How to parse PHP dependency injection

Abstract: High-level modules should not depend on low-level modules, both should rely on abstractions. Abstractions should not depend on details, details should depend on abstractions.

First, let’s look at a piece of code:

class A{        public function echo()
        {                echo 'A'.PHP_EOL;
        }
}class EchoT {        protected  $t;        public function __construct()
        {              $this->t = new A();
        }        public function echo(){                $this->t->echo();
        }
}

Initially, we all use the new method internally, and the EchoT class relies heavily on class A. Whenever class A changes, class EchoT must also change.

Let’s optimize the code

class EchoT {        protected  $t;        public function __construct($t)  //构造器注入由构造器注入到其中        {              $this->t = $t;
        }

You can see that if you do this. On a large scale, we decouple the program. No matter how you change class A, class EchoT does not need to be changed. No longer dependent on A. But a new problem has come again. We only have A now. What if B or CDEFG comes?

面向接口

interface T{        public function echo();
}class A{        public function echo()
        {                echo 'A'.PHP_EOL;
        }
}class B implements T{        public function echo()
        {                echo 'B'.PHP_EOL;
        }
}class EchoT {        protected  $t;        public function __construct(T $t)  //构造器注入由构造器注入到其中        {              $this->t = $t;
        }        public function echo(){                $this->t->echo();
        }
}

将T抽象出为接口，这样，EchoT类中的echo方法变成一个抽象的方法，不到运行那一刻，不知道他们的Method方式是怎么实现的。

工厂

function getT($str) {    if(class_exists($str)){        return new $str();
        }
}

T要使用哪个是不明确的，因此，我们可以将其工厂化。【看上去很简单，在DI实际上有体现】

DI（重点来了）

首先，我们看一下PHP的psr规范。

http://www.php-fig.org/psr/psr-11/

官方定义的接口

Psr\Container\ContainerInterface
包含两个方法function get($id);function has($id);

仔细看上面的工厂，是不是和get($id)很一致，PHP官方将其定义为容器(Container,我个人理解，就是一个复杂的工厂)

dependency injection container

依赖注入容器

namespace Core;use Psr\Container\ContainerInterface;class Container implements ContainerInterface{        protected $instance = [];//对象存储的数组
        public function __construct($path) {                $this->_autoload($path);  //首先我们要自动加载  psr-autoload
        }        public function build($className)
        {                if(is_string($className) and $this->has($className)) {                        return $this->get($className);
                }                //反射
                $reflector = new \ReflectionClass($className);                if (!$reflector->isInstantiable()) {                        throw new \Exception("Can't instantiate ".$className);
                }                // 检查类是否可实例化, 排除抽象类abstract和对象接口interface
                if (!$reflector->isInstantiable()) {                        throw new \Exception("Can't instantiate ".$className);
                }                /** @var \ReflectionMethod $constructor 获取类的构造函数 */
                $constructor = $reflector->getConstructor();                // 若无构造函数，直接实例化并返回
                if (is_null($constructor)) {                        return new $className;
                }                // 取构造函数参数,通过 ReflectionParameter 数组返回参数列表
                $parameters = $constructor->getParameters();                // 递归解析构造函数的参数
                $dependencies = $this->getDependencies($parameters);                // 创建一个类的新实例，给出的参数将传递到类的构造函数。
                $class =  $reflector->newInstanceArgs($dependencies);                $this->instance[$className] = $class;                return $class;
        }        /**
         * @param array $parameters
         * @return array
         */
        public function getDependencies(array $parameters)
        {
                $dependencies = [];                /** @var \ReflectionParameter $parameter */
                foreach ($parameters as $parameter) {                        /** @var \ReflectionClass $dependency */
                        $dependency = $parameter->getClass();                        if (is_null($dependency)) {                                // 是变量,有默认值则设置默认值
                                $dependencies[] = $this->resolveNonClass($parameter);
                        } else {                                // 是一个类，递归解析
                                $dependencies[] = $this->build($dependency->name);
                        }
                }                return $dependencies;
        }        /**
         * @param \ReflectionParameter $parameter
         * @return mixed
         * @throws \Exception
         */
        public function resolveNonClass(\ReflectionParameter $parameter)
        {                // 有默认值则返回默认值
                if ($parameter->isDefaultValueAvailable()) {                        return $parameter->getDefaultValue();
                }                throw new \Exception($parameter->getName().' must be not null');
        }        /**
         * 参照psr-autoload规范
         * @param $path
         */
        public function _autoload($path) {
                spl_autoload_register(function(string $class) use ($path) {
                        $file = DIRECTORY_SEPARATOR.str_replace('\\',DIRECTORY_SEPARATOR, $class).'.php';                        if(is_file($path.$file)) {                                include($path.$file);                                return true;
                        }                        return false;
                });
        }        public function get($id)
        {                if($this->has($id)) {                        return $this->instance[$id];
                }                if(class_exists($id)){                        return $this->build($id);
                }                throw new ClassNotFoundException('class not found');  //实现的PSR规范的异常
        }        public function has($id)
        {                return isset($this->instance[$id]) ? true : false;
        }
}

使用示例

$container = new Container('../');//假设这是路径$echoT = $container->get(\Test\EchoT::class);     //假设echoT类的命名空间是\Test$echoT->echo();

这个时候，会出现一个问题：

  // 检查类是否可实例化, 排除抽象类abstract和对象接口interface
                if (!$reflector->isInstantiable()) {                        throw new \Exception("Can't instantiate ".$className);
                }
因为接口T是无法实例化的，所以，一般在程序内，我们都加上别名（参照laravel框架）
$container->alisa(\Test\T::class,\Test\T\A::class);  //指定接口T使用类A(控制反转)

针对接口

下面是alias方法

      public function alias(string $key, $class, bool $singleton = true) 
        {                if($singleton) {                        $this->singleton[] = $class;
                }                $this->aliases[$key] = $class;                return $this;
        }    //同时，我们需要在build的时候进行判断是否为别名
 public function build($className)
        {                if(is_string($className) and $this->has($className)) {                        return $this->get($className);
                }                if(isset($this->aliases[$className])) {                        if(is_object($this->aliases[$className])) {                               return $this->aliases[$className];
                        }
                        $className = $this->aliases[$className];
                }

就此，一个简单的PHP容器就实现了。

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