---

Human Interface Design Principles

This section presents some key principles critical to the design of elegant, efficient, intuitive, and Aqua-compliant user interfaces. Sometimes overlooked by developers, these principles are as relevant today as when Apple first published them decades ago. In fact, they drive the design of the Mac OS X user interface.

In this section:

Metaphors
Reflect the User’s Mental Model
Explicit and Implied Actions
Direct Manipulation
User Control
Feedback and Communication
Consistency
WYSIWYG (What You See Is What You Get)
Forgiveness
Perceived Stability
Aesthetic Integrity
Modelessness
Managing Complexity in Your Software

Metaphors

Take advantage of people’s knowledge of the world by using metaphors to convey concepts and features of your application. Metaphors are the building blocks in the user’s mental model of a task. Use metaphors that represent concrete, familiar ideas, and make the metaphors obvious, so that users can apply a set of expectations to the computer environment. For example, Mac OS X uses the metaphor of file folders for storing documents; people can organize their hard disks in a way that is analogous to the way they organize file cabinets. Other metaphor examples include iTunes playlists and iPhoto albums, which represent real-world music playlists and photo albums. A Dashboard widget can also be a metaphor for the task it performs because it instantly conveys its purpose to the user. (For Dashboard widget design guidelines, see “Dashboard.”)

Metaphors should suggest a use for a particular element, but that use doesn’t have to limit the implementation of the metaphor. It is important to strike a balance between the metaphor’s suggested use and the computer’s ability to support and extend the metaphor. For example, the number of items a user puts in the Trash is not limited to the number of items a physical wastebasket could hold.

Reflect the User’s Mental Model

The user already has a mental model that describes the task your software is enabling. This model arises from a combination of real-world experiences, experience with other software, and with computers in general. For example, users have real-world experience writing and mailing letters and most users have used email applications to write and send email. Based on this, a user has a conceptual model of this task that includes certain expectations, such as the ability to create a new letter, select a recipient, and send the letter. An email application that ignores the user’s mental model and does not meet at least some of the user’s expectations would be difficult and even unpleasant to use. This is because such an application imposes an unfamiliar conceptual model on its users instead of building on the knowledge and experiences those users already have.

Before you design your application’s user interface, try to discover your users’ mental model of the task your application helps them perform. Be aware of the model’s inherent metaphors, which represent conceptual components of the task. In the letter-writing example, the metaphors include letters, mail boxes, and envelopes. In the mental model of a task related to photography, the metaphors include photographs, cameras, and albums. Strive to reflect the user’s expectations of task components, organization, and workflow in your window layout, menu and toolbar organization, and use of panels.

A good example of how reflecting the appropriate mental model results in a clean, intuitive user interface is the iTunes application. Apple designed iTunes to reflect the mental models people associate with playing music and managing their music collections. In an uncluttered window, iTunes displays individual songs, playlists, and playback and search controls in a song-centric arrangement. The largest pane displays a list of songs, clearly sortable by categories such as title, artist, and album. The smaller pane displays the playlists and collections, which control the list of songs currently displayed, just as the disk and folder icons in the Finder sidebar control the display of files, folders, and applications. The prominent playback controls look like similar controls on radios, CD players, and the iPod. The search field is identical to the search field in Finder, Mail, and countless other Aqua-compliant applications. Because the iTunes user interface reflects a well-defined mental model, instead of forcing users to adopt unfamiliar concepts, even novice users find iTunes intuitive and easy to use.

The mental model your users have should infuse the design of your application’s user interface. It should inform the layout of your application’s windows, the selection and organization of icons and controls in the toolbars, and the functionality of panels. In addition, you should support the user’s mental model by striving to incorporate the following characteristics:

• Familiarity. The user’s mental model is based primarily on experience. When possible, enhance user interface components to reflect the model’s symbology and display labels that use the model’s terminology. Then, where appropriate, use familiar Mac OS X user interface components to offer standard functionality, such as searching and navigating hierarchical sets of data.

