我刚刚开始处理flutter应用程序，并且在按下保存按钮时遇到了这种异常。
方法'call'在null上被调用。接收者：null尝试致电：致电（“产品”的实例）
据我所知，我认为我的工作还不错，但仍然无法找出此错误/异常。
我还将附加代码以供参考。
这是我的主文件
class _MyAppState extends State<MyApp> {
@override
Widget build(BuildContext context) {
// TODO: implement build
return ScopedModel<ProductsModel>(
model: ProductsModel(),
child: MaterialApp(
theme: ThemeData(
brightness: Brightness.light,
primarySwatch: Colors.deepOrange,
accentColor: Colors.deepPurple,
buttonColor: Colors.deepPurple,
),
// home: AuthPage(),
// Either '/' or home stands for starting point of the app
routes: {
'/': (BuildContext context) => AuthPage(),
'/product': (BuildContext context) => ProductsPage(),
'/admin': (BuildContext context) => ProductsAdminPage()
},
onGenerateRoute: (RouteSettings settings) {
final List<String> pathElements = settings.name.split('/');
if (pathElements[0] != '') return null;
if (pathElements[1] == 'product') {
final int index = int.parse(pathElements[2]);
return MaterialPageRoute<bool>(
builder: (BuildContext context) =>
ProductPage(null, null, null, null),
);
}
return null;
},
onUnknownRoute: (RouteSettings settings) {
return MaterialPageRoute(
builder: (BuildContext context) => ProductsPage(),
);
},
),
);
}
并且从这里我导航到添加/编辑页面。
class ProductEditPage extends StatefulWidget {
final Function addProduct;
final Function updateProduct;
final Product product;
final int productIndex;
ProductEditPage(
{this.addProduct, this.updateProduct, this.product, this.productIndex});
@override
State<StatefulWidget> createState() {
return _ProductEditPageState();
}
}
class _ProductEditPageState extends State<ProductEditPage> {
final Map<String, dynamic> _formData = {
'title': null,
'description': null,
'price': null,
'image': 'assets/food.jpg'
};
final GlobalKey<FormState> _globalKey = GlobalKey<FormState>();
final _titleFocusNode = FocusNode();
final _descriptionFocusNode = FocusNode();
final _priceFocusNode = FocusNode();
Widget _buildTitleTextField() {
return EnsureVisibleWhenFocused(
child: TextFormField(
focusNode: _titleFocusNode,
decoration: InputDecoration(labelText: 'Product Title'),
initialValue: widget.product != null ? widget.product.title : "",
validator: (String value) {
if (value.isEmpty || value.length < 5)
return "Title is required and should be 5+ characters long";
else
return null;
},
onSaved: (String value) {
_formData['title'] = value;
},
),
focusNode: _titleFocusNode);
}
Widget _buildDescriptionTextField() {
return EnsureVisibleWhenFocused(
child: TextFormField(
decoration: InputDecoration(labelText: 'Product Description'),
maxLines: 10,
focusNode: _descriptionFocusNode,
initialValue:
widget.product != null ? widget.product.description : "",
validator: (String value) {
if (value.isEmpty || value.length < 10)
return "Description is required and should be 10+ characters long";
else
return null;
},
onSaved: (String value) {
_formData['description'] = value;
},
),
focusNode: _descriptionFocusNode);
}
Widget _buildPriceTextField() {
return EnsureVisibleWhenFocused(
child: TextFormField(
decoration: InputDecoration(labelText: 'Product Price'),
keyboardType:
TextInputType.numberWithOptions(decimal: true, signed: true),
focusNode: _priceFocusNode,
initialValue:
widget.product != null ? widget.product.price.toString() : "",
validator: (String value) {
if (value.isEmpty ||
!RegExp(r'^(?:[1-9]\d*|0)?(?:\.\d+)?$').hasMatch(value))
return "Price is required and should be a number";
else
return null;
},
onSaved: (String value) {
_formData['price'] = double.parse(value);
},
),
focusNode: _priceFocusNode);
}
Widget _buildSubmitButton() {
return ScopedModelDescendant<ProductsModel>(
builder: (BuildContext context, Widget child, ProductsModel model) {
return RaisedButton(
