Flutter 2.8.1用于空值的空校验运算符

Question

我有一个flutter杂货店应用程序，在上次构建之前一直运行良好，但最近的构建给了我以下错误。

我正在一个稳定的频道上工作，已经尝试了cache repair和clean build错误仍然存在。

The following _CastError was thrown building StreamBuilder<List<Product>>(dirty, dependencies: [_InheritedProviderScope<ThemeModel>, MediaQuery], state: _StreamBuilderBaseState<List<Product>, AsyncSnapshot<List<Product>>>#ed27f):
Null check operator used on a null value
The relevant error-causing widget was: 
StreamBuilder<List<Product>> StreamBuilder:file:///Users/Osamah/Downloads/grocery_user_fixed-master/lib/ui/home/cart/cart.dart:68:20
When the exception was thrown, this was the stack: 
#0      Cards.cart (package:grocery/widgets/cards.dart:203:76)
#1      _CartState.build.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:grocery/ui/home/cart/cart.dart:140:48)
#2      new _GrowableList.generate (dart:core-patch/growable_array.dart:133:28)
#3      _CartState.build.<anonymous closure>.<anonymous closure> (package:grocery/ui/home/cart/cart.dart:138:40)
#4      StreamBuilder.build (package:flutter/src/widgets/async.dart:442:81)
#5      _StreamBuilderBaseState.build (package:flutter/src/widgets/async.dart:124:48)
#6      StatefulElement.build (package:flutter/src/widgets/framework.dart:4705:27)
#7      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4588:15)
#8      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4763:11)
#9      Element.rebuild (package:flutter/src/widgets/framework.dart:4311:5)
#10     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2578:33)
#11     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:882:21)
#12     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:363:5)
#13     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1145:15)
#14     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1082:9)
#15     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:996:5)
#19     _invoke (dart:ui/hooks.dart:150:10)
#20     PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:270:5)
#21     _drawFrame (dart:ui/hooks.dart:114:31)
(elided 3 frames from dart:async)

