User Control · An app can make people feel like they’re in control by keeping interactive elements familiar and predictable, confirming destructive actions, and making it easy to cancel operations, even when they’re already underway.
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Depth · Distinct visual layers and realistic motion convey hierarchy, impart vitality, and facilitate understanding. Touch and discoverability heighten delight and enable access to functionality and additional content without losing context. Transitions provide a sense of depth as you navigate through content.
Direct Manipulation · The direct manipulation of onscreen content engages people and facilitates understanding. Users experience direct manipulation when they rotate the device or use gestures to affect onscreen content. Through direct manipulation, they can see the immediate, visible results of their actions.
User Control · Throughout iOS, people—not apps—are in control. An app can suggest a course of action or warn about dangerous consequences, but it’s usually a mistake for the app to take over the decision-making. The best apps find the correct balance between enabling users and avoiding unwanted outcomes.
In America, we may acknowledge Washington and Lincoln as great men, and probably Franklin and Jefferson and maybe Franklin Delano Roosevelt and possibly even several more, but we would probably disagree about precisely what it was that made them great, what it was that enabled them to give a lasting direction to the course of events.