Recomposition

Per-node recomposition counts — the agent-facing performance signal for unnecessary recomposition. Source for heat maps and “this node recomposed N times for one click” reviews.

At a glance


Kind	`compose/recomposition`
Schema version	1
Modules	`:data-recomposition-core` (published) · `:data-recomposition-connector`
Render mode	instrumented
Cost	medium
Token usage	~1 000–1 500 tok per query (`compose/recomposition` ~3–5 KB). See token usage.
Transport	inline
Platforms	Android · Desktop · shared

What it answers

How many times did each composable recompose during this render?
Which subtree is the hot spot — a leaf re-running, or a parent re-running and dragging its children with it?
After a click / scroll / value change, what is the delta in recomposition counts compared to the steady-state snapshot?

Two modes:

Snapshot — a single instrumented render reports cumulative counts.
Click delta — drive the input, then diff before / after counts to attribute work to the interaction.

What it does NOT answer

It does not measure wall-clock time spent in composition — pair with render/trace.
It does not detect the cause (unstable lambda, missing remember, key churn) — it points at the location; reading the source is on the reviewer.
It runs the renderer in instrumented mode, so absolute counts are not directly comparable to a non-instrumented production build.

Use cases

Catch a Modifier factory that captures a new lambda each composition and forces the whole row to recompose.
Verify a derivedStateOf / remember change actually trimmed the recomposition count it was supposed to.
Build a heat-map overlay for a “performance review” PR.

Payload shape

RecompositionPayload, RecompositionNode in :data-recomposition-core.

// compose/recomposition
{
  "mode": "snapshot",         // or "clickDelta"
  "frameStreamId": "f-7a1c",  // when click-delta
  "nodes": [
    { "nodeId": 12, "name": "Row", "count": 3,
      "boundsInScreen": "0,80,1080,160" },
    { "nodeId": 14, "name": "Text", "count": 12,
      "boundsInScreen": "16,96,1064,144" }
  ]
}

Enabling

Producer requires instrumented render mode and runs when its extension is publicly enabled. From an MCP client:

{ "method": "tools/call", "params": { "name": "subscribe_preview_data",
  "arguments": { "uri": "compose-preview://<id>/_module/com.example.Foo",
                 "kind": "compose/recomposition" } } }

For PR-review style “did this PR add unnecessary recomposition?” guidance, see compose-preview-review/design/AGENT_AUDITS.md.

Companion products

Layout inspector — layout/inspector to map nodeId to source.
Render trace — render/trace, render/composeAiTrace for the wall-clock side of the same render.