关于依赖注入的思考

最近几日接触了一个PHP API框架，其中里面用到了依赖注入的思想。于是自己写下了一个小例子来加深印象：

<?php
//https://docs.phalconphp.com/zh/latest/reference/di.html
/**
 * Log接口
 * Interface ILog
 */
interface ILog {
    public function info($str);

    public function error($str);
}

/**
 * 数据库log，实现log接口
 * Class DBLog
 */
class DBLog implements ILog {
    private $_name;

    public function __construct($name) {
        $this->_name = $name;
    }

    public function info($str) {
        echo '<br>[DBLog][' . $this->_name . '][info]call log info function: ' . $str . '<br>';
    }

    public function error($str) {
        echo '<br>[DBLog][' . $this->_name . '][error]call log info function: ' . $str . '<br>';
    }
}

/**
 * 文件log，实现log接口
 * Class FileLog
 */
class FileLog implements ILog {
    private $_name;

    public function __construct($name) {
        $this->_name = $name;
    }

    public function info($str) {
        echo '<br>[FileLog][' . $this->_name . '][info]call log info function: ' . $str . '<br>';
    }

    public function error($str) {
        echo '<br>[FileLog][' . $this->_name . '][error]call log info function: ' . $str . '<br>';
    }
}

/**
 * 依赖注入类
 * 用于存放需要依赖的类的容器
 * Class DI
 */
class DI {
    public $arr = array();
    protected static $instance = NULL;

    public static function getInstance() {//单例加载
        if (self::$instance == NULL) {
            self::$instance = new DI();
        }
        return self::$instance;
    }

    public function __set($name, $value) {//魔法方法 设置私有属性值
        $this->set($name, $value);
    }

    public function __get($name) {
        $this->get($name);
    }

    public function set($key, $value) {
        $this->arr[$key] = $value;
    }

    public function get($key, $default = NULL) {
        return $this->arr[$key];
    }
}

/**
 * 主类
 * Class User
 */
class User {

    private $_di;

    public function __construct($di) {
        //使用依赖注入容器，目的是防止出现多个setter方法分别设置不同依赖项
        $this->_di = $di;
    }

    public function writeLog($str) {
        //从DI容器中获取要操作的类/对象
        $obj = $this->_di->get('logger');
        $obj->info($str);
        $obj->error($str);
    }

    public function run() {
        $obj = $this->_di->get('fun');
        $obj()->callRuntime();
    }
}

class Runtime {
    public function __construct() {
        var_dump('call Runtime construct');
    }

    public function callRuntime() {
        var_dump('call Runtime');
    }
}

//客户端调用
echo '<br>=======begin========<br>';
$di = DI::getInstance();
$di->logger = new DBLog('mydb');//魔法方法 设置私有属性值
$user = new User($di);
$user->writeLog('lalala');

echo '<br>===============<br>';
$di2 = DI::getInstance();
$di2->logger = new FileLog('myfile');

$di2->fun = function () {//延迟加载
    return new Runtime();
};


//$di2->set('logger',new FileLog('myfile')); 与上面的方式等效
$user2 = new User($di2);//切换依赖项，主类（User类）不需要改一行代码（前提是不同依赖项实现相同的接口）
$user2->writeLog('bababa');
$user2->run();
echo '<br>=======end========<br>';

//返回结果：
/*

=======begin========

[DBLog][mydb][info]call log info function: lalala

[DBLog][mydb][error]call log info function: lalala

===============

[FileLog][myfile][info]call log info function: bababa

[FileLog][myfile][error]call log info function: bababa

string 'call Runtime construct' (length=22)

string 'call Runtime' (length=12)


=======end========
*/