UIButton所有的parent class:

UIButton -> UIControl -> UIView -> UIResponder -> NSObject

UIControl: 

The base class for controls, which are visual elements that convey a specific action or intention in response to user interactions.

UIView: 

An object that manages the content for a rectangular area on the screen.

UIResponder: 

An abstract interface for responding to and handling events. (ex: Touch Events、Motion Events、Press Events)

NSObject:

The root class of most Objective-C class hierarchies, from which subclasses inherit a basic interface to the runtime system and the ability to behave as Objective-C objects.

Ref:

https://developer.apple.com/documentation/uikit/uiresponder

About UIResponder:

https://www.readfog.com/a/1633373455331201024

https://www.appcoda.com.tw/responder-chain/

UIView的hitTest和pointInside方法:
https://www.jianshu.com/p/c87de31b3985