Interface Essentials
Most iOS apps are built using components from UIKit, a programming framework that defines common interface elements. This framework lets apps achieve a consistent appearance across the system, while at the same time offering a high level of customization. UIKit elements are flexible and familiar. They’re adaptable, enabling you to design a single app that looks great on any iOS device, and they automatically update when the system introduces appearance changes. The interface elements provided by UIKit fit into three main categories:
大多数iOS app 用UIKit框架的组件来搭建，UIKit-一个定义了通用界面元素的编程框架。这个框架让app拥有一致的外观的，同时提供了高度的可定制化。UIKit 元素灵活而易于理解。它们适配方便，让你设计一个应用却能在所有iOS设备都看上去不错，它们自动升级随着系统自带样式改变。UIKit提供的界面元素分为3个主要类别：
Bars.Tell people where they are in your app, provide navigation, and may contain buttons or other elements for initiating actions and communicating information.
栅栏。告诉人们在app何处，提供导航，包含按钮或其他元素来开始行动和交流信息。
Views.Contain the primary content people see in your app, such as text, graphics, animations, and interactive elements. Views can enable behaviors such as scrolling, insertion, deletion, and arrangement.
视图。包含主要用户在app中看到的主要内容，例如文本，图表，动画，和交互元素。视图支持以下行为：滚动，插入，删除和整理。
Controls.Initiate actions and convey information. Buttons, switches, text fields, and progress indicators are examples of controls.
控制。初始化操作和传递信息。控制器的例子有按钮，转换器，文本框和进度指示器。
In addition to defining the interface of iOS, UIKit defines functionality your app can adopt. Through this framework, for example, your app can respond to gestures on the touchscreen and enable features such as drawing, accessibility, and printing.
除了定义iOS的界面，UIKit还定义你的app能运用的功能。通过这个框架，例如，你的app可以对触摸屏手势应答，支持特性有诸如绘画，可访问性，打印。
iOS tightly integrates with other programming frameworks and technologies too, such as Apple Pay, HealthKit, and ResearchKit, enabling you to design amazingly powerful apps.
iOS高度集成其他框架和技术，例如Apple Pay, HealthKit, and ResearchKit,是你能设计出无比卓越的app。
https://developer.apple.com/ios/human-interface-guidelines/overview/interface-essentials/
