Yii2的深入学习--别名（Aliases）
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：
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 style="color: #000000;">phpYii</span>::setAlias('common', <span style="color: #008080;">dirname</span><span style="color: #000000;">(__DIR__));Yii</span>::setAlias('frontend', <span style="color: #008080;">dirname</span>(<span style="color: #008080;">dirname</span>(__DIR__)) . '/frontend'<span style="color: #000000;">);Yii</span>::setAlias('backend', <span style="color: #008080;">dirname</span>(<span style="color: #008080;">dirname</span>(__DIR__)) . '/backend'<span style="color: #000000;">);Yii</span>::setAlias('console', <span style="color: #008080;">dirname</span>(<span style="color: #008080;">dirname</span>(__DIR__)) . '/console');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 style="color: #000000;">php</span><span style="color: #0000ff;">class</span><span style="color: #000000;"> BaseYii{ </span><span style="color: #008000;">/*</span><span style="color: #008000;">* * @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 style="color: #008000;">*/</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #800080;">$aliases</span> = [[email protected]' =><span style="color: #000000;"> __DIR__]; </span><span style="color: #008000;">/*</span><span style="color: #008000;">* * Translates a path alias into an actual path. * 将别名转化为真实的路径 </span><span style="color: #008000;">*/</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">function</span> getAlias(<span style="color: #800080;">$alias</span>, <span style="color: #800080;">$throwException</span> = <span style="color: #0000ff;">true</span><span style="color: #000000;">) { </span>...<span style="color: #000000;"> } </span><span style="color: #008000;">/*</span><span style="color: #008000;">* * Registers a path alias. * 用一个真实的路径注册一个别名 </span><span style="color: #008000;">*/</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">function</span> setAlias(<span style="color: #800080;">$alias</span>, <span style="color: #800080;">$path</span><span style="color: #000000;">) { </span>...<span style="color: #000000;"> }}</span>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 style="color: #008000;">/*</span><span style="color: #008000;">* * 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 [email protected]' 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. [email protected]/base`). In this case, the path alias will be converted into the * actual path first by calling [[getAlias()]]. * * @throws InvalidParamException if $path is an invalid alias. * @see getAlias() </span><span style="color: #008000;">*/</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">function</span> setAlias(<span style="color: #800080;">$alias</span>, <span style="color: #800080;">$path</span><span style="color: #000000;">) { </span><span style="color: #0000ff;">if</span> (<span style="color: #008080;">strncmp</span>(<span style="color: #800080;">$alias</span>, '@', 1<span style="color: #000000;">)) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果不是以 @ 开头，就将 @ 拼到开头</span> <span style="color: #800080;">$alias</span> = '@' . <span style="color: #800080;">$alias</span><span style="color: #000000;">; } </span><span style="color: #008000;">//</span><span style="color: #008000;"> 获取 / 在 $alias 中首次出现的位置</span> <span style="color: #800080;">$pos</span> = <span style="color: #008080;">strpos</span>(<span style="color: #800080;">$alias</span>, '/'<span style="color: #000000;">); </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果 / 不存在，$root 就是整个 $alias，否则就是 $alias 中 / 前的内容</span> <span style="color: #800080;">$root</span> = <span style="color: #800080;">$pos</span> === <span style="color: #0000ff;">false</span> ? <span style="color: #800080;">$alias</span> : <span style="color: #008080;">substr</span>(<span style="color: #800080;">$alias</span>, 0, <span style="color: #800080;">$pos</span><span style="color: #000000;">); </span><span style="color: #0000ff;">if</span> (<span style="color: #800080;">$path</span> !== <span style="color: #0000ff;">null</span><span style="color: #000000;">) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果 $path 以 @ 开头，使用 getAlias 去获取路径，否则，就去除掉最右边的 /</span> <span style="color: #800080;">$path</span> = <span style="color: #008080;">strncmp</span>(<span style="color: #800080;">$path</span>, '@', 1) ? <span style="color: #008080;">rtrim</span>(<span style="color: #800080;">$path</span>, '\\/') : <span style="color: #0000ff;">static</span>::getAlias(<span style="color: #800080;">$path</span><span style="color: #000000;">); </span><span style="color: #0000ff;">if</span> (!