PHP PSR-4 与 PSR-0 规范

在上一篇文章中，介绍了PSR-0和autoload相关的内容。继PSR-0这个PHP autoloading的规范之后，PHP-FIG又推出了PSR-4，称为改进的autoloading规范。

在PSR-0中，\Symfony\Core\Request会被转换成文件系统的/path/to/project/lib/vendor/Symfony/Core/Request.php这个路径。PSR-4与PSR-0在内容上相差也不大。

在此就不详谈两者的定义了。来看看两者在实际中的一些区别吧。由于Composer的流行，这里对Composer中这两种风格进行比较。

在Composer中，遵循PSR-0标准的典型目录结构是这样的：

vendor/
    vendor_name/
        package_name/
            src/
                Vendor_Name/
                    Package_Name/
                        ClassName.php       # Vendor_Name\Package_Name\ClassName
            tests/
                Vendor_Name/
                    Package_Name/
                        ClassNameTest.php   # Vendor_Name\Package_Name\ClassNameTest

可以看到目录结构有明显的重复而且层次很深。src/和test/目录又重新包含了Vendor和Package目录。

再来看看PSR-4的：

vendor/
    vendor_name/
        package_name/
            src/
                ClassName.php       # Vendor_Name\Package_Name\ClassName
            tests/
                ClassNameTest.php   # Vendor_Name\Package_Name\ClassNameTest

可以看到目录结构更加简洁了。

在PSR-0中目录结构要与命名空间层层对应，无法插入一个单独的目录。Vendor\Package\Class在psr-0会里被直接转换成同样的路径，而PSR-4则没有这样的强制要求。

对比PSR-0，除了PSR-4可以更简洁外，需要注意PSR-0中对下划线(_)是有特殊的处理的，下划线会转换成DIRECTORY_SEPARATOR，这是出于对PHP5.3以前版本兼容的考虑，而PSR-4中是没有这个处理的，这也是两者比较大的一个区别。

此外，PSR-4要求在autoloader中不允许抛出exceptions以及引发任何级别的errors，也不应该有返回值。这是因为可能注册了多个autoloaders，如果一个autoloader没有找到对应的class，应该交给下一个来处理，而不是去阻断这个通道。

PSR-4更简洁更灵活了，但这使得它相对更复杂了。例如通过完全符合PSR-0标准的class name，通常可以明确的知道这个class的路径，而PSR-4可能就不是这样了。

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');

     //此时一个namespace prefix对应到了多个"base directory"

     //autoloader会去加载/path/to/packages/foo-bar/src/Qux/Quux.php
     new \Foo\Bar\Qux\Quux;

     //autoloader会去加载/path/to/packages/foo-bar/tests/Qux/QuuxTest.php
     new \Foo\Bar\Qux\QuuxTest;

下面是如上PSR-4 autoloader的实现：

<?php
namespace Example;

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;
        $base_dir = rtrim($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('\\', DIRECTORY_SEPARATOR, $relative_class)
                  . '.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;
    }
}

单元测试代码：

<?php
namespace Example\Tests;

class MockPsr4AutoloaderClass extends Psr4AutoloaderClass
{
    protected $files = array();

    public function setFiles(array $files)
    {
        $this->files = $files;
    }

    protected function requireFile($file)
    {
        return in_array($file, $this->files);
    }
}

class Psr4AutoloaderClassTest extends \PHPUnit_Framework_TestCase
{
    protected $loader;

    protected function setUp()
    {
        $this->loader = new MockPsr4AutoloaderClass;

        $this->loader->setFiles(array(
            '/vendor/foo.bar/src/ClassName.php',
            '/vendor/foo.bar/src/DoomClassName.php',
            '/vendor/foo.bar/tests/ClassNameTest.php',
            '/vendor/foo.bardoom/src/ClassName.php',
            '/vendor/foo.bar.baz.dib/src/ClassName.php',
            '/vendor/foo.bar.baz.dib.zim.gir/src/ClassName.php',
        ));

        $this->loader->addNamespace(
            'Foo\Bar',
            '/vendor/foo.bar/src'
        );

        $this->loader->addNamespace(
            'Foo\Bar',
            '/vendor/foo.bar/tests'
        );

        $this->loader->addNamespace(
            'Foo\BarDoom',
            '/vendor/foo.bardoom/src'
        );

        $this->loader->addNamespace(
            'Foo\Bar\Baz\Dib',
            '/vendor/foo.bar.baz.dib/src'
        );

        $this->loader->addNamespace(
            'Foo\Bar\Baz\Dib\Zim\Gir',
            '/vendor/foo.bar.baz.dib.zim.gir/src'
        );
    }

    public function testExistingFile()
    {
        $actual = $this->loader->loadClass('Foo\Bar\ClassName');
        $expect = '/vendor/foo.bar/src/ClassName.php';
        $this->assertSame($expect, $actual);

        $actual = $this->loader->loadClass('Foo\Bar\ClassNameTest');
        $expect = '/vendor/foo.bar/tests/ClassNameTest.php';
        $this->assertSame($expect, $actual);
    }

    public function testMissingFile()
    {
        $actual = $this->loader->loadClass('No_Vendor\No_Package\NoClass');
        $this->assertFalse($actual);
    }

    public function testDeepFile()
    {
        $actual = $this->loader->loadClass('Foo\Bar\Baz\Dib\Zim\Gir\ClassName');
        $expect = '/vendor/foo.bar.baz.dib.zim.gir/src/ClassName.php';
        $this->assertSame($expect, $actual);
    }

    public function testConfusion()
    {
        $actual = $this->loader->loadClass('Foo\Bar\DoomClassName');
        $expect = '/vendor/foo.bar/src/DoomClassName.php';
        $this->assertSame($expect, $actual);

        $actual = $this->loader->loadClass('Foo\BarDoom\ClassName');
        $expect = '/vendor/foo.bardoom/src/ClassName.php';
        $this->assertSame($expect, $actual);
    }
}

完。