Self Hiding Child Cannot Hide Its Slot

A self-hiding child cannot hide its slot

Optional controls commonly hide themselves: no platform support, no permission, nothing to connect to — render nothing. That is correct and it is not enough. A caller that gave the control a container, a weight or a divider still draws all three around an empty box.

The component's own contract has to say so, in the same file that declares it:

// adapted
/**
 * Whether [OptionalControl] would render anything. Layouts that give it a container of its own
 * must hide the container too when this is false — the control hides itself, but it cannot hide
 * a wrapper it does not own.
 */
expect fun isOptionalControlAvailable(): Boolean

Traps

Publish the predicate next to the component, not at the call sites. Two call sites re-deriving "is this available" from platform checks will disagree eventually — and the one that disagrees is the one drawing the empty box. One exported predicate, backed by the same platform code the component uses, is the whole fix.

The gate is needed exactly when the caller adds a container, and not otherwise. In this tree the same control appears twice: inside a segmented group it is wrapped in if (available), and in a plain row it is called bare, because there the control's own emptiness is invisible. Both are right. Auditing for "every call site must be gated" would produce a wrong finding.

Removing a slot from a segmented group changes its neighbours' shapes. End caps, corner radii and dividers are assigned by position. Drop a middle slot and the arithmetic still works; drop the first or last and the cap has to move, or the group ends up with two square outer corners. Derive the shapes from the built list rather than hardcoding them per index.

A slot with no click of its own needs a real branch, not a no-op lambda. The clickable and non-clickable forms of a container are different composables with different semantics. Passing onClick = {} keeps the ripple, keeps the accessibility click action and keeps consuming the tap — so the control inside, which owns its own click, never receives it. Branch on onClick != null and emit the plain container in the else.

Declare the predicate and the component as one platform pair, in one file. A target that stubs the control to draw nothing must stub the predicate to false in the same edit; split across files they drift, and the drift is invisible until someone builds that target. Declaring both together also puts the contract — "callers with a container must gate it" — where both implementers read it.

Keep the predicate cheap and pure. It is called during composition, on every recomposition, from inside a layout loop. A constant-returning platform function is fine; anything doing I/O, reading a store or touching a service must be hoisted into state and passed down, or the group stutters.

Decide whether the slot disappears or reserves space, and say which. In a weighted row a removed slot redistributes width to its siblings, which visibly resizes every remaining control. That is usually right for a permanently unavailable feature and wrong for a transiently unavailable one, where the group would twitch each time. Reserve for transient, remove for permanent.

"It renders nothing, so it costs nothing" is false for anything with a weight or a background. An empty container with weight(1f) still claims its share of the row and still paints its container colour; that is precisely the artefact this rule exists to prevent.

Verifying it

1. The availability predicate exists and is declared beside the component it describes:

   grep -rn --include='*.kt' --exclude-dir=build "expect fun is[A-Za-z]*Available(): Boolean" .
   

2. Every call site of that component, each shown with the gate above it if there is one. Read the ungated ones and confirm the caller adds no container:

   F=$(grep -rl --include='*.kt' --exclude-dir=build "expect fun is[A-Za-z]*Available(): Boolean" .)
   C=$(grep -oE "^expect fun [A-Za-z]+" "$F" | awk '{print $3}' | head -1)
   grep -rn --include='*.kt' --exclude-dir=build -B8 "^ *$C(" . | grep -E "$C\(|Available\(\)"
   

3. The slot really branches instead of taking a no-op click — expect two container emissions in the helper, one with a click parameter and one without:

   grep -rn --include='*.kt' --exclude-dir=build -A6 "onClick: (() -> Unit)?" .
   

4. On a device or target where the feature is unavailable: open the screen and count the cells in the group. An extra one, a wider gap, or an outer corner that is square instead of rounded all mean the slot outlived the control.