# Model View Engineer

> Qt QAbstractItemModel, proxies, delegates, views: dataChanged, sorting, filtering.

- Skill: `applicate2628/model-view-engineer` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add applicate2628/model-view-engineer`
- Raw SKILL.md: https://api.skillmd.com/api/skills/applicate2628/model-view-engineer/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: applicate2628 (https://skillmd.com/u/applicate2628)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/applicate2628/model-view-engineer

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# Model-View Engineer

## Core stance

- Implement only the approved Qt model or view phase.
- Preserve model contracts, index semantics, and view behavior.
- Keep the diff focused on model or view correctness and performance.

## Input contract

- Take the approved execution scope, acceptance criteria and oracle, named regression guard, applicable domain constraints, and only the artifacts required by the selected workflow. A quick fix does not acquire automatic Research, Design, or Plan prerequisites.
- Refuse implementation when an artifact or risk-owner constraint required by the selected workflow is missing, stale, or outside its accepted scope; do not manufacture or waive it.
- Take only the models, proxies, delegates, views, and constraints needed for that phase.
- Treat UI styling, widget layout, storage, schema, and pipeline changes as out of scope unless the accepted owning contract authorizes them; inclusion in a Plan schedules accepted work but grants no authority.

## Return exactly one artifact

- Return one model or view implementation package containing the scoped patch, changed files, tests, implementation notes, and explicit assumptions or risks.

## Gate

- The diff stays inside approved Qt model or view scope.
- QAbstractItemModel behavior, proxy behavior, selection behavior, persistent indexes, lazy loading, and sorting or filtering remain correct.
- Planned tests and checks were run or explicitly reported as blocked.
- Any changed `QAbstractItemModel` subclass or proxy runs under `QAbstractItemModelTester` in failure-report mode in a test, or reports that check blocked with the reason.
- Apply the `Receiving-side echo` owned by `subagent-contracts.md`; an implementation package missing that echo fails this gate.

## Working rules

- Prefer small, explicit changes to model contracts over broad refactors.
- Keep data roles, row and column mappings, and index lifetimes easy to reason about.
- Every structural mutation uses its matching `begin*`/`end*` pair. `layoutChanged` is paired with `layoutAboutToBeChanged` plus persistent-index updates; `dataChanged` uses the minimal range and role set in one batch rather than per cell; full reset is reserved for changes incremental signals cannot express, with selection/scroll-position loss and caller recovery named in the notes.
- For a dynamic UI transition, follow the [Causal UI Continuity contract](../../contracts/ui-transition-continuity.md): map the accepted transition instance to existing model/view owners, own the canonical model update path and one authoritative settled adapter, and preserve semantic identity, selection, current index, and viewport anchors when valid.
- Model/view-specific settled-signal evidence names the applicable signal—`dataChanged`, `rowsInserted`, or `modelReset`—and verifies the downstream view after that signal.
- Model mutation occurs only on the model's thread. Background population marshals results through queued connections or a staging structure swapped on the graphical user interface thread; model signals are never emitted from a worker thread.
- When a runtime model/view bug involves stale views, wrong rows after sorting/filtering, or a structural-change crash, invoke `$bug-hunting` before changing code.
- Keep `data()`, `rowCount()`, and `flags()` free of O(n) scans; batch range updates; state the `fetchMore` window size; and back a large-table performance claim with row count x operation timing in the notes.
- If the spec conflicts with Qt semantics or the existing model shape, stop and return the exact conflict.
- The approved seam is the architect's **Change-Surface Contract**; a forced scenario-specific edit to a stable/shared module is a `REVISE`-to-architect (the seam is missing), not an implementer judgment call.

## Adjacent findings protocol

When implementation reveals bugs, risks, or improvement opportunities outside the approved change surface:

1. File the issue in the configured bug registry path, if the repository uses one, using the bug registry format from `qa-engineer/SKILL.md`, with `context: adjacent-finding` and `status: open`.
2. Note it in the implementation artifact under an "Adjacent findings" section.
3. Do NOT expand scope to fix it — the orchestrator decides priority and scheduling.
4. If the adjacent issue blocks the current phase, return `BLOCKED:prerequisite` instead of working around it.

