Add Background Device Monitor

[fotiDim]

Add Background Device Monitor feature to the example app that demonstrates BLE scanning from a foreground service on Android.

[gemini-code-assist]

Summary of Changes

Hello @fotiDim, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the example application by integrating a background BLE device monitoring capability. It leverages Android's foreground services to continuously scan for user-selected Bluetooth devices, even when the app is not actively in use. The changes include the necessary Android manifest updates, a dedicated Dart service for background operations, and modifications to the UI to manage the monitored device list and service state, providing a more robust and feature-rich demonstration of BLE interaction.

Highlights

• Background BLE Monitoring: Introduced a new feature to the example application that enables background monitoring of BLE devices on Android using a foreground service.
• Foreground Service Implementation: Utilizes the flutter_foreground_task package to run a persistent background service that periodically scans for specified BLE devices.
• Persistent Storage for Monitored Devices: Implemented functionality to save and retrieve a list of monitored device IDs using SharedPreferences, ensuring persistence across app sessions.
• User Interface Enhancements: Added UI elements in the scanner screen to start/stop the background monitor and allowed users to long-press on scanned devices to add or remove them from the monitored list.

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[gemini-code-assist]

Code Review

This pull request introduces a background device monitoring feature, which is a great addition to the example app. The implementation uses a foreground service on Android to perform periodic BLE scans, which is the correct approach. The code is well-structured, with a clear separation of concerns between the background service logic, UI management, and data storage. I've made a couple of suggestions: one to improve code maintainability by removing a duplicated constant, and another to fix a potential crash in the error handling logic of the background task. Overall, this is a solid implementation of a complex feature.

[gemini-code-assist]

Using substring(0, 50) will throw a RangeError if the error message string is shorter than 50 characters, which could crash the background service. It's safer to use a method that handles shorter strings gracefully, like characters.take().

Suggested change

	notificationText: 'Scan error: ${e.toString().substring(0, 50)}',
	notificationText: 'Scan error: ${e.toString().characters.take(50)}',

[gemini-code-assist]

The key 'monitored_devices' is also defined in StorageService. To avoid duplicating this string literal and improve maintainability, you should define it as a public constant in StorageService and reuse it here.

First, make the key public in example/lib/data/storage_service.dart:

// line 24
static const String monitoredDevicesKey = 'monitored_devices';

Then, you can remove this local constant and use StorageService.monitoredDevicesKey on lines 29 and 52.

[gemini-code-assist]

This key is also used in BleMonitorTaskHandler. To avoid string duplication and improve maintainability, consider making this constant public so it can be imported and used in other parts of the app.

Suggested change

	static const String _monitoredDevicesKey = 'monitored_devices';
	static const String monitoredDevicesKey = 'monitored_devices';