官方文件：

A UIWindow object provides the backdrop for your app’s user interface and provides important event-handling behaviors. Windows do not have any visual appearance of their own, but they are crucial to the presentation of your app’s views. Every view that appears onscreen is enclosed by a window, and each window is independent of the other windows in your app. Events received by your app are initially routed to the appropriate window object, which in turn forwards those events to the appropriate view. Windows work with your view controllers to implement orientation changes and to perform many other tasks that are fundamental to your app’s operation.

老實講官方文件沒有說得很清楚，從上述大概可以知道幾點：

• Every view that appears onscreen is enclosed by a window
• Events received by your app are initially routed to the appropriate window object, which in turn forwards those events to the appropriate view

文件說我們很少會直接接觸到它，大部分 UIKit 就自己解決的了，看來如果想要更深入瞭解又要去看民間寫的文章了....