参考：
iOS SDK wiki
07年开始 1.x 2017年11.x 一年一个版本 十年了
SDK contents：
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1. IOS SDK
iOS官方参考文档 IOS Developer Library，所有系统SDK及Framework都可从此查看到；要熟悉SDK跟Framework多看官方。
Cocoa Touch
遵守MVC结构设计
Main features:
- UIKit
- Auto Layout
- Storyboards
- Gesture Recognisers
- Standard System View Controllers
- Others
Frameworks:
Foundation Kit/UIKit/MapKit/Address Book UI/EventKit/Message UI/Notification Center/PushKit/iAd/GameKit
Media
- audio/video/image
- Quartz(Core Graphics)/Core Animation/OpenGL
CGContext: Core Graphic Context;
Core Services
- Networking
- Embedded SQLite database
- Core Location
- Threads
- CoreMotion 重力加速度/陀螺仪 Reference
Core OS / IOS kernel
- TCP/IP and Sockets
- Power management
- File system
- Security