  As described above, the iTunes application displays playback controls that use well-known symbols users associate with play, pause, and rewind. Then, to offer searching and help, for example, iTunes uses standard Aqua user interface components. A Mac OS X user automatically knows how to use such standard user interface elements, regardless of the application in which they appear.

• Simplicity. A mental model of a task is typically streamlined and focused on the fundamental components of the task. Although there may be myriad optional details associated with a given task, the basic components should not have to compete with the details for the user’s attention.

  In the iTunes application, for example, the basic task components of playing songs, selecting playlists, and searching are prominently featured. However, these are supplemented by easily accessible menu items and controls that perform additional tasks, such as ejecting a disk, shuffling a playlist, and displaying song artwork.

• Availability. A corollary of simplicity is availability. An uncluttered user interface is essential, but the availability of certain key features and settings the user needs is equally so. Avoid hiding such components too deeply in submenus or making them accessible only from a contextual menu.

  The iCal application, for example, has commands for subscribing to a new calendar and for publishing a calendar in the Calendar menu. These tasks are easily accessible, but are not so frequently performed that they warrant dedicated controls on the application’s main window.

• Discoverability. Encourage your users to discover functionality by providing cues about how to use user interface elements. If an element is clickable, for example, it must appear that way, or a user may never try clicking it. Be sure to use Aqua controls properly and avoid making controls invisible to inexperienced users.

  Aqua buttons, for example, appear three-dimensional, enhancing their resemblance to buttons users see on physical devices. Well-designed toolbar icons make the commands they portray recognizable to users. This familiarity gives users the confidence to explore the functionality of a new application.

  Don’t discourage discovery by making actions difficult to reverse or recover from. For more information on this, see “Forgiveness.”

Explicit and Implied Actions

Each Mac OS X operation involves the manipulation of an object using an action. In the first step of this manipulation, the user sees the desired object onscreen. In the second step, the user selects or designates that object. In the final step, the user performs an action, either using a menu command or by direct manipulation of the object with the mouse or other device. This leads to two paradigms for manipulating objects: explicit and implied actions.

Explicit actions clearly state the result of manipulating an object. For example, menus list the commands that can be performed on the currently selected object. The name of the menu command clearly indicates what the action is and the current state of the command (dimmed or enabled) indicates whether that action is valid in the current context. Explicit actions do not require the user to memorize the commands that can be performed on a given object.

Implied actions convey the result of an action through visual cues or context. A drag-and-drop operation is a common example of an implied action. Dragging one object onto another object constitutes a relationship between the objects and an action to be performed by the drag operation. For example, dragging a file icon to the Trash implies the imminent removal of the underlying file from the file system. For implied actions to be apparent, the user must be able to recognize the objects involved, the manipulation to be performed, and the consequences of the action.

Keep these two paradigms in mind as you design your user interface. Examine the user’s mental model of your application’s task to help you determine when each type of action is appropriate. For example, Automator supports implied actions when the user drags actions into the workflow pane, creating relationships between them. Automator conveys these relationships by displaying connection points between actions, warning of potentially undesirable consequences, and suggesting types of input and output. When it requires the user to provide specific information, however, Automator supports explicit actions with the display of checkboxes and editable text fields.

Direct Manipulation

Direct manipulation is an example of an implied action that allows users to feel that they are controlling the objects represented by the computer. According to this principle, an onscreen object should remain visible while a user performs an action on it, and the impact of the action should be immediately visible. For example, with a drag-and-drop operation (the most common example of direct manipulation) users can move a file by dragging its icon from one location to another, or drag selected text directly into another document. Other examples of direct manipulation are the resizing of a graphic object in a drawing application and the positioning of an object or camera view in a three-dimensional scene.

