feat: 新增隱私優先的社交動態分享

lib/core/location_share.dart

@@ -1,3 +1,4 @@
+import 'package:characters/characters.dart';
 import 'package:geolocator/geolocator.dart';
 
 /// Controls how much coordinate detail is included in a shared map link.
@@ -16,6 +17,36 @@ enum LocationSharePrecision {
   };
 }
 
+/// A short, transit-oriented status included in a social update.
+enum SocialShareActivity {
+  waiting,
+  riding,
+  arriving,
+  meetUp;
+
+  String get label => switch (this) {
+    waiting => '正在等車',
+    riding => '正在搭車',
+    arriving => '即將抵達',
+    meetUp => '想約人同行',
+  };
+}
+
+/// How long the sender considers an update relevant.
+///
+/// This is deliberately described as a viewing deadline, rather than a hard
+/// expiry: content copied to another app cannot be remotely revoked.
+enum SocialShareDuration {
+  fifteenMinutes(Duration(minutes: 15), '15 分鐘'),
+  oneHour(Duration(hours: 1), '1 小時'),
+  oneDay(Duration(days: 1), '24 小時');
+
+  const SocialShareDuration(this.duration, this.label);
+
+  final Duration duration;
+  final String label;
+}
+
 /// Creates a one-time map link for the system share sheet.
 ///
 /// The app deliberately does not persist or upload the coordinates. The
@@ -54,3 +85,60 @@ class LocationShareMessage {
         '這是一次性分享，不會持續追蹤。';
   }
 }
+
+/// Builds a privacy-conscious social update for the system share sheet.
+class SocialShareMessage {
+  const SocialShareMessage._();
+
+  static const int noteMaxLength = 120;
+
+  static String normalizeNote(String note) {
+    final normalized = note.trim().replaceAll(RegExp(r'\s+'), ' ');
+    if (normalized.characters.length <= noteMaxLength) {
+      return normalized;
+    }
+    return normalized.characters.take(noteMaxLength).toString().trimRight();
+  }
+
+  static String compose({
+    required String displayName,
+    required SocialShareActivity activity,
+    required SocialShareDuration duration,
+    Position? position,
+    LocationSharePrecision precision = LocationSharePrecision.approximate,
+    String note = '',
+    DateTime? createdAt,
+  }) {
+    final sender = displayName.trim();
+    final created = (createdAt ?? DateTime.now()).toLocal();
+    final viewUntil = created.add(duration.duration);
+    final normalizedNote = normalizeNote(note);
+    final lines = <String>[
+      '${sender.isEmpty ? '我的' : '$sender 的'}近況｜${activity.label}',
+      if (normalizedNote.isNotEmpty) normalizedNote,
+      if (position != null) ...[
+        '',
+        '位置（${precision.label}）',
+        LocationShareMessage.mapUrl(
+          latitude: position.latitude,
+          longitude: position.longitude,
+          precision: precision,
+        ),
+        precision == LocationSharePrecision.exact
+            ? '含有精確座標，請只分享給信任的人。'
+            : '位置已簡化為約 100 公尺範圍。',
+      ],
+      '',
+      '分享時間：${_formatDateTime(created)}',
+      '建議查看至：${_formatDateTime(viewUntil)}（${duration.label}）',
+      '這是一次性分享；複製或轉傳到其他 App 後無法遠端撤回。',
+    ];
+    return lines.join('\n');
+  }
+
+  static String _formatDateTime(DateTime value) {
+    String twoDigits(int number) => number.toString().padLeft(2, '0');
+    return '${value.year}/${twoDigits(value.month)}/${twoDigits(value.day)} '
+        '${twoDigits(value.hour)}:${twoDigits(value.minute)}';
+  }
+}