使用 sqlite 和颤振检索数据

Question

我正在开发一个 Flutter 项目并使用 Sqflite 数据库。我已经设法将数据保存在 db 中，我正在尝试显示保存在 sqllite 中的数据，我正在尝试使用 listview builder 但它抛出错误The getter 'length' was called on null.。但我从数据库中获取数据没有问题

错误

The following NoSuchMethodError was thrown building FutureBuilder<List<CartModel>>(dirty, state: _FutureBuilderState<List<CartModel>>#31bfe):
The getter 'length' was called on null.
Receiver: null
Tried calling: length

从数据库中检索数据以查看

  Widget Coupon_List() {
    return FutureBuilder<List<CartModel>>(
      future: productFromDatabase(),
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          return new ListView.builder(
              itemCount: snapshot.data.length,
              itemBuilder: (context, index) {
                return new Column(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: <Widget>[
                      new Text(snapshot.data[index].boon_name,
                          style: new TextStyle(
                              fontWeight: FontWeight.bold, fontSize: 18.0)),
                      new Divider()
                    ]);
              });
        } else if (snapshot.data.length == 0) {
          return new Text("No Data found");
        }
        return new Container(
          alignment: AlignmentDirectional.center,
          child: new CircularProgressIndicator(),
        );
      },
    );
  }

获取要查看的数据库函数

Future<List<CartModel>> productFromDatabase() async {
    Future<List<CartModel>> product = dbHandler.getCartList();
    return product;
  }

用于检索数据的 DbHelper 类

  Future<List<CartModel>> getCartList() async {
    List<Map> list = await _db.rawQuery('SELECT * FROM Cart');
    List<CartModel> productList = new List();
    for (int i = 0; i < list.length; i++) {
      var data=list[i];
      productList.add(CartModel.fromJson(data));
    }
    print(productList.length);
    return productList;
  }

这是我从 db 获取的数据

[{id: 1, boon_id: 3, product_id: 15, boon_name: iPhone 11 Pro Max, product_name: Boon Polo Tshirt, product_price: ₹599.00, quantity: 0, product_size: M, product_color: Blck, product_image: https://www.boonways.com//images/gallery/1593989052.jpg, boon_image: https://www.boonways.com//images/coupon/coupon_15_20200706040643.jpg, percentage: 14.43}]

Answer 1

改变这个功能

Future<List<CartModel>> productFromDatabase() async {
    Future<List<CartModel>> product = dbHandler.getCartList();
    return product;
  }

到

Future<List<CartModel>> productFromDatabase() async {
    List<CartModel> product = await dbHandler.getCartList();
    return product;
  }

也改变这个

if (snapshot.hasData) {
          return new ListView.builder(
              itemCount: snapshot.data.length,
              itemBuilder: (context, index) {
                return new Column(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: <Widget>[
                      new Text(snapshot.data[index].boon_name,
                          style: new TextStyle(
                              fontWeight: FontWeight.bold, fontSize: 18.0)),
                      new Divider()
                    ]);
              });
        } else if (snapshot.data.length == 0) {
          return new Text("No Data found");
        }

进入

if (snapshot.hasData && snapshot.data.length > 0) {
          return new ListView.builder(
              itemCount: snapshot.data.length,
              itemBuilder: (context, index) {
                return new Column(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: <Widget>[
                      new Text(snapshot.data[index].boon_name,
                          style: new TextStyle(
                              fontWeight: FontWeight.bold, fontSize: 18.0)),
                      new Divider()
                    ]);
              });
        } else if (snapshot.hasData && snapshot.data.length == 0) {
          return new Text("No Data found");
        }

让我知道发生了什么。

Answer 2

乍一看，您似乎是在构建器中执行此操作（伪代码）：

if there is data then do
   return widget
else if the data-length is equal to zero then do
   return widget
end if-else

如果没有返回数据并且snapshot.data 为空（else if 部分），这可能会给您一个错误，这可以解释此消息：

在 null 上调用了 getter 'length'。

但我不确定错误发生在哪里。您可能应该更改该条件并进行验证。也许是这样的：

if (snapshot.hasData) {
    if(snapshot.data.length == 0) {
        return Text('No data');
    }
    return Text('Data is here');
} else if(snapshot.hasError){
    return Text('Something went wrong');
} else {
    return CircularProgressIndicator();
}