<span style="color: #0000ff;">isset</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span><span style="color: #000000;">])) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果不存在这个 $root 的别名</span> <span style="color: #0000ff;">if</span> (<span style="color: #800080;">$pos</span> === <span style="color: #0000ff;">false</span><span style="color: #000000;">) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 没有 /，就将 $path 直接赋值以为 $root 别名对应的路径</span> <span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>] = <span style="color: #800080;">$path</span><span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 否则，就将 $path 直接赋值为 $root 下的 $alias 的路径</span> <span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>] = [<span style="color: #800080;">$alias</span> => <span style="color: #800080;">$path</span><span style="color: #000000;">]; } } </span><span style="color: #0000ff;">elseif</span> (<span style="color: #008080;">is_string</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span><span style="color: #000000;">])) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果存在，而且是个string类型</span> <span style="color: #0000ff;">if</span> (<span style="color: #800080;">$pos</span> === <span style="color: #0000ff;">false</span><span style="color: #000000;">) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 没有 /，意味着 $alias 就是 $root，直接覆盖即可</span> <span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>] = <span style="color: #800080;">$path</span><span style="color: #000000;">; } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 否则，就合并到一起</span> <span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>] =<span style="color: #000000;"> [ </span><span style="color: #800080;">$alias</span> => <span style="color: #800080;">$path</span>, <span style="color: #800080;">$root</span> => <span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>],<span style="color: #000000;"> ]; } } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 这种，正常是个 array 类型 // 直接添加进去即可</span> <span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>][<span style="color: #800080;">$alias</span>] = <span style="color: #800080;">$path</span><span style="color: #000000;">; </span><span style="color: #008000;">//</span><span style="color: #008000;"> krsort — 对数组按照键名逆向排序<br> // 可以做到优先匹配长的别名</span> <span style="color: #008080;">krsort</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span><span style="color: #000000;">]); } } </span><span style="color: #0000ff;">elseif</span> (<span style="color: #0000ff;">isset</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span><span style="color: #000000;">])) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> $path 为空且对应的别名有值存在，就是要移除相应的别名</span> <span style="color: #0000ff;">if</span> (<span style="color: #008080;">is_array</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span><span style="color: #000000;">])) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果 $root 的别名对应一个 array，就只移除掉对应的别名即可</span> <span style="color: #0000ff;">unset</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>][<span style="color: #800080;">$alias</span><span style="color: #000000;">]); } </span><span style="color: #0000ff;">elseif</span> (<span style="color: #800080;">$pos</span> === <span style="color: #0000ff;">false</span><span style="color: #000000;">) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果 $root 的别名对应不是一个 array 而且 $root 就是 $alias，就移除这个 $root 的别名</span> <span style="color: #0000ff;">unset</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span><span style="color: #000000;">]); } } }</span>下面举几个例子来说明，别名写入后，$aliases 中的内容变化。
<span style="color: #008000;">//</span><span style="color: #008000;"> 初始 BaseYii::aliases[[email protected]'] = 'path/to/foo'</span>Yii::setAlias([email protected]', 'path/to/foo'<span style="color: #000000;">);</span><span style="color: #008000;">//</span><span style="color: #008000;"> 直接覆盖 BaseYii::aliases[[email protected]'] = 'path/to/foo2'</span>Yii::setAlias([email protected]', 'path/to/foo2'<span style="color: #000000;">);</span><span style="color: #008000;">/*</span><span style="color: #008000;">** 新增* BaseYii::aliases[[email protected]'] = [* [email protected]/bar' => 'path/to/foo/bar',* [email protected]' => 'path/to/foo2',* ];</span><span style="color: #008000;">*/</span><span style="color: #000000;">Yii</span>::setAlias([email protected]/bar', 'path/to/foo/bar'<span style="color: #000000;">);</span><span style="color: #008000;">//</span><span style="color: #008000;"> 初始 BaseYii::aliases[[email protected]'] = [[email protected]/qux' => 'path/to/bar/qux'];</span>Yii::setAlias([email protected]/qux', 'path/to/bar/qux'<span style="color: #000000;">);</span><span style="color: #008000;">//</span><span style="color: #008000;"> 直接覆盖 BaseYii::aliases[[email protected]'] = [[email protected]/qux' => 'path/to/bar/qux2'];</span>Yii::setAlias([email protected]/qux', 'path/to/bar/qux2'<span style="color: #000000;">);</span><span style="color: #008000;">/*</span><span style="color: #008000;">** 新增* BaseYii::aliases[[email protected]'] = [* [email protected]/foo' => 'path/to/bar/foo',* [email protected]/qux' => 'path/to/bar/qux2',* ];</span><span style="color: #008000;">*/</span><span style="color: #000000;">Yii</span>::setAlias([email protected]/foo', 'path/to/bar/foo'<span style="color: #000000;">);</span><span style="color: #008000;">/*</span><span style="color: #008000;">** 新增* BaseYii::aliases[[email protected]'] = [* [email protected]/foo' => 'path/to/bar/foo',* [email protected]/qux' => 'path/to/bar/qux2',* [email protected]' => 'path/to/bar',* ];</span><span style="color: #008000;">*/</span><span style="color: #000000;">Yii</span>::setAlias([email protected]', 'path/to/bar'<span style="color: #000000;">);</span><span style="color: #008000;">/*</span><span style="color: #008000;">** 删除* BaseYii::aliases[[email protected]'] = [* [email protected]/foo' => 'path/to/bar/foo',* [email protected]' => 'path/to/bar',* ];</span><span style="color: #008000;">*/</span><span style="color: #000000;">Yii</span>::setAlias([email protected]/qux', <span style="color: #0000ff;">null</span><span style="color: #000000;">);</span><span style="color: #008000;">/*</span><span style="color: #008000;">** 删除* BaseYii::aliases[[email protected]'] = [* [email protected]/foo' => 'path/to/bar/foo',* ];</span><span style="color: #008000;">*/</span><span style="color: #000000;">Yii</span>::setAlias([email protected]', <span style="color: #0000ff;">null</span>);
