'components' => [
  'view' => [
    'theme' => [
      'pathMap' => ['@app/views' => '@app/themes/basic'],
      'baseUrl' => '@web/themes/basic',
    ],
  ],
],

$pathMap=[
    '@app/a' => '@app/aaa',
    '@app/b' => '@app/bbb',
    '@app/c' => [
        '@app/ccc/xxx',
        '@app/ccc/yyy',
      ],
];

$pathMap=[
    '@app/c' => [
        '@app/ccc/blank',
        '@app/ccc/default',
      ],
];

'pathMap' =>[
    '@app/views' => [
        '@app/themes/blank',
        '@app/themes/default',
    ],
    '@app/modules' => '@app/themes/default/modules',
    '@app/widgets' => '@app/themes/default/widgets'
],

public function init()
{
    parent::init();
    //如果没有设置$pathMap映射，则使用$basePath，
    if (empty($this->pathMap)) {
        /*
         * 如果$basePath也没有设置，则出异常。
         * 也就是说 $pathMap和$basePath至少要设置一个，如果两个都设置了，优先使用$pathMap
         */
      if (($basePath = $this->getBasePath()) === null) {
        throw new InvalidConfigException('The "basePath" property must be set.');
      }
      //设置当前模块的路径和$basePath的映射关系
      $this->pathMap = [Yii::$app->getBasePath() => [$basePath]];
    }
}

//这个就是按照 $pathMap中的定义的映射关系来对$path进行替换字符串的
public function applyTo($path)
{
    //对路径中的"/"、“\”进行统一替换
    $path = FileHelper::normalizePath($path);
    foreach ($this->pathMap as $from => $tos) {
       //映射数组中的来源（旧值）
      $from = FileHelper::normalizePath(Yii::getAlias($from)) . DIRECTORY_SEPARATOR;
      //如果在$path中有可替换的旧值
      if (strpos($path, $from) === 0) {
        $n = strlen($from);
        //对目标值循环，
        foreach ((array) $tos as $to) {
          $to = FileHelper::normalizePath(Yii::getAlias($to)) . DIRECTORY_SEPARATOR;
          //把$path中的$from替换为$to
          $file = $to . substr($path, $n);
          //如果是文件，直接返回
          if (is_file($file)) {
            return $file;
          }
        }
      }
    }
    return $path;
}