The foundational class for all visual elements defined in UIKit is the UIView, or view.
UIKit defines dozens of special UIView subclasses that perform specific tasks. For example, UILabel displays text
“To display a view onscreen, you need to give it a frame—which consists of a size and a position—and add it to the view hierarchy. The area within the view is its bounds. A view is transparent by default, so you'll also need to set a background color property. Here are some of the attributes you can change when working with views: ”
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Excerpts From: Apple Education. “App Development with Swift.” Apple Inc. - Education, 2017. iBooks. https://itunes.apple.com/au/book/app-development-with-swift/id1219117996?mt=11