再来看一下 getAlias 方法，其内容如下：
<span style="color: #008000;">/*</span><span style="color: #008000;">* * 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 [email protected]' is registered as the alias to the Yii framework directory, * say '/path/to/yii'. The alias [email protected]/web' would then be translated into '/path/to/yii/web'. * * If you have registered two aliases [email protected]' and [email protected]/bar'. Then translating [email protected]/bar/config' * would replace the part [email protected]/bar' (instead of [email protected]') with the corresponding registered path. * This is because the longest alias takes precedence. * * However, if the alias to be translated is [email protected]/barbar/config', then [email protected]' will be replaced * instead of [email protected]/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 $throwException is true. * @see setAlias() </span><span style="color: #008000;">*/</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">function</span> getAlias(<span style="color: #800080;">$alias</span>, <span style="color: #800080;">$throwException</span> = <span style="color: #0000ff;">true</span><span style="color: #000000;">) { </span><span style="color: #008000;">/*</span><span style="color: #008000;">* * strncmp — 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 $alias 不是以 '@' 开头的，就不是一个 Yii 的别名 </span><span style="color: #008000;">*/</span> <span style="color: #0000ff;">if</span> (<span style="color: #008080;">strncmp</span>(<span style="color: #800080;">$alias</span>, '@', 1<span style="color: #000000;">)) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> not an alias</span> <span style="color: #0000ff;">return</span> <span style="color: #800080;">$alias</span><span style="color: #000000;">; } </span><span style="color: #008000;">//</span><span style="color: #008000;"> 获取 / 在 $alias 中首次出现的位置</span> <span style="color: #800080;">$pos</span> = <span style="color: #008080;">strpos</span>(<span style="color: #800080;">$alias</span>, '/'<span style="color: #000000;">); </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果 / 不存在，$root 就是整个 $alias，否则就是 $alias 中 / 前的内容</span> <span style="color: #800080;">$root</span> = <span style="color: #800080;">$pos</span> === <span style="color: #0000ff;">false</span> ? <span style="color: #800080;">$alias</span> : <span style="color: #008080;">substr</span>(<span style="color: #800080;">$alias</span>, 0, <span style="color: #800080;">$pos</span><span style="color: #000000;">); </span><span style="color: #008000;">//</span><span style="color: #008000;"> 如果存在 $root 的别名</span> <span style="color: #0000ff;">if</span> (<span style="color: #0000ff;">isset</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span><span style="color: #000000;">])) { </span><span style="color: #0000ff;">if</span> (<span style="color: #008080;">is_string</span>(<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span><span style="color: #000000;">])) { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 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 $root 后，剩下的字符串</span> <span style="color: #0000ff;">return</span> <span style="color: #800080;">$pos</span> === <span style="color: #0000ff;">false</span> ? <span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>] : <span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>] . <span style="color: #008080;">substr</span>(<span style="color: #800080;">$alias</span>, <span style="color: #800080;">$pos</span><span style="color: #000000;">); } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #008000;">//</span><span style="color: #008000;"> 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 $root 后，剩下的字符串</span> <span style="color: #0000ff;">foreach</span> (<span style="color: #0000ff;">static</span>::<span style="color: #800080;">$aliases</span>[<span style="color: #800080;">$root</span>] <span style="color: #0000ff;">as</span> <span style="color: #800080;">$name</span> => <span style="color: #800080;">$path</span><span style="color: #000000;">) { </span><span style="color: #0000ff;">if</span> (<span style="color: #008080;">strpos</span>(<span style="color: #800080;">$alias</span> . '/', <span style="color: #800080;">$name</span> . '/') === 0<span style="color: #000000;">) { </span><span style="color: #0000ff;">return</span> <span style="color: #800080;">$path</span> . <span style="color: #008080;">substr</span>(<span style="color: #800080;">$alias</span>, <span style="color: #008080;">strlen</span>(<span style="color: #800080;">$name</span><span style="color: #000000;">)); } } } } </span><span style="color: #0000ff;">if</span> (<span style="color: #800080;">$throwException</span><span style="color: #000000;">) { </span><span style="color: #0000ff;">throw</span> <span style="color: #0000ff;">new</span> InvalidParamException("Invalid path alias: <span style="color: #800080;">$alias</span>"<span style="color: #000000;">); } </span><span style="color: #0000ff;">else</span><span style="color: #000000;"> { </span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">false</span><span style="color: #000000;">; } }</span>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。
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