# Cancel Flutter timers and streams when the screen is disposed

Checking mounted suppresses callbacks but does not release the resources producing them. Cancel the subscription and timer when the State owner ends so navigation does not accumulate hidden work.

Save this requirement in Perch and check it after each agent edit. The rule gives your agent a named check to run, a finding to repair, and the same command to recheck.

## Implement the Flutter boundary

Save this illustrative integration module as `lib/order_feed.dart`.

```dart
import 'dart:async';
import 'package:flutter/widgets.dart';

class OrderFeed extends StatefulWidget {
  final Stream<String> updates;
  const OrderFeed({super.key, required this.updates});
  @override State<OrderFeed> createState() => OrderFeedState();
}
class OrderFeedState extends State<OrderFeed> {
  StreamSubscription<String>? subscription;
  Timer? timer;
  String latest = '';
  int generation = 0;
  void subscribe() {
    final current = ++generation;
    subscription = widget.updates.listen((value) {
      if (mounted && current == generation) setState(() { latest = value; });
    });
  }
  @override void initState() {
    super.initState();
    subscribe();
    timer = Timer.periodic(const Duration(seconds:30), (_) { if (mounted) setState(() {}); });
  }
  @override void didUpdateWidget(covariant OrderFeed oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.updates != widget.updates) {
      generation++; subscription?.cancel(); latest = ''; subscribe();
    }
  }
  @override void dispose() {
    generation++;
    subscription?.cancel();
    timer?.cancel();
    super.dispose();
  }
  @override Widget build(BuildContext context) => Text(latest);
}
```

The order, inventory, or resource-lifetime scenario is adapted from Perch’s public [order-service source at commit 4af67af](https://github.com/lakeday-org/perch/blob/4af67af46d76ac99398d61c7ad0a45441760d4dc/test/fixtures/order-service/storage.py). This Flutter implementation is an illustrative adaptation.

## Integrate with your application

This widget owns the subscription and timer, but not the shared source stream. didUpdateWidget cancels and invalidates the old subscription before subscribing to the replacement stream. StreamSubscription.cancel returns a Future; dispose cannot await it, so adapters with asynchronous cleanup should expose and test that completion elsewhere. Session changes must close or reauthorize the underlying feed.



## Exercise the boundary and failure paths

| Scenario | Expected outcome |
| --- | --- |
| Mounted feed | Current updates shown |
| Screen removed | Timer and subscription canceled |
| Late buffered callback | mounted check prevents update |
| Stream dependency changes | Old subscription replaced in didUpdateWidget |

Add regression tests for these cases in your existing Flutter test project. Use separate connections or deliberately reordered completion where the case involves concurrency; a sequential happy-path test does not exercise that failure. Run the test command after creating the matching test file or suite:

```sh
flutter test test/order_feed_test.dart
```

The scenarios above specify tests to implement. A completed Perch source check below is separate from running these application integration tests.

## Introduce the specific agent regression

In a throwaway copy, replace this source span:

```dart
subscription?.cancel();
    timer?.cancel();
```

with:

```dart
// Remove the source span shown above.
```

Run the relevant negative test and the Perch command on that broken copy. Restore the corrected source before committing the application. The supplied mutation reproduces the exact regression for the rule check.

## Preserve the requirement as a Perch rule

Save this in `.perch/rules/perch-guide-flutter-owned-subscription-disposal.yaml` at the repository root, keeping your existing `perch.yaml`:

```yaml
- name: screen-disposes-owned-work
  where: "lib/order_feed.dart"
  ensure: >-
    dispose must cancel both the stream subscription and periodic timer before calling super.dispose.
```

Install and authenticate the CLI once, then check the current working file:

```sh
npm install -g @lakeday/perch
perch login
perch check lib/order_feed.dart --rules screen-disposes-owned-work --json
```

Use Node 22 or newer. The framework can use another language; Perch’s CLI remains a Node command. A gated finding exits 3. Exits 1 or 2 mean the check did not complete and must be reported as a blocker.

## Inspect the captured rule check

On 2026-10-14, Perch using jev-latest checked the exact fixed and broken source provided here. The capture records SHA-256 hashes of both sources and the requirement.

| Source control | Exit | Outcome |
| --- | --- | --- |
| Broken | 3 | Rule finding |
| Corrected | 0 | No rule finding |

The rule distinguished these controls in this run. [Raw Perch capture](/owned-subscription-disposal/capture.json) · [Exact sources and mutation](/owned-subscription-disposal/source.json).

## Install the rule pack and configure your agent

[Download this rule pack ZIP](/owned-subscription-disposal/rule-pack.zip) · [Download the YAML rules](/owned-subscription-disposal/rules.yaml) · [Agent task](/owned-subscription-disposal/agent-task.txt). The pack contains the rule, its example paths and the repair/recheck instructions. Review the `where` selector before using it in an existing project. Keep your root `perch.yaml`; install this rule as a separate file under `.perch/rules/`.

1. Install Perch using Node 22 or newer and sign in once:

```sh
npm install -g @lakeday/perch
perch login
```

2. Download the pack, inspect it, and extract it from your repository root. The extraction preserves any existing files:

```sh
curl -fL 'https://reporules.fyi/flutter/owned-subscription-disposal/rule-pack.zip' -o 'perch-guide-flutter-owned-subscription-disposal.zip'
unzip -n 'perch-guide-flutter-owned-subscription-disposal.zip'
```

The installed rule is `.perch/rules/perch-guide-flutter-owned-subscription-disposal.yaml`. Its selector matches `lib/order_feed.dart` in this example; adapt the selector to your application file before checking an existing project.

3. Configure the coding agent you use. Run one of these commands:

Claude Code:

```sh
perch setup claude-code
```

Codex:

```sh
perch setup codex
```

Cursor:

```sh
perch setup cursor
```

Pi:

```sh
perch setup pi
```

4. Ask the agent to edit the implementation and check the saved requirement:

> Use the installed Perch skill while editing lib/order_feed.dart. Preserve screen-disposes-owned-work. Run perch check lib/order_feed.dart --rules screen-disposes-owned-work --json after the edit. Read each finding and the affected code, fix the implementation, and run the identical check again after the fix. Keep the rule and regression tests intact. Run the application's relevant regression tests and report the commands and actual results. Exit 3 means a rule finding; exit 1 or 2 means the check did not complete. Report an unavailable or incomplete check as blocked. Do not claim the application tests ran if only the Perch check ran.

5. After repairing a finding, run the same check again and inspect the result:

```sh
perch check lib/order_feed.dart --rules screen-disposes-owned-work --json
```

Commit the downloaded rule and the installed agent configuration with your application. The rule file adds no guarantee of correctness; keep the regression tests from this guide.

With the base reference fetched, use `perch scan --since origin/main --filter type=lint` in CI.

## Framework references and verification scope

- [api.flutter.dev reference](https://api.flutter.dev/flutter/widgets/State/dispose.html)
- [api.dart.dev reference](https://api.dart.dev/dart-async/StreamSubscription/cancel.html)

The dated capture records the source check shown above. Implement the adapters, schema and regression tests described in this guide in your application. Unchanged requests reuse cached answers; fresh model judgments can vary. Model analysis sends relevant source to the configured service.