错误：位置参数过多：预期为0，但发现3

Question

I am getting this error: Too many positional arguments: 0 expected, but 3 found. (extra_positional_arguments_could_be_named at itemBuilder...and can not figure out what could be causing it.

the error is appearing here ->

itemBuilder: (ctx, i) => ProductItem(
          loadedProducts[i].id,
          loadedProducts[i].title,
          loadedProducts[i].imageUrl,
        ),

class ProductListPage extends StatelessWidget {
  ProductListPage({this.context});

  final List<Product> loadedProducts = [
    Product(
      id: 'p1',
      title: "Michael Kora",
      description: 'this is cool',
      price: 699,
      imageUrl:
          "https://n1.sdlcdn.com/imgs/c/9/8/Lambency-Brown-Solid-Casual-Blazers-SDL781227769-1-1b660.jpg",
    ),
    Product(
      id: 'p1',
      title: "Michael Kora",
      description: 'this is cool',
      price: 699,
      imageUrl:
          "https://n1.sdlcdn.com/imgs/c/9/8/Lambency-Brown-Solid-Casual-Blazers-SDL781227769-1-1b660.jpg",
    ),

  ];

  final BuildContext context;

//  @override
  Widget build(BuildContext context) {
    return Scaffold(

      body: GridView.builder(
        padding: const EdgeInsets.all(10.0),
        itemCount: loadedProducts.length,
        itemBuilder: (ctx, i) => ProductItem(
          loadedProducts[i].id,
          loadedProducts[i].title,
          loadedProducts[i].imageUrl,
        ),

    );
  }
}

ProductItem is defined as follows:

class ProductItem extends StatelessWidget { 
  ProductItem({this.id, this.imageUrl, this.title}); 

  final String id;
  final String title;
  final String imageUrl;

  @override
  Widget build(BuildContext context) {
    return GridTile(
      child: Image.network(imageUrl),
    );
  } 
}

Answer 1

Short answer:

ProductItem expects named , not positional arguments. Initialize it like so:

ProductItem(
  id: loadedProducts[i].id,
  title: loadedProducts[i].title,
  imageUrl: loadedProducts[i].imageUrl,
)

Longer answer:

The error informs us:

Too many positional arguments: 0 expected, but 3 found.

Which suggests that ProductItem constructor did not expect positional arguments. It does not know what to do with them. Why? Let's inspect ProductItem class definition:

class ProductItem extends StatelessWidget { 
  ProductItem({this.id, this.imageUrl, this.title}); 
  ...
}

The parameters are enclosed in {} . In Dart, this means that they are optional named parameters. That is, if you decide to pass them in, you must pass them in like so:

ProductItem(id: 'id', imageUrl: 'url', title: 'title')

Note that each argument is preceded by its name - hence it is called a named parameter. In contrast, positional arguments are differentiated only by the position they take in a call to a constructor.

Class definition informs us that constructor of ProductItem should not be called with positional arguments. Instead, named arguments should be used. ProductItem should be constructed this way:

ProductItem(
  id: loadedProducts[i].id,
  title: loadedProducts[i].title,
  imageUrl: loadedProducts[i].imageUrl,
)

To read more about types of parameters, refer to Dart documentation . Alternatively, explore the differences in DartPad .