Yii2的深入学习--别名（Aliases），yii2aliases

在之前自动加载机制的文章中，我们有提到别名，提到 getAlias 方法，大家当时可能不太清楚，这到底是什么，今天我们就来说一下别名。

别名用来表示文件路径和 URL，这样就避免了将一些文件路径、URL以硬编码的方式写入代码中，或者多处出现一长串的文件路径、URL。

在 Yii2 中，一个别名必须以 @ 字符开头，Yii2 预定义了大量可用的别名，预定义的别名如下：

@yii 表示Yii框架所在的目录，也是 BaseYii.php 文件所在的位置
@app 表示正在运行的应用的根目录
@vendor 表示Composer 第三方库所在目录，一般是 @app/vendor 或 @app/../vendor
@bower 表示 Bower 第三方库所在目录，一般是 @vendor/bower
@npm 表示 NPM 第三方库所在目录，一般是 @vendor/npm
@runtime 表示正在运行的应用的运行时用于存放运行时文件的目录，一般是 @app/runtime
@webroot 表示正在运行的应用的入口文件 index.php 所在的目录，一般是 @app/web
@web URL别名，表示当前应用的根URL，主要用于前端
@common 表示通用文件夹
@frontend 表示前台应用所在的文件夹
@backend 表示后台应用所在的文件夹
@console 表示命令行应用所在的文件夹
其他使用Composer安装的Yii扩展注册的二级别名

其中的 @common @frontend @backend 和 @console 在 baisc 的项目中是不会存在的。在 advanced 的项目中通常是定义在 common\config\bootstrap.php 文件中，其内容如下：

<?<span>php
Yii</span>::setAlias('common', <span>dirname</span><span>(__DIR__));
Yii</span>::setAlias('frontend', <span>dirname</span>(<span>dirname</span>(__DIR__)) . '/frontend'<span>);
Yii</span>::setAlias('backend', <span>dirname</span>(<span>dirname</span>(__DIR__)) . '/backend'<span>);
Yii</span>::setAlias('console', <span>dirname</span>(<span>dirname</span>(__DIR__)) . '/console');

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Yii2 中关于别名的设置和获取的方法都放在 BaseYii 类中，其结构基本如下：

<?<span>php
</span><span>class</span><span> BaseYii
{
    </span><span>/*</span><span>*
     * @var array registered path aliases
     * @see getAlias()
     * @see setAlias()
     * Yii 的路径别名的 Map, 默认 @yii 指向当前目录
     </span><span>*/</span>
    <span>public</span> <span>static</span> <span>$aliases</span> = ['@yii' =><span> __DIR__];

    </span><span>/*</span><span>*
     * Translates a path alias into an actual path.
     * 将别名转化为真实的路径
     </span><span>*/</span>
    <span>public</span> <span>static</span> <span>function</span> getAlias(<span>$alias</span>, <span>$throwException</span> = <span>true</span><span>)
    {
        </span>...<span>
    }

    </span><span>/*</span><span>*
     * Registers a path alias.
     * 用一个真实的路径注册一个别名
     </span><span>*/</span>
    <span>public</span> <span>static</span> <span>function</span> setAlias(<span>$alias</span>, <span>$path</span><span>)
    {
        </span>...<span>
    }
}</span>

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这是简化之后的 BaseYii 类的结构，其中有一个重要的变量 $aliases，两个重要的方法 getAlias 和 setAlias。$aliases 是存储 Yii2 路径别名的一个数组，key 是别名，value 是真实路径。getAlias 方法是根据别名获取到真实的地址，setAlias 是用一个真实的地址去注册一个别名。