这是代码部分，据说可以在其中识别错误cart.art

import 'package:flutter/material.dart';
import 'package:flutter_svg/flutter_svg.dart';
import 'package:grocery/blocs/cart_bloc.dart';
import 'package:grocery/models/data_models/cart_item.dart';
import 'package:grocery/models/state_models/home_model.dart';
import 'package:grocery/models/data_models/product.dart';
import 'package:grocery/models/state_models/theme_model.dart';
import 'package:grocery/ui/home/cart/checkout/checkout.dart';
import 'package:grocery/ui/product_details/product_details.dart';
import 'package:grocery/widgets/buttons.dart';
import 'package:grocery/widgets/cards.dart';
import 'package:grocery/widgets/dialogs.dart';
import 'package:grocery/widgets/fade_in.dart';
import 'package:grocery/widgets/texts.dart';
import 'package:provider/provider.dart';
class Cart extends StatefulWidget {
final CartBloc bloc;
const Cart({required this.bloc});
static Widget create() {
return Consumer<CartBloc>(
builder: (context, bloc, _) {
return Cart(
bloc: bloc,
);
},
);
}
@override
_CartState createState() => _CartState();
}
class _CartState extends State<Cart> with TickerProviderStateMixin {
@override
Widget build(BuildContext context) {
final themeModel = Provider.of<ThemeModel>(context);
double width = MediaQuery.of(context).size.width;
return StreamBuilder<List<CartItem>>(
stream: widget.bloc.cartItems,
builder: (context, snapshot) {
if (snapshot.hasData) {
List<CartItem> cartItems = snapshot.data!;
if (cartItems.length == 0) {
return FadeIn(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
SvgPicture.asset(
'images/state_images/empty_cart.svg',
width: width * 0.5,
fit: BoxFit.cover,
),
Padding(
padding: EdgeInsets.only(top: 30),
child: Texts.headline3(
'Nothing found herenGo and enjoy shopping!',
themeModel.accentColor,
alignment: TextAlign.center),
)
]),
);
} else {
return StreamBuilder<List<Product>>(
stream: widget.bloc.getProducts(cartItems),
builder: (context, snapshot) {
if (snapshot.hasData) {
List<Product> products = snapshot.data!;
cartItems = cartItems.where((cartItem) {
if (products.where((product) {
if (cartItem.reference == product.reference) {
cartItem.product = product;
return true;
} else {
return false;
}
}).length ==
0) {
return false;
} else {
return true;
}
}).toList();
if (cartItems.length == 0) {
return FadeIn(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
SvgPicture.asset(
'images/state_images/empty_cart.svg',
width: width * 0.5,
fit: BoxFit.cover,
),
Padding(
padding: EdgeInsets.only(top: 30),
child: Texts.headline3(
'Nothing found herenGo and enjoy shopping!',
themeModel.accentColor,
alignment: TextAlign.center),
)
]),
);
} else {
return ListView(
padding: EdgeInsets.only(
left: 20, right: 20, top: 40, bottom: 80),
children: [
Padding(
padding: EdgeInsets.only(bottom: 20, top: 20),
child: Align(
alignment: Alignment.center,
child:
Texts.headline3("Cart", themeModel.textColor),
),
),
Padding(
padding: EdgeInsets.only(bottom: 20),
child: Align(
alignment: Alignment.center,
child: Texts.descriptiveItems(
"All orders over R600 recieve free delivery",
themeModel.textColor),
),
),
///List of cart items
AnimatedSize(
vsync: this,
duration: Duration(milliseconds: 300),
child: Column(
children:
List.generate(cartItems.length, (index) {
return FadeIn(
child: Cards.cart(context,
goToProduct: () {
ProductDetails.create(
context, products[index])
.then((value) {
if (value != null) {
final homeModel =
Provider.of<HomeModel>(context,
listen: false);
homeModel.goToPage(0);
showDialog(
context: context,
builder: (context) => Dialogs.success(
context,
message:
"Congratulations!nYour order is placed!"),
).then((value) {
widget.bloc.removeCart();
});
}
});
},
cartItem: cartItems[index],
updateQuantity:
widget.bloc.updateQuantity,
updateUnit: widget.bloc.updateUnit,
delete: () async {
await widget.bloc.removeFromCart(
cartItems[index].reference);
}),
);
}),
),
),
///Checkout button
FadeIn(
duration: Duration(milliseconds: 400),
child: Buttons.button(
color: themeModel.accentColor,
widget: Row(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(
Icons.arrow_forward,
color: Colors.white,
),
Padding(
padding: EdgeInsets.only(left: 10),
child: Texts.headline3(
"Checkout", Colors.white),
)
],
),
onPressed: () async {
final checkoutResult =
await Checkout.create(context);
if (checkoutResult ?? false) {
final homeModel = Provider.of<HomeModel>(
context,
listen: false);
homeModel.goToPage(0);
showDialog(
context: context,
builder: (context) => Dialogs.success(
context,
message:
"Congratulations!nYour order is placed!"),
).then((value) {
widget.bloc.removeCart();
});
}
}),
)
],
);
}
} else if (snapshot.hasError) {
return FadeIn(
child: Center(
child: SvgPicture.asset(
'images/state_images/error.svg',
width: width * 0.5,
fit: BoxFit.cover,
),
),
);
} else {
return Center(
child: CircularProgressIndicator(),
);
}
});
}
} else if (snapshot.hasError) {
return FadeIn(
child: Center(
child: SvgPicture.asset(
'images/state_images/error.svg',
width: width * 0.5,
fit: BoxFit.cover,
),
),
);
} else {
return Center(
child: CircularProgressIndicator(),
);
}
},
);
}
}

这是我的颤振医生输出

[✓] Flutter (Channel stable, 2.8.1, on macOS 11.4 20F71 darwin-x64, locale en-PK)
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 13.2.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 4.1)
[✓] VS Code (version 1.64.2)
[✓] Connected device (1 available)
• No issues found!

