A level indicator graphically represents of a specific value within a range of numeric values. It’s similar to a slider (see Sliders) in purpose, but more visual and doesn’t contain a distinct control for selecting a value—clicking and dragging across the level indicator itself to select a value is supported, however. A level indicator can also include tick marks, making it easy for the user to pinpoint a specific value in the range. There are three different level indicator styles, each with a different appearance, for communicating capacity, rating, and relevance.
Use a level indicator in a view, not a window frame. Level indicators aren’t intended for use within portions of window frames, such as in toolbars and status bars.
For developer guidance, see NSLevelIndicator.
A capacity indicator illustrates the current level in relation to a finite capacity. Capacity indicators are often used when communicating factors like disk and battery usage. Mail, for example, uses a capacity indicator to show the percentage of data used in relation to an email account’s quota.
There are two styles of capacity indicator.
A rating indicator uses a series of horizontally arranged graphical symbols to communicate a ranking level. The default symbol is a star. In iTunes, for example, you can assign star ratings to individual songs and podcast episodes. These ratings may be referenced for the purposes of searching, sorting, and creating smart playlists.
A rating indicator doesn’t display partial symbols—its value is rounded in order to display complete symbols only. Within a rating indicator, symbols are always the same distance apart and don't expand or shrink to fit the control.
Facilitate efficient ranking adjustments. When presenting a list of ranked items, let the user adjust the rank of individual items inline without the need to navigate to a separate editing screen.
If you must replace the star with a custom symbol, ensure that its purpose is clear. The star is a very recognizable ranking symbol. Other symbols are not as easily associated with a rating scale.
For developer guidance, see the level indicator style rating.
A relevance indicator communicates relevancy using a series of vertical bars. It often appears in a list of search results for reference when sorting and comparing multiple items. iTunes uses relevance indicators to denote the popularity of songs and podcast episodes.
For developer guidance, see the level indicator style relevancy.