Support direct manipulation when users are likely to expect it. Avoid forcing users to use controls to manipulate data. For example, an application that manages a virtual library might allow the user to drag a book icon onto a patron’s name to check it out. Such direct manipulation supports the user’s mental model of the task and is much more natural than opening a window, selecting a book title, selecting a patron name, and clicking a Check Out button. (For more information on the concept of a mental model, see “Reflect the User’s Mental Model.”)

User Control

Allow the user, not the computer, to initiate and control actions. Some applications attempt to assist the user by offering only those alternatives deemed good for the user or by protecting the user from having to make detailed decisions. Because this approach puts the computer, not the user, in control, it is best confined to parts of the user interface aimed at novice users. Provide the level of user control that is appropriate for your audience (see “Know Your Audience” for more information on ways to determine the audience for your application). For some suggestions on how to provide the appropriate level of detail in your user interface, see “Managing Complexity in Your Software.”

The key is to provide users with the capabilities they need while helping them avoid dangerous, irreversible actions. For example, in situations where the user might destroy data accidentally, you should always provide a warning, but allow the user to proceed if they choose.

Feedback and Communication

Feedback and communication encompass far more than merely displaying alerts when something goes wrong. Instead, it involves keeping users informed about what’s happening by providing appropriate feedback and enabling communication with your application.

When a user initiates an action, always provide an indication that your application has received the user’s input and is operating on it. Users want to know that a command is being carried out. If a command can’t be carried out, they want to know why it can’t and what can be done instead. When used sparingly, animation is one of the best ways to show a user that a requested action is being carried out. For example, when a user clicks an icon in the Dock, the icon bounces to let the user know that the application is in the process of opening.

Often, you can use animation to make clear the relationships between objects and the consequences of actions. Mac OS X uses animation to subtly but clearly communicate with the user in many different ways, a few of which are listed here:

• When a user minimizes a window, it doesn’t just disappear. Instead, it smoothly slips into the Dock, clearly telling the user where to find it again.

• To communicate the relationship between a sheet and a window, the sheet unfurls from the window’s title bar.

• To emphasize the relationship between a drawer and a window, the drawer slides out from beneath the window, displaying shadowing that makes it look like a desk drawer.

You should consider using subtle animation effects such as these to enhance feedback in your user interface.

For potentially lengthy operations, use a progress indicator to provide useful information about how long the operation will take. Users don’t need to know precisely how many seconds an operation will take, but an estimate is helpful. For example, Mac OS X uses statements such as “about a minute remains” to indicate an approximate time frame. It can also be helpful to communicate the total number of steps needed to complete a task—for example, you might include text that says “Copying 30 of 850 files.”

Provide direct, simple feedback that people can understand. For example, error messages should spell out exactly what situation caused the error (“There’s not enough space on that disk to save the document”) and possible actions the user can take to rectify it (“Try saving the document in another location”). For more information on how to compose useful alert messages, see “Writing Good Alert Messages.”

If your application consists of a foreground process that displays a user interface and a background process that performs some or all of the application’s main tasks, take special care to conduct all communication with the user through the user interface of the foreground process. In particular, a background process should never display a dialog or window in which the user is required to change settings or supply information. If a background process must communicate with the user, it should start or bring forward the foreground application. This is important because the user may not know (or remember) that a background process is running and receiving communication from it would be confusing.

For example, consider a backup application consisting of a foreground process that displays a user interface and a background process that performs the scheduled backups. The user starts the application, sets the backup frequency and provides the data and backup locations, and quits the application, secure in the knowledge that backups will proceed as scheduled. If, at some time in the future, the backup disk becomes full, the background process must tell the user immediately; otherwise, the user may lose data. To do this, the background process should start the application and cause its Dock icon to bounce. Drawing the user’s attention to a familiar application, instead of displaying an alert from an invisible process, prepares the user to receive the information and take appropriate action.