Answer 1

不是因为snapshot有数据，所以data不是null。尝试检查snapshot.hasdata和snapshot.data != null

class _CartState extends State<Cart> with TickerProviderStateMixin {
@override
Widget build(BuildContext context) {
final themeModel = Provider.of<ThemeModel>(context);
double width = MediaQuery.of(context).size.width;
return StreamBuilder<List<CartItem>>(
stream: widget.bloc.cartItems,
builder: (context, snapshot) {
if (snapshot.hasData && snapshot.data != null) {
List<CartItem> cartItems = snapshot.data!;
if (cartItems.length == 0) {
return FadeIn(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
SvgPicture.asset(
'images/state_images/empty_cart.svg',
width: width * 0.5,
fit: BoxFit.cover,
),
Padding(
padding: EdgeInsets.only(top: 30),
child: Texts.headline3(
'Nothing found herenGo and enjoy shopping!',
themeModel.accentColor,
alignment: TextAlign.center),
)
]),
);
} else {
return StreamBuilder<List<Product>>(
stream: widget.bloc.getProducts(cartItems),
builder: (context, snapshot) {
if (snapshot.hasData && snapshot.data != null) {
List<Product> products = snapshot.data!;
cartItems = cartItems.where((cartItem) {
if (products.where((product) {
if (cartItem.reference == product.reference) {
cartItem.product = product;
return true;
} else {
return false;
}
}).length ==
0) {
return false;
} else {
return true;
}
}).toList();
if (cartItems.length == 0) {
return FadeIn(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
SvgPicture.asset(
'images/state_images/empty_cart.svg',
width: width * 0.5,
fit: BoxFit.cover,
),
Padding(
padding: EdgeInsets.only(top: 30),
child: Texts.headline3(
'Nothing found herenGo and enjoy shopping!',
themeModel.accentColor,
alignment: TextAlign.center),
)
]),
);
} else {
return ListView(
padding: EdgeInsets.only(
left: 20, right: 20, top: 40, bottom: 80),
children: [
Padding(
padding: EdgeInsets.only(bottom: 20, top: 20),
child: Align(
alignment: Alignment.center,
child:
Texts.headline3("Cart", themeModel.textColor),
),
),
Padding(
padding: EdgeInsets.only(bottom: 20),
child: Align(
alignment: Alignment.center,
child: Texts.descriptiveItems(
"All orders over R600 recieve free delivery",
themeModel.textColor),
),
),
///List of cart items
AnimatedSize(
vsync: this,
duration: Duration(milliseconds: 300),
child: Column(
children:
List.generate(cartItems.length, (index) {
return FadeIn(
child: Cards.cart(context,
goToProduct: () {
ProductDetails.create(
context, products[index])
.then((value) {
if (value != null) {
final homeModel =
Provider.of<HomeModel>(context,
listen: false);
homeModel.goToPage(0);
showDialog(
context: context,
builder: (context) => Dialogs.success(
context,
message:
"Congratulations!nYour order is placed!"),
).then((value) {
widget.bloc.removeCart();
});
}
});
},
cartItem: cartItems[index],
updateQuantity:
widget.bloc.updateQuantity,
updateUnit: widget.bloc.updateUnit,
delete: () async {
await widget.bloc.removeFromCart(
cartItems[index].reference);
}),
);
}),
),
),
///Checkout button
FadeIn(
duration: Duration(milliseconds: 400),
child: Buttons.button(
color: themeModel.accentColor,
widget: Row(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Icon(
Icons.arrow_forward,
color: Colors.white,
),
Padding(
padding: EdgeInsets.only(left: 10),
child: Texts.headline3(
"Checkout", Colors.white),
)
],
),
onPressed: () async {
final checkoutResult =
await Checkout.create(context);
if (checkoutResult ?? false) {
final homeModel = Provider.of<HomeModel>(
context,
listen: false);
homeModel.goToPage(0);
showDialog(
context: context,
builder: (context) => Dialogs.success(
context,
message:
"Congratulations!nYour order is placed!"),
).then((value) {
widget.bloc.removeCart();
});
}
}),
)
],
);
}
} else if (snapshot.hasError) {
return FadeIn(
child: Center(
child: SvgPicture.asset(
'images/state_images/error.svg',
width: width * 0.5,
fit: BoxFit.cover,
),
),
);
} else {
return Center(
child: CircularProgressIndicator(),
);
}
});
}
} else if (snapshot.hasError) {
return FadeIn(
child: Center(
child: SvgPicture.asset(
'images/state_images/error.svg',
width: width * 0.5,
fit: BoxFit.cover,
),
),
);
} else {
return Center(
child: CircularProgressIndicator(),
);
}
},
);
}
}

Answer 2

• 当我们使用null检查运算符(！(并且使用它的值为null时，实际上会发生此错误
• ！运算符用于表示使用它的变量永远不会为null，但在您的情况下，它可能具有null值
• 发生此错误的原因可能是snapshot.data的值为null。另一个原因可能是您项目中的另一个文件具有null值和！我想问题也可能出现在Cards.dart文件中。所以，也要检查一下。并确保您收到的值不能为空！操作员