/**
* Constructor modification point for MagentoFrameworkAppAction.
*
* All context classes were introduced to allow for backwards compatible constructor modifications
* of classes that were supposed to be extended by extension developers.
*
* Do not call methods of this class directly.
*
* As Magento moves from inheritance-based APIs all such classes will be deprecated together with
* the classes they were introduced for.
*
* @api
* @since 100.0.2
*/

总体来说，这个类是给控制器提供一些默认的类(相当于公共类)，便于调用，把公共常用的类放在这个类里，你打开这个文件看下就知道了，构造函数里面都是些常用的类。
貌似以后会抛弃这个类，不再使用，因为以后都只走api。