Consistency

Consistency in the interface allows users to transfer their knowledge and skills from one application to another. Use the standard elements of the Aqua interface to ensure consistency within your application and to benefit from consistency across applications. Ask yourself the following questions when thinking about consistency in your product:

• Is it consistent with Mac OS X standards? For example, does the application use the reserved and recommended keyboard equivalents (see “Keyboard Shortcuts Quick Reference”) for their correct purposes? Is it Aqua-compliant? Does it use the solutions to standard tasks Mac OS X provides? (For more information on these solutions, see “Using Mac OS X Technologies.”)

• Is it consistent within itself? Does it use consistent terminology for labels and features? Do icons mean the same thing every time they are used? Are concepts presented in similar ways across all modules? Are similar controls and other user interface elements located in similar places in windows and dialogs?

• Is it consistent with earlier versions of the product? Have the terms and meanings remained the same between releases? Are the fundamental concepts essentially unchanged?

• Is it consistent with people’s expectations? Does it meet the needs of the user without extraneous features? Does it conform to the user’s mental model? (For more information on this concept, see “Reflect the User’s Mental Model.”)

Meeting everyone’s expectations is the most difficult kind of consistency to achieve, especially if your product is likely to be used by an audience with a wide range of expertise. You can address this problem by carefully weighing the consistency issues in the context of your target audience and their needs. See “Know Your Audience” for more information on how to define your audience.

WYSIWYG (What You See Is What You Get)

In applications in which users can format data for printing, publish to the web, or write to film, DVD, or other formats, make sure there are no significant differences between what users see onscreen and what they receive in the final output. When the user makes changes to a document, display the results immediately; the user shouldn’t have to wait for the final output or make mental calculations about how the document will look later. Use a preview function if necessary.

People should be able to find all the available features in your application. Don’t hide features by failing to make commands available in a menu. Menus present lists of commands so that people can see their choices rather than try to remember command names. Avoid providing access to features only in toolbars or contextual menus. Because toolbars and contextual menus may be hidden, the commands they contain should always be available in menu bar menus as well.

Forgiveness

Encourage people to explore your application by building in forgiveness—that is, making most actions easily reversible. People need to feel that they can try things without damaging the system or jeopardizing their data. Create safety nets, such as the Undo and Revert to Saved commands, so that people will feel comfortable learning and using your product.

Warn users when they initiate a task that will cause irreversible loss of data. If alerts appear frequently, however, it may mean that the product has some design flaws. When options are presented clearly and feedback is timely, using an application should be relatively error-free.

Anticipate common problems and alert users to potential side effects. Provide extensive feedback and communication at every stage so users feel that they have enough information to make the right choices. For an overview of different types of feedback you can provide, see “Feedback and Communication.”

Perceived Stability

The Aqua interface is designed to provide an understandable, familiar, and predictable environment. To give users a visual sense of stability, the interface defines many standard graphical elements, such as the menu bar, window controls, and so on. These standard elements provide users with a familiar environment in which they know how things behave and what to do with them.

To give users a conceptual sense of stability, the interface provides a clear, finite set of objects and a set of actions to perform on those objects. For example, when a menu command doesn’t apply to a selected object or to the object in its current state, the command is dimmed rather than omitted.

To help convey the perception of stability, preserve user-modifiable settings such as window dimensions and locations. When a user sets up his or her onscreen environment to have a certain layout, the settings should stay that way until the user changes them.

Providing status and feedback also contributes to perceived stability by letting users know that the application is performing the specified task.

Aesthetic Integrity

Aesthetic integrity means that information is well organized and consistent with principles of good visual design. Your product should look pleasant on the screen, even when viewed for a long time.

Keep graphics simple, and use them only when they truly enhance usability. Don’t overload windows and dialogs with dozens of icons or buttons. Don’t use arbitrary symbols to represent concepts; they may confuse or distract users. The overall layout of your windows and design of user interface elements should reflect the user’s mental model of the task your application performs. See “Reflect the User’s Mental Model” for more information on this concept.

