psr-4 规范(php的类自动加载规范)

psr-4 规范(php 的类自动加载规范)

FIG 制定的 PHP 规范，简称 PSR，是 PHP 开发的事实标准。

PSR 原本有四个规范，分别是：

PSR-0 自动加载
PSR-1 基本代码规范
PSR-2 代码样式
PSR-3 日志接口
2013 年底，新出了第 5 个规范—— PSR-4 。

PSR-4 规范了如何指定文件路径从而自动加载类定义，同时规范了自动加载文件的位置。这个乍一看和 PSR-0 重复了，实际上，在功能上确实有所重复。区别在于 PSR-4 的规范比较干净，去除了兼容 PHP 5.3 以前版本的内容，有一点 PSR-0 升级版的感觉。当然，PSR-4 也不是要完全替代 PSR-0，而是在必要的时候补充 PSR-0——当然，如果你愿意，PSR-4 也可以替代 PSR-0。PSR-4 可以和包括 PSR-0 在内的其他自动加载机制共同使用。

PSR-4 和 PSR-0 最大的区别是对下划线（underscore)的定义不同。PSR-4 中，在类名中使用下划线没有任何特殊含义。而 PSR-0 则规定类名中的下划线 _ 会被转化成目录分隔符。

代码样例
以下代码展示了遵循 PSR-4 的类定义，这个类处理多个命名空间：

<?php
namespace Example;
 
/**
 * An example of a general-purpose implementation that includes the optional
 * functionality of allowing multiple base directories for a single namespace
 * prefix.
 *
 * Given a foo-bar package of classes in the file system at the following
 * paths ...
 *
 *     /path/to/packages/foo-bar/
 *         src/
 *             Baz.php             # Foo\Bar\Baz
 *             Qux/
 *                 Quux.php        # Foo\Bar\Qux\Quux
 *         tests/
 *             BazTest.php         # Foo\Bar\BazTest
 *             Qux/
 *                 QuuxTest.php    # Foo\Bar\Qux\QuuxTest
 *
 * ... add the path to the class files for the \Foo\Bar\ namespace prefix
 * as follows:
 *
 *      <?php
 *      // instantiate the loader
 *      $loader = new \Example\Psr4AutoloaderClass;
 *
 *      // register the autoloader
 *      $loader->register();
 *
 *      // register the base directories for the namespace prefix
 *      $loader->addNamespace('Foo\Bar', '/path/to/packages/foo-bar/src');
 *      $loader->addNamespace('Foo\Bar', '/path/to/packages/foo-bar/tests');
 *
 * The following line would cause the autoloader to attempt to load the
 * \Foo\Bar\Qux\Quux class from /path/to/packages/foo-bar/src/Qux/Quux.php:
 *
 *      <?php
 *      new \Foo\Bar\Qux\Quux;
 *
 * The following line would cause the autoloader to attempt to load the
 * \Foo\Bar\Qux\QuuxTest class from /path/to/packages/foo-bar/tests/Qux/QuuxTest.php:
 *
 *      <?php
 *      new \Foo\Bar\Qux\QuuxTest;
 */
class Psr4AutoloaderClass
{
    /**
     * An associative array where the key is a namespace prefix and the value
     * is an array of base directories for classes in that namespace.
     *
     * @var array
     */
    protected $prefixes = array();
 
    /**
     * Register loader with SPL autoloader stack.
     *
     * @return void
     */
    public function register()
    {
        spl_autoload_register(array($this, 'loadClass'));
    }
 
    /**
     * Adds a base directory for a namespace prefix.
     *
     * @param string $prefix The namespace prefix.
     * @param string $base_dir A base directory for class files in the
     * namespace.
     * @param bool $prepend If true, prepend the base directory to the stack
     * instead of appending it; this causes it to be searched first rather
     * than last.
     * @return void
     */
    public function addNamespace($prefix, $base_dir, $prepend = false)
    {
        // normalize namespace prefix
        $prefix = trim($prefix, '\\') . '\\';
 
        // normalize the base directory with a trailing separator
        $base_dir = rtrim($base_dir, DIRECTORY_SEPARATOR) . '/';
 
        // initialize the namespace prefix array
        if (isset($this->prefixes[$prefix]) === false) {
            $this->prefixes[$prefix] = array();
        }
 
        // retain the base directory for the namespace prefix
        if ($prepend) {
            array_unshift($this->prefixes[$prefix], $base_dir);
        } else {
            array_push($this->prefixes[$prefix], $base_dir);
        }
    }
 
    /**
     * Loads the class file for a given class name.
     *
     * @param string $class The fully-qualified class name.
     * @return mixed The mapped file name on success, or boolean false on
     * failure.
     */
    public function loadClass($class)
    {
        // the current namespace prefix
        $prefix = $class;
 
        // work backwards through the namespace names of the fully-qualified
        // class name to find a mapped file name
        while (false !== $pos = strrpos($prefix, '\\')) {
 
            // retain the trailing namespace separator in the prefix
            $prefix = substr($class, 0, $pos + 1);
 
            // the rest is the relative class name
            $relative_class = substr($class, $pos + 1);
 
            // try to load a mapped file for the prefix and relative class
            $mapped_file = $this->loadMappedFile($prefix, $relative_class);
            if ($mapped_file) {
                return $mapped_file;
            }
 
            // remove the trailing namespace separator for the next iteration
            // of strrpos()
            $prefix = rtrim($prefix, '\\');
        }
 
        // never found a mapped file
        return false;
    }
 
    /**
     * Load the mapped file for a namespace prefix and relative class.
     *
     * @param string $prefix The namespace prefix.
     * @param string $relative_class The relative class name.
     * @return mixed Boolean false if no mapped file can be loaded, or the
     * name of the mapped file that was loaded.
     */
    protected function loadMappedFile($prefix, $relative_class)
    {
        // are there any base directories for this namespace prefix?
        if (isset($this->prefixes[$prefix]) === false) {
            return false;
        }
 
        // look through base directories for this namespace prefix
        foreach ($this->prefixes[$prefix] as $base_dir) {
 
            // replace the namespace prefix with the base directory,
            // replace namespace separators with directory separators
            // in the relative class name, append with .php
            $file = $base_dir
                  . str_replace('\\', '/', $relative_class)
                  . '.php';
 
            // if the mapped file exists, require it
            if ($this->requireFile($file)) {
                // yes, we're done
                return $file;
            }
        }
 
        // never found it
        return false;
    }
 
    /**
     * If a file exists, require it from the file system.
     *
     * @param string $file The file to require.
     * @return bool True if the file exists, false if not.
     */
    protected function requireFile($file)
    {
        if (file_exists($file)) {
            require $file;
            return true;
        }
        return false;
    }
}