child: Text('SAVE'),
textColor: Colors.white,
onPressed: () =>
_onSavePressed(model.addProduct, model.updateProduct),
);
},
);
}
Widget _buildPageContent(BuildContext context) {
final double deviceWidth = MediaQuery.of(context).size.width;
final double targetWidth =
MediaQuery.of(context).orientation == Orientation.portrait
? deviceWidth * 0.95
: deviceWidth * 0.75;
final double targetPadding = deviceWidth - targetWidth;
return GestureDetector(
onTap: () {
FocusScope.of(context).requestFocus(FocusNode());
},
child: Container(
margin: EdgeInsets.all(10.0),
child: Form(
key: _globalKey,
child: ListView(
padding: EdgeInsets.symmetric(horizontal: targetPadding / 2),
children: <Widget>[
_buildTitleTextField(),
_buildDescriptionTextField(),
_buildPriceTextField(),
SizedBox(
height: 10.0,
),
_buildSubmitButton(),
// GestureDetector(
// onTap: _onSavePressed,
// child: Container(
// color: Colors.green,
// padding: EdgeInsets.all(5.0),
// child: Text('SAVE'),
// ),
// )
],
),
),
),
);
}
void _onSavePressed(Function addProduct, Function updateProduct) {
if (!_globalKey.currentState.validate()) {
return;
}
_globalKey.currentState.save();
if (widget.product == null) {
widget.addProduct(
Product(
title: _formData['title'],
image: _formData['image'],
price: _formData['price'],
description: _formData['description']),
);
} else {
widget.updateProduct(
widget.productIndex,
Product(
title: _formData['title'],
image: _formData['image'],
price: _formData['price'],
description: _formData['description']));
}
Navigator.pushNamed(context, '/product');
}
@override
Widget build(BuildContext context) {
final Widget pageContent = _buildPageContent(context);
return widget.product == null
? pageContent
: Scaffold(
appBar: AppBar(
title: Text('Edit Product'),
),
body: pageContent,
);
}
这里是产品型号类别
class Product {
final String title;
final String description;
final double price;
final String image;
Product(
{@required this.title,
@required this.description,
@required this.price,
@required this.image});
}
也是这里的日志
════════ Exception caught by gesture ═══════════════════════════════════════════════════════════════
The following NoSuchMethodError was thrown while handling a gesture:
The method 'call' was called on null.
Receiver: null
Tried calling: call(Instance of 'Product')
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1 _ProductEditPageState._onSavePressed (package:flutter_app/pages/product_edit.dart:157:14)
#2 _ProductEditPageState._buildSubmitButton.<anonymous closure>.<anonymous closure> (package:flutter_app/pages/product_edit.dart:104:15)
#3 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:654:14)
#4 _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:729:32)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#1e958
debugOwner: GestureDetector
state: ready
won arena
finalPosition: Offset(95.0, 358.0)
finalLocalPosition: Offset(94.0, 23.0)
sent tap down
════════════════════════════════════════════════════════════════════════════════════════════════════
任何人！！！
错误意味着您正在尝试将null变量作为函数调用。
在_onSavePressed方法上，您正在调用widget.addProduct函数，该函数实际上为空。您应该调用作为参数传递的addProduct函数，如下所示：
void _onSavePressed(Function addProduct, Function updateProduct) {
if (!_globalKey.currentState.validate()) {
return;
}
_globalKey.currentState.save();
if (widget.product == null) {
addProduct( // change here
Product(
title: _formData['title'],
image: _formData['image'],
price: _formData['price'],
description: _formData['description']),
);
} else {
updateProduct( // change here
widget.productIndex,
Product(
title: _formData['title'],
image: _formData['image'],
price: _formData['price'],
description: _formData['description']));
}
Navigator.pushNamed(context, '/product');
}