main.dart

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:riverpod/riverpod.dart';

// 1つ目のStreamProvider
final authUserProvider =
    StreamProvider<User?>((ref) => FirebaseAuth.instance.userChanges());

// 2つ目のStreamProvider
// authUserProviderをwatchした値を使ってFirestoreから得られるStreamを返す
final userSnapshotProvider = StreamProvider((ref) async* {
  final uid =
      await ref.watch(authUserProvider.selectAsync((user) => user?.uid));
  if (uid == null) {
    // 値を返す時は`yield`を使う
    yield null;
    return;
  }
  // Streamを返すときは`yield*`を使う
  // (実際には`.withConverter`挟むのが良い)
  yield* FirebaseFirestore.instance.collection('users').doc(uid).snapshots();
});

// こういう時はFutureProviderで済む
// (元のコード例では不自然にStreamProviderになっていた)
final isSignedInProvider = FutureProvider<bool>((ref) async {
  final user = await ref.watch(authUserProvider.future);
  return user != null;
});

// 別解
final isSignedInProvider2 = FutureProvider<bool>(
  (ref) => ref.watch(authUserProvider.selectAsync((user) => user != null)),
);