Đọc và Ghi File

🎯 Mục tiêu: File I/O với dart:io.

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💡 Đọc file

import 'dart:io';

// Sync
String content = File('data.txt').readAsStringSync();

// Async
String content = await File('data.txt').readAsString();

// Lines
List<String> lines = await File('data.txt').readAsLines();

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📝 Ghi file

// Write (overwrite)
await File('output.txt').writeAsString('Hello Dart');

// Append
await File('log.txt').writeAsString('New line\n', mode: FileMode.append);

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🔧 Kiểm tra file

var file = File('test.txt');

await file.exists();     // File exists?
await file.length();     // Size in bytes
file.path;              // File path

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⚠️ Lưu ý

[!WARNING] dart:io không có trong Flutter web. Dùng packages như universal_io hoặc APIs khác cho cross-platform.

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✅ Checklist

• Import dart:io
• Async methods cho non-blocking
• Handle file not found exceptions

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