware and produces a new link that is appened to the head of the linked list of mw links. As you can see, the middleware on the second link is executed first.

Meta Middleware¶

Custom Context and invocation is a very useful feature; however, it requires special consideration if you are creating your own Meta Middleware. Meta middleware are middleware that accept other middleware and inject middleware into the chain of middleware.

mw\group
mw\lazy
mw\filter

These are all meta middleware. To allow all middleware to be properly linked and have access to the context, these meta middleware need to learn how to properly inject middleware into the mw link.

Here’s an example:

<?php

use Krak\Mw;

// maybe middleware will only invoke the middleware if the parameter is < 10
function maybe($mw) {
    return function($i, $next) use ($mw) {
        if ($i < 10) {
            /** NOTE - this is the crucial part where we prepend the `$mw` onto the link. Now, when we execute `$next`,
                the `$mw` func will be first to be executed */
            $next = $next->chain($mw);
        }

        return $next($i);
    };
}

function loggingInvoke() {
    return function($func, ...$params) {
        echo "Invoking Middleware with Param: $params[0]\n";
        return call_user_func($func, ...$params);
    };
}

$handler = mw\compose([
    function() { return 1; },
    maybe(function($i, $next) {
        return $next($i) + 100;
    })
], new Mw\Context\StdContext(loggingInvoke()));

echo $handler(1) . PHP_EOL;
echo $handler(10) . PHP_EOL;

/*
Outputs:

Invoking Middleware with Param: 1
Invoking Middleware with Param: 1
Invoking Middleware with Param: 1
101
Invoking Middleware with Param: 10
Invoking Middleware with Param: 10
1
*/

Middleware Functions¶

Closure compose(array $mws, Context $ctx = null, callable $last = null, $link_class = Link::class)

Composes a set of middleware into a handler.

<?php

$handler = mw\compose([
    function($a, $b, $next) {
        return $a . $b;
    },
    function($a, $b, $next) {
        return $next($a . 'b', $b . 'd');
    }
]);

$res = $handler('a', 'c');
assert($res === 'abcd');

The middleware stack passed in is executed in LIFO order. So the last middleware will be executed first, and the first middleware will be executed last.

After composing the stack of middleware, the resulting handler will share the same signature as the middleware except that it won’t have the $next.

$ctx will default to Context\StdContext if none is supplied, and it will be the context that is passed to the start link (see: Avanced Usage for more details).

$last represents the last middleware to be executed if no other middleware handle the parameters. This typically will throw an exception in that case, but it might be advantageous to set this to something else for your needs.

$link_class is the class that will be constructed for the middleware link. It must be or extend Krak\Mw\Link (see: Avanced Usage for more details).

Closure group(array $mws)

Creates a new middleware composed as one from a middleware stack.

Internally, this calls the compose function, so the same behaviors will apply to this function.

<?php

/** some middleware that will append values to the parameter */
function appendMw($c) {
    return function($s, $next) use ($c) {
        return $next($s . $c);
    };
}

$handler = mw\compose([
    function($s) { return $s; },
    append('d'),
    mw\group([
        append('c'),
        append('b'),
    ]),
    append('a'),
]);

$res = $handler('');
assert($res === 'abcd');

On the surface, this doesn’t seemv very useful, but the ability group middleware into one allows you to then apply other middleware onto a group.

For example, you can do something like:

$grouped = mw\group([
    // ...
]);
mw\filter($grouped, $predicate);

In this example, we just filted an entire group of middleware

Closure lazy(callable $mw_gen)

Lazily creates and executes middleware when it’s executed. Useful if the middleware needs to be generated from a container or if it has expensive dependencies that you only want initialized if the middleware is going to be executed.

<?php

$mw = lazy(function() {
    return expensiveMw($expensive_service_that_was_just_created);
});

The expensive service won’t be created until the $mw is actually executed

Closure filter(callable $mw, callable $predicate)

Either applies the middleware or skips it depending on the result of the predicate. This if very useful for building conditional middleware.

<?php

$mw = function() { return 2; };
$handler = mw\compose([
    function() { return 1; },
    mw\filter($mw, function($v) {
        return $v == 4;
    })
]);
assert($handler(5) == 1 && $handler(4) == 2);

In this example, the stack of middleware always returns 1, however, the filtered middleware gets executed if the value is 4, and in that case, it returns 2 instead.

Invoke Functions¶

Closure pimpleAwareInvoke(Pimple\Container $c, $invoke = ‘call_user_func’)

invokes middleware while checking if the mw is a service defined in the pimple container

Closure methodInvoke(string $method_name, $allow_callable = true, $invoke = ‘call_user_func’)

This will convert the middleware into a callable array like [$obj, $method_name] and invoke it. The $allow_callable parameter will allow the stack to either invoke objects with the given method or invoke callables. If you want to only allow objects with that method to be invokable, then set $allow_callable to false.

Stack Functions¶

MwStack stack($name, array $entries = [], Context $ctx = null, $link_class = Link::class)

Creates a MwStack instance. Every stack must have a name which is just a personal identifier for the stack. It’s primary use is for errors/exceptions that help the user track down which stack has an issue. $ctx and $link_class are forwarded to the MwStack constructor.

