C H A P T E R 1 5
Controls
Asynchronous Progress Indicator Implementation
Asynchronous progress indicators are available in Interface Builder. In the Attributes pane of the inspector, select Spinning for the style and be sure the Indeterminate checkbox is selected. To create an asynchronous progress indicator using Application Kit programming interfaces, use the NSProgressIndicator class with style NSProgressIndicatorSpinningStyle.
Level Indicators
A level indicator provides graphical information about the level or amount of something. Level indicators can be configured to display different colors to warn users when values are reaching critical levels.
There are three styles of level indicator:
■Capacity
■Rating
■Relevancy
Capacity Indicators
A capacityindicator,asitsnamesuggests,providesgraphicalinformationaboutthecurrentstateofsomething that has a finite capacity, such as storage space or battery charge. There are two styles of capacity indicator, continuous and discrete. A continuous capacity indicator is a translucent track that is filled with a colored bar that indicates the current value. For example, Figure 15-61 shows a continuous capacity indicator that represents how much space has been used (and how much space is left) in the user’s mail account.
Figure 15-61 A continuous capacity indicator shows a fine-grained representation of current capacity
A continuous capacity indicator
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A discrete capacity indicator is a row of separate, rectangular segments equal in number to the maximum value set for the control. Figure 15-62 shows a discrete capacity indicator that represents the battery charge in a Bluetooth mouse.
Figure 15-62 A discrete capacity indicator shows a medium-grained representation of current capacity
A discrete capacity indicator
Capacity Indicator Usage
Use a capacity indicator to provide information about the level or amount of something that has well defined minimum and maximum values. You can configure a capacity indicator to display a different color fill when the current value enters a warning or critical range that you define (for more on this, see “Capacity Indicator Contents and Labeling”). Because capacity indicators provide a clear, easily understood picture of a current state, they’re especially useful in dialogs and preferences windows that users tend to view briefly.
Note that the discrete capacity indicator displays the current value rounded to the nearest integer, and the segments are stretched or shrunk to a uniform width to fit the specified length of the control. This means that the segments in the discrete capacity indicator are either completely filled or empty, never partially filled. This makes a discrete capacity indicator better for showing coarser-grained values than a continuous capacity indicator.
Capacity Indicator Contents and Labeling
The default color of the fill in both capacity indicator styles is green. If you define a value for a warning level, the fill color changes to yellow when it reaches that value. If you define a value for a critical level, the fill color changes to red when it reaches that value.
You can specify which end of the indicator is critical (red) by setting appropriate values. If you define a critical value that is greater than the warning value, the fill is green at values less than the warning level, yellow between the warning and critical levels, and red above the critical level. This orientation is useful if you need to display a warning when a capacity is approaching the maximum value, such as the limit of storage space.
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Controls
If you define a critical value that is less than the warning value, the fill is red below the critical value, yellow between the critical and warning values, and green above the warning value (up to the maximum). This orientation is useful if you need to warn the user when a capacity is approaching the minimum value, such as the end of battery charge.
For example, Figure 15-63 shows different states of a continuous capacity indicator. Below it, Figure 15-64 shows different states of a discrete capacity indicator.
Figure 15-63 A continuous capacity indicator displaying values in three different ranges
Level in standard range
Level in warning range
Level in critical range
Figure 15-64 A discrete capacity indicator displaying values in three different ranges
Level in standard range
Level in warning range
Level in critical range
Both continuous and discrete capacity indicators allow the display of tick marks above or below the indicator control to give context to the level shown by the fill. However, only the continuous capacity indicator should display the tick marks because the number and width of the segments in the discrete capacity indicator provide similar context, making tick marks redundant. If you find that you need to display very small segments in a discrete capacity indicator to appropriately represent the scale of values, you might want to use a continuous capacity indicator instead.
You should label at least the first and last tick marks to define the scale of the control and provide context for the user.
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Capacity Indicator Specifications
Control sizes: Both styles of capacity indicators are available in regular size only (the bar or segment has a height of 16 pixels), but you determine the horizontal width of the control. Note that as you stretch a discrete levelindicator,thenumberofsegmentsremainsconstant,butthewidthofeachsegmentstretchesaccordingly.
Label spacing and fonts: If you want to provide labels for tick marks in a continuous capacity indicator, use the label font (see “Fonts” (page 125) for more information about this font). These labels should be placed 3 pixels below the lower edge of the tick marks.
Capacity Indicator Implementation
Continuous and discrete capacity indicators are available in Interface Builder. You can change the style from discrete to continuous in the Attributes pane of the inspector. To create one using Application Kit programming interfaces, use the NSLevelIndicator class with style NSDiscreteCapacityLevelIndicatorStyle or NSContinuousCapacityLevelIndicatorStyle.
Rating Indicators
A rating indicator displays a number of stars that corresponds to the rating of something. For example, Figure 15-65 shows the rating a user assigned an item in iTunes.
Figure 15-65 A rating indicator shows the user-assigned rating for an item
A rating indicator 
Figure 15-66 shows rating indicators displaying different values.
Figure 15-66 Rating indicators showing different ratings
Rating Indicator Usage
Use a rating indicator to provide a graphic representation of the rating of an object. Because a rating indicator conveys the ranking of one item relative to all other items in a category, such as favorite images or most-visited webpages, it’s most useful in a list or table view that contains many items.
You might want to allow the user to set ranking criteria (perhaps in a preferences window) that you use to determine what rating should be displayed for each item as it appears. Or, you can make a rating indicator editable so the user can increase or decrease the ranking of an item in a table or list.
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