Flutter - 无法从REST API获取数据

Question

I am trying to load data from my REST API and load them into the UI. Below is my code

 FutureBuilder loadProductSellAds() {
    return FutureBuilder<ProductSellAdvertisement>(
      future: DataFetch().loadProductSellSingleAd(
          AppNavigation.getAPIUrl() +
              "sellAdvertisement/getAdByID/" +
              widget._advertisementID.toString()),
      builder: (context, snapshot) {
        if (snapshot.hasError) print(snapshot.error);

        if(snapshot.hasData)
        {
          ProductSellAdvertisement p = snapshot.data;

          setValuesToProductDataSection(
            type: p.type,
            location: p.location,
            quantity: p.quantity.toString(),
            stocksAvailable: p.stocksAvailableTill.toString(),
            unitPrice: p.unitPrice.toString()

          );
          displayProductInfoSection = true;
        }
      },
    );
  }

datafetch.dart

/**
   * Use to Load Product Sell Advertisements
   * 
   **/
  Future<ProductSellAdvertisement> loadProductSellSingleAd(String url) async {
    final response = await http.get(url);
    // Use the compute function to run parsePhotos in a separate isolate
    final parsed =
        convert.json.decode(response.body).cast<Map<String, dynamic>>();

    ProductSellAdvertisement obj = parsed
        .map<ProductSellAdvertisement>(
            (json) => new ProductSellAdvertisement.fromJson(json));
    return obj;
  }

and finally, the model class, product_sell_advertisement.dart

import './product.dart';
import './product_unit.dart';
import './user.dart';

import 'package:json_annotation/json_annotation.dart';


part 'product_sell_advertisement.g.dart';

@JsonSerializable()

class ProductSellAdvertisement
{
     int idproductSellAdvertisement;
     Product product;
     ProductUnit productUnits;
     User user;
     String type;
     String grade;
     double unitPrice;
     double quantity;
     String location;
     int stocksAvailableTill;
     String extraInformation;
     int expireOn;
     int deleteTimestamp;
     int dateCreated;
     int lastUpdated;

     ProductSellAdvertisement(this.idproductSellAdvertisement, this.product, this.productUnits, this.user, this.type,
     this.grade, this.unitPrice, this.quantity, this.location, this.stocksAvailableTill, this.extraInformation, this.expireOn,
     this.deleteTimestamp,this.dateCreated, this.lastUpdated);


  factory ProductSellAdvertisement.fromJson(Map<String, dynamic> json) => _$ProductSellAdvertisementFromJson(json);


  Map<String, dynamic> toJson() => _$ProductSellAdvertisementToJson(this);
}

When I execute the loadProductSellAds method (the first code in this post), I run into the below error

    [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: NoSuchMethodError: Class '_InternalLinkedHashMap<String, dynamic>' has no instance method 'cast' with matching arguments.
E/flutter ( 4670): Receiver: _LinkedHashMap len:15
E/flutter ( 4670): Tried calling: cast<Map<String, dynamic>>()
E/flutter ( 4670): Found: cast<RK, RV>() => Map<RK, RV>
E/flutter ( 4670): #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
E/flutter ( 4670): #1      DataFetch.loadProductSellSingleAd 
package:xxx/custom_functions/data_fetch.dart:101
E/flutter ( 4670): <asynchronous suspension>
E/flutter ( 4670): #2      SingleSellAdvertisementState.loadProductSellAds 
package:xxx/pages/single_sell_advertisement_page.dart:225
E/flutter ( 4670): #3      SingleSellAdvertisementState.initState 
package:xxx/pages/single_sell_advertisement_page.dart:5

Why is this happening?

Answer 1

By definition that cast() is a method introduced in Dart 2 on Iterable that actually creates a new iterable of type Iterable (or subclass List) filled with the values of the original interable. Your source object is not an instance if Iterable.