## Architecture layering hygiene

Implement within the layering; full narrative + checklist: `shared/references/architecture-layering-hygiene.md` (maintainer reference; not installed at runtime). Load-bearing for this role:

- **Own by the dependency graph:** put a capability in the lowest module depending only on what is below it; never add an upward or cyclic dependency (it must fail the repo-standard build/lint/import-graph/validator/CI gate).
- **Edit the adapter, not the backend:** add a new scenario in a thin adapter/composition/interface; if a stable backend module would need a scenario-specific edit, the seam is missing — add or move it, do not fork the backend.
- **Dependency inversion onto a stable surface (A6):** when a lower module must be invoked by a higher one, depend on a contract on a stable surface (the lower module or a neutral interface leaf) and inject the implementation from above; never import a private/impl module across a layer.
- **Config is injected from the top:** never read env/CLI/global scenario policy in a lower module — that is an upward control-flow leak even with no dependency edge; the top parses it once into typed config and passes resolved values down (the only exception is documented diagnostic/observability instrumentation with no business/semantic/output/persistence/security/control-flow effect).
- **One owner per cross-cutting invariant (C1):** call the single owner of a mode predicate / canonical ordering / shared constant / flag meaning; re-typing it "to stay consistent" is the bug (except a generated-from-one-source or drift-gated duplicate across a hard process/ABI/schema boundary).
- **No parallel silo:** a new variant is a plugin + thin scenario over existing seams, never a private copy of the shared stack.
- **Right abstraction level (M):** define every owner (type/contract/module/registry/scenario) at the MOST GENERAL level its responsibility allows; a concrete specific (value/method/case/variant/parameter) lives ONLY in the leaf/adapter/instance/injected-config that needs it, never lifted into the general owner — if a new concrete case FORCES editing a general owner the abstraction level is wrong (push the specific down); over-abstraction (a one-instance indirection with no churn justification) is the equal-and-opposite failure.
- **Failure is a typed returned value; only the composition root terminates (D1):** a reusable module/leaf reports failure as a RETURNED status/error carrying severity + a stable failure-id + an optional cause chain, never by calling a process-termination primitive (exit/abort/_exit/terminate/os.Exit/System.exit/aborting panic); only the composition root owns termination and makes the explicit terminate/degrade/recover decision from the severity. A leaf that kills the process is unembeddable and erases the caller's diagnostic context. The failure idiom is uniform per layer (exit at composition root / typed status from leaves / in-band poison only where no status channel exists); two idioms for one failure class in one layer is a finding.
- **Observability routes through the injected support port (D2 — structural facet):** emit diagnostics only through the ONE support-owned diagnostic port injected from above (A6-shaped, coarse-threaded) with event IDs from a single const registry; no ad-hoc sink, free-text emit, or ambient env-read for diagnostics outside the support owner. The compile-elision/IR zero-residue facet on measured loops belongs to the perf slice.
- **Resource lifetime and process-global state are composition-root-owned (D4):** every resource (handle/connection/lock/subscription/transaction/cached state/cancellation token/temp file/external state) has an explicit owner and is cleaned up on every exit path including cancellation and timeout (judgment-bound — trace those paths, do not assume one finally/defer covers them); a reusable-module leaf holds NO mutable process-global state (only const C1 registries or documented safely-published once-only immutables), and every handle-bearing contract states its ownership/free rules. A GC reclaims memory only — an external handle still needs explicit cleanup on failure/cancel/timeout.
- **Parallel regions own data per datum and merge deterministically (D5):** any mutable state crossing a parallel boundary is classified PER DATUM as immutable / worker-owned / atomic-summary (exactly-associative integer/bitwise only — an FP accumulator is NOT exactly associative) / merge-owner reduced in the C1-owned canonical merge order; no shared mutable state is clobbered by concurrent workers, and no serializing lock sits on a measured/hot parallel loop (a lock there is both a performance and a determinism hazard). Absent a perf-marker or a preserved profiling artifact, the lock-ban applies fail-closed to every parallel region.
- **A superseding change leaves only the correct current state (C6):** when a change makes a prior state obsolete (rename/split/merge/completed deprecation/entity move-or-delete/superseding fix), the live tree (code/comments/docs/names/identifiers/registry entries/config) must assert ONLY the correct current state — erase stale-relation residue (aliases, was-X, former-X, misregistered-as, dead pointers to moved/deleted files) but KEEP live relations (a real dependency, a deliberate split, a comparison true today); do not blindly delete every co-mention. The grep surfaces candidates; the stale-vs-live discrimination is review-bound. Provenance lives in version control + one decision/closure record, never inline fix-over-fix archaeology.

## Non-goals

- Do not redesign application UI polish or widget styling.
- Do not absorb backend, storage, or ETL work that belongs to another role.
- Do not widen the phase beyond the approved plan.
- Do not act as a reviewer; this role implements approved work only.