How to implement reflective injection in php interview

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 precisely because of class reflection 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. 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 an associative array consisting of the constant name and value

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

Get properties through reflection

$reflectionClass->getProperties();

Return an array composed of ReflectionProperty 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" } }

Reflect out the methods defined in the class

$reflectionClass->getMethods();

Returns 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(), Its return value is a ReflectionMethod object.

$constructor = $reflectionClass->getConstructor();

Reflection method parameters

$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, pass the class name to the make function and return the object of the class, in make Here it will help us inject the dependencies of the class, that is, in this case, 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 the object 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)*/

Through the above In the example, I briefly described how to use the reflection of PHP classes to implement dependency injection. Laravel's dependency injection is also implemented through this idea, but the design is more sophisticated and makes extensive use of closure callbacks to deal with various complex dependency injections. .

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