如何在 Flutter 的 Shared Preferences 中获取和设置两种不同数据类型的值？

Question

I have a map named cardDetails whose details are fetched from firestore. Once the data is fetched, I want to used shared_preferences to save the details locally by converting the map to string and set the key to be true. When the user opens the app again then, I check if the value has been read by using getBool(key) and then fetch the data from the string using getString(key) .

The problem is that getString(key) is returning bool value instead of a string owing to which I cannot decode the string to map again

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following assertion was thrown building:
type 'bool' is not a subtype of type 'String'

Declaration and initialisation

Map<String, dynamic> cardDetails;
  var keys;
  bool _initialSharedValue;

  @override
  void initState() {
    super.initState();

    SharedPreferences.getInstance().then((SharedPreferences sp) {
      sharedPreferences = sp;
      _initialSharedValue = sharedPreferences.getBool("homepage");
      // will be null if never previously saved
      if (_initialSharedValue == null) {
        _initialSharedValue = false;
        persist(_initialSharedValue); // set an initial value
      }
      setState(() {});
    });
  }

  Future<void> persist(bool value) async {
    setState(() {
      _initialSharedValue = value;
    });
    await sharedPreferences?.setBool("homepage", value);
  }

  storeCardInSharedPreference(Map cardDetails) async {
    await sharedPreferences?.setString("homepage", json.encode(cardDetails));
  }

In UI for building cards

    if (sharedPreferences.getBool("homepage"))
      ListView.builder(
        itemBuilder: (context, index) {
          cardDetails = json.decode(sharedPreferences.getString("homepage"));
          return HomepageCards(
            user: widget.user,
            cardDetails: cardDetails[cardDetails.keys.toList()[index]],
          );
        },
        itemCount: 3,
        scrollDirection: Axis.vertical,
        controller: _controller,
        shrinkWrap: true,
      )
    else
      StreamBuilder<DocumentSnapshot>(
        stream: Firestore()
            .collection('homepage')
            .document(widget.user.uid)
            .collection('h')
            .document('28032020')
            .snapshots(),
        builder: (context, snapshot) {
          if (snapshot.data != null) {
            cardDetails = {};
            snapshot.data.data.forEach((index, individualDetail) {
              cardDetails[index] = individualDetail;
            });
            storeCardInSharedPreference(cardDetails);
            sharedPreferences.setBool("homepage", true);
            keys = snapshot.data.data.keys;
          } else {
            // TODO: Convert it to Shimmer with card skeletal layout
            CircularProgressIndicator();
          }

          return ListView.builder(
            itemBuilder: (context, index) {
              return HomepageCards(
                user: widget.user,
                cardDetails:
                    cardDetails[cardDetails.keys.toList()[index]],
              );
            },
            itemCount: keys.length,
            scrollDirection: Axis.vertical,
            controller: _controller,
            shrinkWrap: true,
          );
        },
      )

Answer 1

I solved the issue by using ? in the if condition to make it a null-aware method invocation.

if (sharedPreferences?.getBool("homepage"))