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1	# Parameter resolvers
2
3	Extending the behavior of the `Invoker` is easy and is done by implementing a [](https://github.com/PHP-DI/Invoker/blob/master/src/ParameterResolver/ParameterResolver.php`ParameterResolver`](https://github.com/PHP-DI/Invoker/blob/master/src/ParameterResolver/ParameterResolver.php](https://github.com/PHP-DI/Invoker/blob/master/src/ParameterResolver/ParameterResolver.php):
4
5	```php
6	interface ParameterResolver
7	{
8	public function getParameters(
9	ReflectionFunctionAbstract $reflection,
10	array $providedParameters,
11	array $resolvedParameters
12	);
13	}
14	```
15
16	- `$providedParameters` contains the parameters provided by the user when calling `$invoker->call($callable, $parameters)`
17	- `$resolvedParameters` contains parameters that have already been resolved by other parameter resolvers
18
19	An `Invoker` can chain multiple parameter resolvers to mix behaviors, e.g. you can mix "named parameters" support with "dependency injection" support. This is why a `ParameterResolver` should skip parameters that are already resolved in `$resolvedParameters`.
20
21	Here is an implementation example for dumb dependency injection that creates a new instance of the classes type-hinted:
22
23	```php
24	class MyParameterResolver implements ParameterResolver
25	{
26	public function getParameters(
27	ReflectionFunctionAbstract $reflection,
28	array $providedParameters,
29	array $resolvedParameters
30	) {
31	foreach ($reflection->getParameters() as $index => $parameter) {
32	if (array_key_exists($index, $resolvedParameters)) {
33	// Skip already resolved parameters
34	continue;
35	}
36
37	$class = $parameter->getClass();
38
39	if ($class) {
40	$resolvedParameters[$index] = $class->newInstance();
41	}
42	}
43
44	return $resolvedParameters;
45	}
46	}
47	```
48
49	To use it:
50
51	```php
52	$invoker = new Invoker\Invoker(new MyParameterResolver);
53
54	$invoker->call(function (ArticleManager $articleManager) {
55	$articleManager->publishArticle('Hello world', 'This is the article content.');
56	});
57	```
58
59	A new instance of `ArticleManager` will be created by our parameter resolver.
60
61	## Chaining parameter resolvers
62
63	The fun starts to happen when we want to add support for many things:
64
65	- named parameters
66	- dependency injection for type-hinted parameters
67	- ...
68
69	This is where we should use the [](https://github.com/PHP-DI/Invoker/blob/master/src/ParameterResolver/ResolverChain.php`ResolverChain`](https://github.com/PHP-DI/Invoker/blob/master/src/ParameterResolver/ResolverChain.php](https://github.com/PHP-DI/Invoker/blob/master/src/ParameterResolver/ResolverChain.php). This resolver implements the [](http://en.wikipedia.org/wiki/Chain-of-responsibility_patternChain of responsibility](http://en.wikipedia.org/wiki/Chain-of-responsibility_pattern](http://en.wikipedia.org/wiki/Chain-of-responsibility_pattern) design pattern.
70
71	For example the default chain is:
72
73	```php
74	$parameterResolver = new ResolverChain([
75	new NumericArrayResolver,
76	new AssociativeArrayResolver,
77	new DefaultValueResolver,
78	]);
79	```
80
81	It allows to support even the weirdest use cases like:
82
83	```php
84	$parameters = [];
85
86	// First parameter will receive "Welcome"
87	$parameters[] = 'Welcome';
88
89	// Parameter named "content" will receive "Hello world!"
90	$parameters['content'] = 'Hello world!';
91
92	// $published is not defined so it will use its default value
93	$invoker->call(function ($title, $content, $published = true) {
94	// ...
95	}, $parameters);
96	```
97
98	We can put our custom parameter resolver in the list and created a super-duper invoker that also supports basic dependency injection:
99
100	```php
101	$parameterResolver = new ResolverChain([
102	new MyParameterResolver, // Our resolver is at the top for highest priority
103	new NumericArrayResolver,
104	new AssociativeArrayResolver,
105	new DefaultValueResolver,
106	]);
107
108	$invoker = new Invoker\Invoker($parameterResolver);
109	```
110