先来看下 setAlias 方法，其内容如下：

    <span>/*</span><span>*
     * Registers a path alias.
     *
     * 用一个真实的路径注册一个别名
     *
     * A path alias is a short name representing a long path (a file path, a URL, etc.)
     * For example, we use '@yii' as the alias of the path to the Yii framework directory.
     *
     * A path alias must start with the character '@' so that it can be easily differentiated
     * from non-alias paths.
     *
     * Note that this method does not check if the given path exists or not. All it does is
     * to associate the alias with the path.
     *
     * Any trailing '/' and '\' characters in the given path will be trimmed.
     *
     * @param string $alias the alias name (e.g. "@yii"). It must start with a '@' character.
     * It may contain the forward slash '/' which serves as boundary character when performing
     * alias translation by [[getAlias()]].
     * @param string $path the path corresponding to the alias. If this is null, the alias will
     * be removed. Trailing '/' and '\' characters will be trimmed. This can be
     *
     * - a directory or a file path (e.g. `/tmp`, `/tmp/main.txt`)
     * - a URL (e.g. `http://www.yiiframework.com`)
     * - a path alias (e.g. `@yii/base`). In this case, the path alias will be converted into the
     *   actual path first by calling [[getAlias()]].
     *
     * @throws InvalidParamException if $path is an invalid alias.
     * @see getAlias()
     </span><span>*/</span>
    <span>public</span> <span>static</span> <span>function</span> setAlias(<span>$alias</span>, <span>$path</span><span>)
    {
        </span><span>if</span> (<span>strncmp</span>(<span>$alias</span>, '@', 1<span>)) {
            </span><span>//</span><span> 如果不是以 @ 开头，就将 @ 拼到开头</span>
            <span>$alias</span> = '@' . <span>$alias</span><span>;
        }
        </span><span>//</span><span> 获取 / 在 $alias 中首次出现的位置</span>
        <span>$pos</span> = <span>strpos</span>(<span>$alias</span>, '/'<span>);
        </span><span>//</span><span> 如果 / 不存在，$root 就是整个 $alias，否则就是 $alias 中 / 前的内容</span>
        <span>$root</span> = <span>$pos</span> === <span>false</span> ? <span>$alias</span> : <span>substr</span>(<span>$alias</span>, 0, <span>$pos</span><span>);
        </span><span>if</span> (<span>$path</span> !== <span>null</span><span>) {
            </span><span>//</span><span> 如果 $path 以 @ 开头，使用 getAlias 去获取路径，否则，就去除掉最右边的 /</span>
            <span>$path</span> = <span>strncmp</span>(<span>$path</span>, '@', 1) ? <span>rtrim</span>(<span>$path</span>, '\\/') : <span>static</span>::getAlias(<span>$path</span><span>);
            </span><span>if</span> (!<span>isset</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span><span>])) {
                </span><span>//</span><span> 如果不存在这个 $root 的别名</span>
                <span>if</span> (<span>$pos</span> === <span>false</span><span>) {
                    </span><span>//</span><span> 没有 /，就将 $path 直接赋值以为 $root 别名对应的路径</span>
                    <span>static</span>::<span>$aliases</span>[<span>$root</span>] = <span>$path</span><span>;
                } </span><span>else</span><span> {
                    </span><span>//</span><span> 否则，就将 $path 直接赋值为 $root 下的 $alias 的路径</span>
                    <span>static</span>::<span>$aliases</span>[<span>$root</span>] = [<span>$alias</span> => <span>$path</span><span>];
                }
            } </span><span>elseif</span> (<span>is_string</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span><span>])) {
                </span><span>//</span><span> 如果存在，而且是个string类型</span>
                <span>if</span> (<span>$pos</span> === <span>false</span><span>) {
                    </span><span>//</span><span> 没有 /，意味着 $alias 就是 $root，直接覆盖即可</span>
                    <span>static</span>::<span>$aliases</span>[<span>$root</span>] = <span>$path</span><span>;
                } </span><span>else</span><span> {
                    </span><span>//</span><span> 否则，就合并到一起</span>
                    <span>static</span>::<span>$aliases</span>[<span>$root</span>] =<span> [
                        </span><span>$alias</span> => <span>$path</span>,
                        <span>$root</span> => <span>static</span>::<span>$aliases</span>[<span>$root</span>],<span>
                    ];
                }
            } </span><span>else</span><span> {
                </span><span>//</span><span> 这种，正常是个 array 类型
                // 直接添加进去即可</span>
                <span>static</span>::<span>$aliases</span>[<span>$root</span>][<span>$alias</span>] = <span>$path</span><span>;
                </span><span>//</span><span> krsort &mdash; 对数组按照键名逆向排序<br />　　　　　　　　　 // 可以做到优先匹配长的别名</span>
                <span>krsort</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span><span>]);
            }
        } </span><span>elseif</span> (<span>isset</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span><span>])) {
            </span><span>//</span><span> $path 为空且对应的别名有值存在，就是要移除相应的别名</span>
            <span>if</span> (<span>is_array</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span><span>])) {
                </span><span>//</span><span> 如果 $root 的别名对应一个 array，就只移除掉对应的别名即可</span>
                <span>unset</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span>][<span>$alias</span><span>]);
            } </span><span>elseif</span> (<span>$pos</span> === <span>false</span><span>) {
                </span><span>//</span><span> 如果 $root 的别名对应不是一个 array 而且 $root 就是 $alias，就移除这个 $root 的别名</span>
                <span>unset</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span><span>]);
            }
        }
    }</span>

