Human Interface Principles

http://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/MobileHIG/Principles/Principles.html

Human Interface Principles

A great user interface follows human interface design principles that are based on the way people—users—think and work, not on the capabilities of the device. A user interface that is unattractive, convoluted, or illogical can make even a great application seem like a chore to use. But a beautiful, intuitive, compelling user interface enhances an application’s functionality and inspires a positive emotional attachment in users.
Aesthetic Integrity

Aesthetic integrity is not a measure of how beautiful an application is. It’s a measure of how well the appearance of the app integrates with its function. For example, an app that enables a productive task generally keeps decorative elements subtle and in the background, while giving prominence to the task by providing standard controls and behaviors. Such an app gives users a clear, unified message about its purpose and its identity. If, on the other hand, the app enables the productive task within a UI that seems whimsical or frivolous, people might not know how to interpret these contradictory signals.

Similarly, in an app that encourages an immersive task, such as a game, users expect a beautiful appearance that promises fun and encourages discovery. Although people don’t expect to accomplish a serious or productive task in a game, they still expect the game’s appearance to integrate with the experience.
Consistency

Consistency in the interface allows people to transfer their knowledge and skills from one application to another. A consistent application is not a slavish copy of other applications. Rather, it is an application that takes advantage of the standards and paradigms people are comfortable with.

To determine whether an app follows the principle of consistency, think about these questions:

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Is the application consistent with iOS standards? Does it use system-provided controls, views, and icons correctly? Does it incorporate device features in a reliable way?
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Is the application consistent within itself? Does text use uniform terminology and style? Do the same icons always mean the same thing? Can people predict what will happen when they perform the same action in different places? Do custom UI elements look and behave the same throughout the app?
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Within reason, is the application consistent with its earlier versions? Have the terms and meanings remained the same? Are the fundamental concepts essentially unchanged?

Direct Manipulation

When people directly manipulate onscreen objects instead of using separate controls to manipulate them, they're more engaged with the task and they more readily understand the results of their actions. iOS users enjoy a heightened sense of direct manipulation because of the Multi-Touch interface. Using gestures gives people a greater affinity for, and sense of control over, the objects they see onscreen, because they're able to touch them without using an intermediary, such as a mouse.

For example, instead of tapping zoom controls, people can use the pinch gestures to directly expand or contract an area of content. And in a game, players move and interact directly with onscreen objects. For example, a game might display a combination lock that users can spin to open.

In an iOS application, people can experience direct manipulation when they:

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Rotate or otherwise move the device to affect onscreen objects
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Use gestures to manipulate onscreen objects
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Can see that their actions have immediate, visible results

Feedback

Feedback acknowledges people’s actions and assures them that processing is occurring. People expect immediate feedback when they operate a control, and they appreciate status updates during lengthy operations.

The built-in iOS applications respond to every user action with some perceptible change. For example, list items highlight briefly when people tap them. During operations that last more than a few seconds, a control shows elapsing progress, and if appropriate, the app displays an explanatory message.

Subtle animation can give people meaningful feedback that helps clarify the results of their actions. For example, lists can animate the addition of a new row to help people track the change visually.

Sound can also give people useful feedback. But sound shouldn’t be the primary or sole feedback mechanism because people may use their devices in places where they can’t hear or where they must turn off the sound.
Metaphors

When virtual objects and actions in an application are metaphors for objects and actions in the real world, users quickly grasp how to use the app. The classic example of a software metaphor is the folder: People put things in folders in the real world, so they immediately understand the idea of putting files into folders on a computer.

The most appropriate metaphors suggest a usage or experience without enforcing the limitations of the real-world object or action on which they’re based. For example, people can fill software folders with much more content than would fit in a physical folder.

iOS provides great scope for metaphors because it supports rich graphical images and gestures. People physically interact with realistic onscreen objects, in many cases operating them as if they were real-world objects. Metaphors in iOS include:

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Tapping iPod playback controls
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Dragging, flicking, or swiping objects in a game
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Sliding On/Off switches
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Flicking through pages of photos
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Spinning picker wheels to make choices

In general, metaphors work best when they’re not stretched too far. For example, the usability of software folders would decrease if they had to be organized into a virtual filing cabinet.

User Control

People, not applications, should initiate and control actions. Although an application can suggest a course of action or warn about dangerous consequences, it’s usually a mistake for the app to take decision-making away from the user. The best apps find the correct balance between giving people the capabilities they need while helping them avoid dangerous outcomes.

Users feel more in control of an app when behaviors and controls are familiar and predictable. And, when actions are simple and straightforward, users can easily understand and remember them.

People expect to have ample opportunity to cancel an operation before it begins, and they expect to get a chance to confirm their intention to perform a potentially destructive action. Finally, people expect to be able to gracefully stop an operation that’s underway.

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