<?php

$stack = mw\stack('demo stack');
$stack->push($mw)
    ->unshift($mw1);

// compose into handler
$handler = $stack->compose();
// or, use as a grouped middleware
$handler = mw\compose([
    $mw2,
    $stack
]);

array stackEntry(callable $mw, $sort = 0, $name = null)

Creates an entry for the MwStack. This is only used if you want to initialize a stack with entries, else, you’ll just be using the stack methods to create stack entries.

<?php

$stack = mw\stack('demo stack', [
    stackEntry($mw1, 0, 'mw1'),
    stackEntry($mw2),
    stackEntry($mw3, 5, 'mw3'),
]);
// equivalent to
$stack = mw\stack('demo stack')
    ->push($mw1, 0, 'mw1')
    ->push($mw2)
    ->push($mw3, 5, 'mw3');

MwStack stackMerge(...$stacks)

Merges stacks into one another. The resulting stack has the same name as the first stack in the set. The values from the later stacks will override the values from the earlier stacks.

<?php

$a = mw\stack('stack', [
    mw\stackEntry($mw1),
    mw\stackEntry($mw2),
    mw\stackEntry($mw3, 0, 'mw')
]);
$b = mw\stack('stack', [
    mw\stackEntry($mw4, 0, 'mw'),
]);
$c = mw\stackMerge($a, $b);
// stack $c is equivalent to
$c = mw\stack('stack')
    ->push($mw1)
    ->push($mw2)
    ->push($mw4, 0, 'mw')

Utility Functions¶

array splitArgs(array $args)

Splits arguments between the parameters and middleware.

<?php

use Krak\Mw

function middleware() {
    return function(...$args) {
        list($args, $next) = Mw\splitArgs($args);
        return $next(...$args);
    };
}

class MwStack implements Countable¶

The stack presents a mutable interface into a stack of middleware. Middleware can be added with a name and priority. Only one middleware with a given name may exist. Middleware that are last in the stack will be executed first once the stack is composed.

__construct($name, Context $ctx = null, $link_class = Link::class)

Creates the mw stack with a name. The $ctx and $link_class are forwarded to mw\compose once the stack is composed.

string getName()

returns the name of the middleware

MwStack push($mw, $sort = 0, $name = null)

Pushes a new middleware on the stack. The sort determines the priority of the middleware. Middleware pushed at the same priority will be pushed on like a stack.

MwStack unshift($mw, $sort = 0, $name = null)

Similar to push except it prepends the stack at the beginning.

MwStack on($name, $mw, $sort = 0)

Simply an alias of push; however, the argument order lends it nicer for adding/replacing named middleware.

MwStack before($name, $mw, $mw_name = null)

Inserts a middleware right before the given middleware.

MwStack after($name, $mw, $mw_name = null)

Inserts a middleware right after the given middleware.

array shift($sort = 0)

Shifts the stack at the priority given by taking an element from the front/bottom of the stack. The shifted stack entry is returned as a tuple.

array pop($sort = 0)

Pops the stack at the priority given be taking an element from the back/top of the stack. The popped stack entry is returned as a tuple.

array remove($name)

Removes a named middleware. The removed middleware is returned as a tuple.

array normalize()

Normalizes the stack into an array of middleware that can be used with mw\compose

mixed __invoke(...$params)

Allows the middleware stack to be used as middleware itself.

Closure compose(callable $last = null)

Composes the stack into a handler.

Generator getEntries()

Yields the raw stack entries in the order they were added.

MwStack static createFromEntries($name, $entries)

Creates a stack with a set of entries. mw\stack internally calls this.

class Link¶

Represents a link in the middleware chain. A link instance is passed to every middleware as the last parameter which allows the next middleware to be called. See Avanced Usage for more details.

__construct($mw, Context $ctx, Link $next = null)

Creates a link. If $next is provided, then the created link will be the new head of that linked list.

__invoke(...$params)

Invokes the middleware. It forwards the params to the middleware and additionaly adds the next link to the end of argument list for the middleware.

chain($mw)

Creates a new link to be the head of the current list of links. The context is copied from the current link.

getContext()

returns the context instance apart of the link.

interface Context¶

Represents the middleware context utilized by the internal system.

getInvoke()

Returns the invoker configured for this context.

class Context\StdContext implements Context¶

The default context for the mw system. It simply holds the a value to the invoker for custom invocation.

__construct($invoke = ‘call_user_func’)

class Context\PimpleContext implements Context¶

Provides nice pimple integeration by allowing the context to act like a pimple container and it provides pimple invocation by default.

View the Pimple Middleware for example on this.

__construct(Container $container, $invoke = null)

The pimple contianer and an optional invoker if you don’t want to use the pimpleAwareInvoke

“No middleware returned a response” Error¶

When you get this error or something similar, this means that no middleware in the set of middleware returned a response.

You can get this error if you:

• Forget to put a return statement in your middleware so the chain breaks and no response is returned.
• Have a logic error where no middleware actually accepts the response

If you are having trouble finding which handler is causing the issue, you can use a MwStack instead. These provide better error messages because each middleware stack has a name which can help track down which middlware stack is causing the problem.

“Middleware cannot be invoked because it does not contain the ‘’ method”¶

This exception is thrown when using the methodInvoke for composing your middleware. This means that one of the middleware on your stack doesn’t have the proper method to be called.

To fix this, you should check your middleware stack and verify that every middleware has the proper method. The stack trace should also show you which class instance caused the problem to help you track down the problem.