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下面举几个例子来说明，别名写入后，$aliases 中的内容变化。

<span>//</span><span> 初始 BaseYii::aliases['@foo'] = 'path/to/foo'</span>
Yii::setAlias('@foo', 'path/to/foo'<span>);

</span><span>//</span><span> 直接覆盖 BaseYii::aliases['@foo'] = 'path/to/foo2'</span>
Yii::setAlias('@foo', 'path/to/foo2'<span>);

</span><span>/*</span><span>*
* 新增
* BaseYii::aliases['@foo'] = [
*     '@foo/bar' => 'path/to/foo/bar',
*     '@foo' => 'path/to/foo2',
* ];
</span><span>*/</span><span>
Yii</span>::setAlias('@foo/bar', 'path/to/foo/bar'<span>);

</span><span>//</span><span> 初始 BaseYii::aliases['@bar'] = ['@bar/qux' => 'path/to/bar/qux'];</span>
Yii::setAlias('@bar/qux', 'path/to/bar/qux'<span>);

</span><span>//</span><span> 直接覆盖 BaseYii::aliases['@bar'] = ['@bar/qux' => 'path/to/bar/qux2'];</span>
Yii::setAlias('@bar/qux', 'path/to/bar/qux2'<span>);

</span><span>/*</span><span>*
* 新增
* BaseYii::aliases['@bar'] = [
*     '@bar/foo' => 'path/to/bar/foo',
*     '@bar/qux' => 'path/to/bar/qux2',
* ];
</span><span>*/</span><span>
Yii</span>::setAlias('@bar/foo', 'path/to/bar/foo'<span>);

</span><span>/*</span><span>*
* 新增
* BaseYii::aliases['@bar'] = [
*     '@bar/foo' => 'path/to/bar/foo',
*     '@bar/qux' => 'path/to/bar/qux2',
*     '@bar' => 'path/to/bar',
* ];
</span><span>*/</span><span>
Yii</span>::setAlias('@bar', 'path/to/bar'<span>);

</span><span>/*</span><span>*
* 删除
* BaseYii::aliases['@bar'] = [
*     '@bar/foo' => 'path/to/bar/foo',
*     '@bar' => 'path/to/bar',
* ];
</span><span>*/</span><span>
Yii</span>::setAlias('@bar/qux', <span>null</span><span>);

</span><span>/*</span><span>*
* 删除
* BaseYii::aliases['@bar'] = [
*     '@bar/foo' => 'path/to/bar/foo',
* ];
</span><span>*/</span><span>
Yii</span>::setAlias('@bar', <span>null</span>);