When implementing your user interface, there are many things you can do to ensure high quality. For example:

• All icons should be rendered at the highest quality (see “Icons” for extensive guidelines for icon design).

• All text should be anti-aliased, which is automatic when you use the standard system fonts (see “Fonts” for more information).

• The font size and type should be consistent within a window (see “Text” for more information on the font sizes and styles available to you).

• The control size should be consistent within a window—for example, don’t mix small and standard controls (see “Controls” for more information on the controls Mac OS X supplies).

Match a graphic element with a user’s likely expectations of its behavior. Don’t change the meaning or behavior of standard items. For example:

• Always use checkboxes for multiple choices, not for mutually exclusive choices

• Use push buttons for immediate commands such as “Open”

• Avoid using push buttons to display pop-up menus or serve as tabs

• Avoid using bevel buttons as tabs

Modelessness

As much as possible, allow users to do whatever they want at all times. Avoid using modes that lock them into one operation and prevent them from working on anything else until that operation is completed.

Mac OS X supports enhanced modelessness with drawers and sheets. Drawers provide additional functionality while allowing continued access to the parent window. Sheets are modal dialogs attached to a parent window, replacing the use of application-modal dialogs. For more information about drawers, see “Drawers.” For more information about sheets, see “Sheets (Document-Modal Dialogs).”

Most acceptable uses of modes fall into one of the following categories:

• Short-term modes in which the user must constantly do something to maintain the mode. Examples are holding down the mouse button to scroll text or holding down the Shift key to extend a text selection.

• Alerts, in which the user must rectify an unusual situation before proceeding. Keep these to a minimum.

• Installers and Assistants whose sole purpose is to guide users through important tasks.

Other modes are acceptable if they do one of the following:

• They emulate a familiar real-life situation that is itself modal. For example, choosing different tools in a graphics application resembles the real-life choice of physical drawing tools.

• They change only the attributes of something, not its behavior. The boldface and underline modes of text entry are examples.

• They block most other normal operation of the system to emphasize the modality. An example is a dialog that makes all menu commands unavailable except Cut, Copy, and Paste.

If an application uses modes, there must be a clear visual indicator of the current mode, and it should be very easy for users to get into and out of the mode. For example, in many graphics applications, the pointer can look like a pencil, a cross, a paintbrush, or an eraser, depending on the function (the mode) the user selects. Segmented controls are also useful to indicate modes, as is done in iPhoto.

Managing Complexity in Your Software

The best approach to developing easy-to-use software is to keep the design as simple as possible. In other words, a simple design is a good design and the best tools are those that users are not even aware they are using. The more you can do to simplify the interface of your application for your users, the more likely it is that you will build a product that meets their needs and is enjoyable to use.

The more complex your application’s task, the more important it is to keep the user interface simple and focused. Be sure your design reflects the user’s mental model (see “Reflect the User’s Mental Model” for more information on this concept). In addition to creating a streamlined design, you can also manage complexity in the following ways:

• Progressive disclosure presents the most common choices to the user first and provides an option that allows the user to view additional information and choices. This technique makes it easy for novice users to understand your user interface while still giving power users the advanced features they want.

  You can implement progressive disclosure using disclosure triangles (see “Disclosure Triangles”) or disclosure buttons (see “Disclosure Buttons”), depending on the context.

• Inspector windows (and, to a lesser extent, Info windows) reduce the clutter of a user interface by placing additional information and settings in a separate window that can be hidden or shown by the user.

  For more information on the differences between Info windows and inspector windows and how to use inspector windows, see “Inspector Windows.”

• Preferences reduce the complexity of the user interface by giving users the ability to customize what they see on the display screen and, to some extent, how the application performs. By providing preferences, you allow both novice and expert users to mold your application to fit their needs.

  For more information on how to craft a useful set of preferences, see “Preferences.”

---