Level indicator

LevelIndicator is a readout for a capacity: a disk, a battery, a memory pool, a budget.

The reading is Value between Minimum and Maximum, as for every other range control. What this one adds is two lines drawn across that range — Warning and Critical — and a Level that says which side of them the reading is on.

Doc 49 § 7.1 ranks it sixth of the controls this set was missing, and the reason is that ProgressBar was the only readout in it. The two draw nearly the same picture and mean opposite things:

⚠ The accessible role is the substance of that table, not the colour. A screen reader told "progress bar, eighty-seven per cent" about a disk has been told a job is nearly done. LevelIndicator reports AccessibleRole.Meter — ARIA's meter, which exists for exactly this distinction — and announces the reading rather than a fraction, because a meter's bounds are the capacity and "446" is the number the listener wanted, not "0.87".

⚠ With two thresholds, their order is the direction. A disk gets worse as it fills; a battery gets worse as it empties. Both are level indicators, and with both lines set the control tells them apart from the numbers alone:

Leave either threshold unset — they default to float.NaN — and nothing on that line ever fires.

⚠ One line has no order, so with one line you say which way is worse. Left to itself a single threshold reads upward, because a lone line on a capacity is a ceiling — and so a battery given only Critical="0.1" reads critical at a full charge. Direction is how it says otherwise:

Direction is Inferred unless it is set, which is the pair's order when there are two lines and upward when there is one, so an indicator that never mentions it behaves exactly as it always has. A stated direction is obeyed even against the pair. That is deliberate: a falling direction over a disk's 0.7/0.9 reads critical at every ordinary reading, which is loud, whereas a property that was quietly ignored whenever two lines were set would be the silent kind of wrong.

The level reaches the stylesheet as a class, so the theme owns the colours:

ControlTheme.vcss writes exactly those two rules. Warning and danger are not the same word: danger says something has gone wrong or is about to be destroyed; warning says a reading is heading somewhere and there is still time.

⚠ --warning is new to the control set's palette and not new to the tree. The editor has declared its own --warning for as long as it has had a console — #9a6200 dark, #d99a3c light — and the editor's declaration wins wherever the editor is running, because ControlTheme's palette sits in the base layer and EditorTheme is wholly in components. Layer beats load order, so this holds whichever sheet is installed first. What the control set adds is the default underneath: #a26507, for an application that installs only the control set. Write a warning-coloured rule against the token rather than against either literal.

Set Segments above zero to draw blocks instead of a bar — signal strength, a rating, a battery with cells:

⚠ The lit block count rounds rather than truncating. Four blocks at seven-tenths is three, not two. A truncating version shows its last block only at the very top of the range, so a meter resting anywhere near a boundary reads one block low for as long as it rests there. The gap between blocks is --segment-gap and is taken out of each block rather than added between them, so a segmented indicator and a continuous one of the same width end in the same place.

Set TickCount to draw a ruler across the bar — AppKit's numberOfTickMarks, counted the same way, so five ticks mark the quarters with the first and last at the ends:

⚠ The ticks are drawn over the fill, not under it, the opposite of a Slider. A slider's thumb stands where the value is; a level indicator has no thumb, so the marks are read against the fill and a fill that covered them would hide every tick the reading has passed. The last tick sits on the bar's last pixel column rather than the one after it, which would be outside the element and clipped.

Level is computed from the three numbers every time it is read rather than stored, so it cannot come to disagree with them — the failure a cached level has is that it is right until somebody moves a threshold and never touches the value again.

A disk-space row in a settings panel, a signal strength, and a battery — three readings, three directions, one control:

ControlTheme.vcss already stretches a level-indicator inside a key-value-value, alongside every other control that belongs in that half of a row.

• It is not focusable and takes no input. A readout is not operated; ProgressBar makes the same choice for the same reason. Bind it to a value and let something else change that value.
• It has no text. A number beside a bar is a LabeledContent or a KeyValueList row with this in it, and the words belong to the application — "446 GB of 512 GB" is not a sentence this assembly can write without knowing what is being measured.
• It does not warn about itself. Passing Critical adds a class and changes an announcement; it raises no event and shows no dialog. What to do about a full disk is the application's decision.

• Accessibility — the role tree these readings are announced through, and why a meter is not a progressbar.
• Key-value list — where a reading usually sits, beside the word for what it is.
• Labeled content — the same, for a form row rather than a table of facts.
• Gauge — the same reading, thresholds and levels, drawn as a dial.