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再来看一下 getAlias 方法，其内容如下：

    <span>/*</span><span>*
     * Translates a path alias into an actual path.
     * 将别名转化为真实的路径
     *
     * The translation is done according to the following procedure:
     *
     * 1. If the given alias does not start with '@', it is returned back without change;
     * 2. Otherwise, look for the longest registered alias that matches the beginning part
     *    of the given alias. If it exists, replace the matching part of the given alias with
     *    the corresponding registered path.
     * 3. Throw an exception or return false, depending on the `$throwException` parameter.
     *
     * For example, by default '@yii' is registered as the alias to the Yii framework directory,
     * say '/path/to/yii'. The alias '@yii/web' would then be translated into '/path/to/yii/web'.
     *
     * If you have registered two aliases '@foo' and '@foo/bar'. Then translating '@foo/bar/config'
     * would replace the part '@foo/bar' (instead of '@foo') with the corresponding registered path.
     * This is because the longest alias takes precedence.
     *
     * However, if the alias to be translated is '@foo/barbar/config', then '@foo' will be replaced
     * instead of '@foo/bar', because '/' serves as the boundary character.
     *
     * Note, this method does not check if the returned path exists or not.
     *
     * @param string $alias the alias to be translated.
     * @param boolean $throwException whether to throw an exception if the given alias is invalid.
     * If this is false and an invalid alias is given, false will be returned by this method.
     * @return string|boolean the path corresponding to the alias, false if the root alias is not previously registered.
     * @throws InvalidParamException if the alias is invalid while $throwException is true.
     * @see setAlias()
     </span><span>*/</span>
    <span>public</span> <span>static</span> <span>function</span> getAlias(<span>$alias</span>, <span>$throwException</span> = <span>true</span><span>)
    {
        </span><span>/*</span><span>*
         * strncmp &mdash; 二进制安全比较字符串开头的若干个字符
         * int strncmp ( string $str1 , string $str2 , int $len )
         * 如果 $alias 不是以 '@' 开头的，就不是一个 Yii 的别名
         </span><span>*/</span>
        <span>if</span> (<span>strncmp</span>(<span>$alias</span>, '@', 1<span>)) {
            </span><span>//</span><span> not an alias</span>
            <span>return</span> <span>$alias</span><span>;
        }

        </span><span>//</span><span> 获取 / 在 $alias 中首次出现的位置</span>
        <span>$pos</span> = <span>strpos</span>(<span>$alias</span>, '/'<span>);
        </span><span>//</span><span> 如果 / 不存在，$root 就是整个 $alias，否则就是 $alias 中 / 前的内容</span>
        <span>$root</span> = <span>$pos</span> === <span>false</span> ? <span>$alias</span> : <span>substr</span>(<span>$alias</span>, 0, <span>$pos</span><span>);

        </span><span>//</span><span> 如果存在 $root 的别名</span>
        <span>if</span> (<span>isset</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span><span>])) {
            </span><span>if</span> (<span>is_string</span>(<span>static</span>::<span>$aliases</span>[<span>$root</span><span>])) {
                </span><span>//</span><span> 如果 $root 对应的别名是一个字符串，之直接返回 $aliases[$root] 或者 $aliases[$root] . substr($alias, $pos)
                // 当 $root 就是 $alias 返回 $aliases[$root]， 否则就在拼接上 $alias 除去 $root 后，剩下的字符串</span>
                <span>return</span> <span>$pos</span> === <span>false</span> ? <span>static</span>::<span>$aliases</span>[<span>$root</span>] : <span>static</span>::<span>$aliases</span>[<span>$root</span>] . <span>substr</span>(<span>$alias</span>, <span>$pos</span><span>);
            } </span><span>else</span><span> {
                </span><span>//</span><span> 否则，要遍历整个 $aliases[$root] 数组，找到 $name 与 $alias 相同的值，返回 $path . substr($alias, strlen($name))
                // 其实是返回了 $path 拼接上 $alias 除去 $root 后，剩下的字符串</span>
                <span>foreach</span> (<span>static</span>::<span>$aliases</span>[<span>$root</span>] <span>as</span> <span>$name</span> => <span>$path</span><span>) {
                    </span><span>if</span> (<span>strpos</span>(<span>$alias</span> . '/', <span>$name</span> . '/') === 0<span>) {
                        </span><span>return</span> <span>$path</span> . <span>substr</span>(<span>$alias</span>, <span>strlen</span>(<span>$name</span><span>));
                    }
                }
            }
        }

        </span><span>if</span> (<span>$throwException</span><span>) {
            </span><span>throw</span> <span>new</span> InvalidParamException("Invalid path alias: <span>$alias</span>"<span>);
        } </span><span>else</span><span> {
            </span><span>return</span> <span>false</span><span>;
        }
    }</span>

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好了，关于别名就先说这么多～～

对 Yii2 源码有兴趣的同学可以关注项目 yii2-2.0.3-annotated，现在在上面已经添加了不少关于 Yii2 源码的注释，之后还会继续添加～

有兴趣的同学也可以参与进来，提交 Yii2 源码的注释。