Detailed explanation of the dependency injection process of PHP class reflection implementation

PHP has a complete reflection API, providing the ability to reverse engineer classes, interfaces, functions, methods and extensions. Through the capabilities provided by class reflection, we can know how the class is defined, what attributes it has, what methods it has, what parameters the method has, what the path to the class file is, and other very important information. It is also because of the reflection of classes that many PHP frameworks can implement dependency injection to automatically resolve the dependencies between classes, which brings great convenience to our daily development.

This article mainly explains how to use class reflection to implement dependency injection (Dependency Injection). It will not describe every API in PHP Reflection one by one. For detailed API reference information, please refer to the official documentation. This article is mainly and We will introduce the PHP class reflection to realize the dependency injection process and share related knowledge points. Friends who are interested in this can follow the editor to learn. I hope it can help everyone.

In order to better understand, let's look at class reflection through an example and how to implement dependency injection.

The following class represents a point in the coordinate system and has two attributes, the abscissa x and the ordinate y.

/**
 * Class Point
 */
class Point
{
  public $x;
  public $y;

  /**
   * Point constructor.
   * @param int $x horizontal value of point's coordinate
   * @param int $y vertical value of point's coordinate
   */
  public function __construct($x = 0, $y = 0)
  {
    $this->x = $x;
    $this->y = $y;
  }
}

The next class represents a circle. You can see that there is a parameter in its constructor that is of the Point class, that is, the Circle class is dependent on the Point class. .

class Circle
{
  /**
   * @var int
   */
  public $radius;//半径

  /**
   * @var Point
   */
  public $center;//圆心点

  const PI = 3.14;

  public function __construct(Point $point, $radius = 1)
  {
    $this->center = $point;
    $this->radius = $radius;
  }
  
  //打印圆点的坐标
  public function printCenter()
  {
    printf('center coordinate is (%d, %d)', $this->center->x, $this->center->y);
  }

  //计算圆形的面积
  public function area()
  {
    return 3.14 * pow($this->radius, 2);
  }
}

ReflectionClass

Next we use reflection to reverse engineer the Circle class.

Pass the name of the Circle class to reflectionClass to instantiate an object of the ReflectionClass class.

$reflectionClass = new reflectionClass(Circle::class);
//返回值如下
object(ReflectionClass)#1 (1) {
 ["name"]=>
 string(6) "Circle"
}

Reflect the constants of the class

$reflectionClass->getConstants();

Return a constant name and value The associative array composed of

array(1) {
 ["PI"]=>
 float(3.14)
}

Gets properties through reflection

$reflectionClass->getProperties();

Returns a ReflectionProperty An array of objects

array(2) {
 [0]=>
 object(ReflectionProperty)#2 (2) {
  ["name"]=>
  string(6) "radius"
  ["class"]=>
  string(6) "Circle"
 }
 [1]=>
 object(ReflectionProperty)#3 (2) {
  ["name"]=>
  string(6) "center"
  ["class"]=>
  string(6) "Circle"
 }
}

Reflects the method defined in the class

$reflectionClass->getMethods();

Return An array of ReflectionMethod objects

array(3) {
 [0]=>
 object(ReflectionMethod)#2 (2) {
  ["name"]=>
  string(11) "__construct"
  ["class"]=>
  string(6) "Circle"
 }
 [1]=>
 object(ReflectionMethod)#3 (2) {
  ["name"]=>
  string(11) "printCenter"
  ["class"]=>
  string(6) "Circle"
 }
 [2]=>
 object(ReflectionMethod)#4 (2) {
  ["name"]=>
  string(4) "area"
  ["class"]=>
  string(6) "Circle"
 }
}

We can also obtain the constructor method of the class separately through getConstructor(), and its return value is a ReflectionMethod object.

$constructor = $reflectionClass->getConstructor();

The parameters of the reflection method

$parameters = $constructor->getParameters();

The return value is an array composed of ReflectionParameter objects.

array(2) {
 [0]=>
 object(ReflectionParameter)#3 (1) {
  ["name"]=>
  string(5) "point"
 }
 [1]=>
 object(ReflectionParameter)#4 (1) {
  ["name"]=>
  string(6) "radius"
 }
}

Dependency Injection

Okay next we write a function named make, passing the class name to The make function returns the object of the class. In make, it will help us inject the dependencies of the class, that is, in this example, it will help us inject the Point object into the constructor of the Circle class.

//构建类的对象
function make($className)
{
  $reflectionClass = new ReflectionClass($className);
  $constructor = $reflectionClass->getConstructor();
  $parameters = $constructor->getParameters();
  $dependencies = getDependencies($parameters);
  
  return $reflectionClass->newInstanceArgs($dependencies);
}

//依赖解析
function getDependencies($parameters)
{
  $dependencies = [];
  foreach($parameters as $parameter) {
    $dependency = $parameter->getClass();
    if (is_null($dependency)) {
      if($parameter->isDefaultValueAvailable()) {
        $dependencies[] = $parameter->getDefaultValue();
      } else {
        //不是可选参数的为了简单直接赋值为字符串0
        //针对构造方法的必须参数这个情况
        //laravel是通过service provider注册closure到IocContainer,
        //在closure里可以通过return new Class($param1, $param2)来返回类的实例
        //然后在make时回调这个closure即可解析出对象
        //具体细节我会在另一篇文章里面描述
        $dependencies[] = '0';
      }
    } else {
      //递归解析出依赖类的对象
      $dependencies[] = make($parameter->getClass()->name);
    }
  }

  return $dependencies;
}

After defining the make method, we use it to help us instantiate objects of the Circle class:

$circle = make('Circle');
$area = $circle->area();
/*var_dump($circle, $area);
object(Circle)#6 (2) {
 ["radius"]=>
 int(1)
 ["center"]=>
 object(Point)#11 (2) {
  ["x"]=>
  int(0)
  ["y"]=>
  int(0)
 }
}
float(3.14)*/

Related recommendations:

Example analysis of Laravel's dependency injection

Detailed explanation of the method of automatic dependency injection in PHP based on the reflection mechanism

Methods for parsing PHP dependency injection

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