Laravel实现构造函数自动依赖注入的方法_PHP

在Laravel的构造函数中可以实现自动依赖注入，而不需要实例化之前先实例化需要的类，如代码所示：

<?php
namespace Lio\Http\Controllers\Forum;
use Lio\Forum\Replies\ReplyRepository;
use Lio\Forum\Threads\ThreadCreator;
use Lio\Forum\Threads\ThreadCreatorListener;
use Lio\Forum\Threads\ThreadDeleterListener;
use Lio\Forum\Threads\ThreadForm;
use Lio\Forum\Threads\ThreadRepository;
use Lio\Forum\Threads\ThreadUpdaterListener;
use Lio\Http\Controllers\Controller;
use Lio\Tags\TagRepository;
class ForumThreadsController extends Controller implements ThreadCreatorListener, ThreadUpdaterListener, ThreadDeleterListener
{
 protected $threads;
 protected $tags;
 protected $currentSection;
 protected $threadCreator;
 public function __construct(
  ThreadRepository $threads,
  ReplyRepository $replies,
  TagRepository $tags,
  ThreadCreator $threadCreator
 ) {
  $this->threads = $threads;
  $this->tags = $tags;
  $this->threadCreator = $threadCreator;
  $this->replies = $replies;
 }
}

注意构造函数中的几个类型约束，其实并没有地方实例化这个Controller并把这几个类型的参数传进去，Laravel会自动检测类的构造函数中的类型约束参数，并自动识别是否初始化并传入。

源码vendor/illuminate/container/Container.php中的build方法：

$constructor = $reflector->getConstructor();
dump($constructor);

这里会解析类的构造函数，在这里打印看：

它会找出构造函数的参数，再看完整的build方法进行的操作：

public function build($concrete, array $parameters = [])
{
 // If the concrete type is actually a Closure, we will just execute it and
 // hand back the results of the functions, which allows functions to be
 // used as resolvers for more fine-tuned resolution of these objects.
 if ($concrete instanceof Closure) {
  return $concrete($this, $parameters);
 }
 $reflector = new ReflectionClass($concrete);
 // If the type is not instantiable, the developer is attempting to resolve
 // an abstract type such as an Interface of Abstract Class and there is
 // no binding registered for the abstractions so we need to bail out.
 if (! $reflector->isInstantiable()) {
  $message = "Target [$concrete] is not instantiable.";
  throw new BindingResolutionContractException($message);
 }
 $this->buildStack[] = $concrete;
 $constructor = $reflector->getConstructor();
 // If there are no constructors, that means there are no dependencies then
 // we can just resolve the instances of the objects right away, without
 // resolving any other types or dependencies out of these containers.
 if (is_null($constructor)) {
  array_pop($this->buildStack);
  return new $concrete;
 }
 $dependencies = $constructor->getParameters();
 // Once we have all the constructor's parameters we can create each of the
 // dependency instances and then use the reflection instances to make a
 // new instance of this class, injecting the created dependencies in.
 $parameters = $this->keyParametersByArgument(
  $dependencies, $parameters
 );
 $instances = $this->getDependencies(
  $dependencies, $parameters
 );
 array_pop($this->buildStack);
 return $reflector->newInstanceArgs($instances);
}

具体从容器中获取实例的方法：

protected function resolveClass(ReflectionParameter $parameter)
{
 try {
  return $this->make($parameter->getClass()->name);
 }
 // If we can not resolve the class instance, we will check to see if the value
 // is optional, and if it is we will return the optional parameter value as
 // the value of the dependency, similarly to how we do this with scalars.
 catch (BindingResolutionContractException $e) {
  if ($parameter->isOptional()) {
   return $parameter->getDefaultValue();
  }
  throw $e;
 }
}

框架底层通过Reflection反射为开发节省了很多细节，实现了自动依赖注入。这里不做继续深入研究了。

写了一个模拟这个过程的类测试：

<?php
class kulou
{
 //
}
class junjun
{
 //
}
class tanteng
{
 private $kulou;
 private $junjun;
 public function __construct(kulou $kulou,junjun $junjun)
 {
  $this->kulou = $kulou;
  $this->junjun = $junjun;
 }
}
//$tanteng = new tanteng(new kulou(),new junjun());
$reflector = new ReflectionClass('tanteng');
$constructor = $reflector->getConstructor();
$dependencies = $constructor->getParameters();
print_r($dependencies);exit;

原理是通过ReflectionClass类解析类的构造函数，并且取出构造函数的参数，从而判断依赖关系，从容器中取，并自动注入。

转自：小谈博客 http://www.tantengvip.com/2016/01/laravel-construct-ioc/

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