TableCalendar in Flutter Web: O(1) Event Lookup + share_plus BuildContext Fix

What We Shipped

Two features in one session for 自分株式会社:

1. Calendar View — Notion Database parity using table_calendar: ^3.1.2 + calendar-events Edge Function
2. Guitar Studio lint fixes — use_build_context_synchronously, undefined_identifier, and unnecessary_import resolved

Calendar View: O(1) Event Lookup with Date-Key HashMap

The Date Key Pattern

TableCalendar's eventLoader receives a DateTime day. The simplest fast lookup: convert to a YYYY-MM-DD string key and store events in a Map:

final Map<String, List<Map<String, dynamic>>> _eventsByDate = {};

static String _dateKey(DateTime d) =>
    '${d.year}-${d.month.toString().padLeft(2, '0')}-${d.day.toString().padLeft(2, '0')}';

Pass it to TableCalendar:

TableCalendar(
  eventLoader: (day) => _eventsByDate[_dateKey(day)] ?? [],
  ...
)

O(1) lookup per day — no iteration over the full event list.

Fetch Per Month, Cache Locally

Only fetch when the user swipes to a new month. Already-fetched months stay in _eventsByDate:

onPageChanged: (focusedDay) {
  _focusedDay = focusedDay;
  _fetchMonth(focusedDay);
},

On back-navigation, the cached data renders instantly without a network round-trip.

GET Request to Edge Function

supabase_flutter's functions.invoke supports GET + query parameters:

final res = await _supabase.functions.invoke(
  'calendar-events',
  method: HttpMethod.get,
  queryParameters: {
    'view': 'month',
    'year': month.year.toString(),
    'month': month.month.toString(),
  },
);

No body needed for GET — pass everything as query params.

No-Package Color Picker

5-color circle picker without any third-party package:

Wrap(
  spacing: 8,
  children: colors.map((c) => GestureDetector(
    onTap: () => setDialogState(() => selectedColor = c),
    child: Container(
      width: 28, height: 28,
      decoration: BoxDecoration(
        color: Color(int.parse('FF${c.replaceFirst('#', '')}', radix: 16)),
        shape: BoxShape.circle,
        border: selectedColor == c
            ? Border.all(color: Colors.black87, width: 2.5)
            : null,
      ),
    ),
  )).toList(),
),

'FF${hexString}' with radix: 16 converts a CSS hex color to a Flutter Color constant.

Guitar Studio: Three Lint Fixes

Fix 1: use_build_context_synchronously

Flutter 3.33+ enforces this strictly. Using context after any await without a mounted check is an error:

// BEFORE — lint error
await someFuture();
ScaffoldMessenger.of(context).showSnackBar(...);  // context used after async gap

// AFTER — correct
await someFuture();
if (!mounted) return;
ScaffoldMessenger.of(context).showSnackBar(...);

Add if (!mounted) return immediately after every await that precedes a context use.

Fix 2: undefined_identifier + prefer_final_fields

Referencing a field before declaring it, or declaring a mutable field that's never mutated:

// BEFORE — undefined
final level = _audioLevel;  // _audioLevel not declared

// AFTER — declare with final
final double _audioLevel = 0.0;

prefer_final_fields requires final on any field that's set once and never reassigned.

Fix 3: unnecessary_import

share_plus re-exports XFile from cross_file. Importing cross_file directly is redundant:

// BEFORE — redundant import
import 'package:cross_file/cross_file.dart';
import 'package:share_plus/share_plus.dart';

// AFTER — one import is enough
import 'package:share_plus/share_plus.dart';

flutter analyze catches this as unnecessary_import. The re-export is stable across share_plus versions.

Why Notion Calendar Parity Was Cheap to Implement

The calendar-events Edge Function already existed. table_calendar was already in pubspec.yaml. The Flutter page was mostly wiring:

1. Add onPageChanged → call _fetchMonth
2. Add eventLoader → return from HashMap
3. Style markers with CalendarBuilders

Total new Flutter code: ~150 lines. The Edge Function: 0 changes needed.

Pattern: Before building something new, check what infrastructure already exists. Features that look complex on the surface are often just UI over existing data.

Try it: 自分株式会社

buildinpublic #Flutter #Supabase #Dart #webdev