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json - 将http中的json数据解码为列表，但列表在颤动中为空

问题描述

我正在尝试将 json 数据从 http 解码到一个列表中，但是在我设置数据后列表为空。我还有其他方法可以做到吗？我正在使用有状态的小部件并在 initstate 中调用该函数。http 请求获取 json 数据，但我使用 ListView.builder 的构建函数没有执行。脚手架是空的。不打印 ListView 内的打印语句。我什至在列表视图中尝试了纯文本，即使它没有显示屏幕。 List body;

import 'dart:async';
import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;


class SellerSign extends StatefulWidget {

  @override
  _SellerSignState createState() => _SellerSignState();
}

class _SellerSignState extends State<SellerSign> {
  List body;
  @override
  void initState(){
    this.shopment();
  }
  shopment() async {
    var headers = {
      'API-Key': 'TEST_f+y9/tHG+yVxEq3uS3H1ogfezHSCWSq5MsIXUOnIV+Q',
      'Content-Type': 'application/json',
    };

    var data = '{ "rate_options": { "carrier_ids": [ "se-634124" ] }, "shipment": { "validate_address": "no_validation", "ship_to": { "name": "Amanda Miller", "phone": "555-555-5555", "address_line1": "525 S Winchester Blvd", "city_locality": "San Jose", "state_province": "CA", "postal_code": "95128", "country_code": "US", "address_residential_indicator": "yes" }, "ship_from": { "company_name": "Example Corp.", "name": "John Doe", "phone": "111-111-1111", "address_line1": "4009 Marathon Blvd", "address_line2": "Suite 300", "city_locality": "Austin", "state_province": "TX", "postal_code": "78756", "country_code": "US", "address_residential_indicator": "no" }, "packages": [ { "weight": { "value": 1.0, "unit": "ounce" } } ] } }';

    var res = await http.post('https://api.shipengine.com/v1/rates', headers: headers, body: data);
    if (res.statusCode != 200) throw Exception('http.post error: statusCode= ${res.statusCode}');
    print(res.body);
    setState(() {
      body = json.decode(res.body);
    });
  }
  @override
  Widget build(BuildContext context){
    return new Scaffold(
      appBar: new AppBar(title: new Text("Listviews"), backgroundColor: Colors.blue),
      body: new ListView.builder(
        itemCount: body == null ? 0 : body.length,
        itemBuilder: (BuildContext context, int index){
          print("kbhkjb${body[index]["rate_id"]}");
          return new Card(
            child: new Text(body[index]["rate_id"]),
          );
        },
      ),
    );
  }
}

这是打印语句。

I/flutter (26156): {
I/flutter (26156):   "rate_response": {
I/flutter (26156):     "rates": [
I/flutter (26156):       {
I/flutter (26156):         "rate_id": "se-720961365",
I/flutter (26156):         "rate_type": "shipment",
I/flutter (26156):         "carrier_id": "se-634124",
I/flutter (26156):         "shipping_amount": {
I/flutter (26156):           "currency": "usd",
I/flutter (26156):           "amount": 0.5100
I/flutter (26156):         },
I/flutter (26156):         "insurance_amount": {
I/flutter (26156):           "currency": "usd",
I/flutter (26156):           "amount": 0.0
I/flutter (26156):         },
I/flutter (26156):         "confirmation_amount": {
I/flutter (26156):           "currency": "usd",
I/flutter (26156):           "amount": 0.0
I/flutter (26156):         },
I/flutter (26156):         "other_amount": {
I/flutter (26156):           "currency": "usd",
I/flutter (26156):           "amount": 0.0
I/flutter (26156):         },
I/flutter (26156):         "zone": 7,
I/flutter (26156):         "package_type": "letter",
I/flutter (26156):         "delivery_days": 3,
I/flutter (26156):         "guaranteed_service": false,
I/flutter (26156):         "estimated_delivery_date": "2021-06-28T00:00:00Z",
I/flutter (26156):         "carrier_delivery_days": "3",
I/flutter (26156):         "ship_date": "2021-06-24T00:00:00Z",
I/flutter (26156):         "negotiated_rate": false,
I/flutter (26156):         "service_type": "USPS First Class Mail",
I/flutter (26156):         "service_code": "usps_first_class_mail",
I/flutter (26156):         "trackable": false,
I/flutter (26156):         "carrier_code": "stamps_com",
I/flutter (26156):     ```

标签： jsonflutterhttpdart

解决方案

jsonDecodenull如果接收到的 JSON 为 null（即 JSON 字符串只是），将返回'null'：

final json = jsonDecode('null');
expect(json, null); // passes

确保 API 没有返回nulljson（尽管根据我的经验，这并不常见）。

另一个可能的错误来源是如果您在设置之前尝试使用body它。

当你创建时List body;，假设你没有在空安全模式下运行，它被初始化为null;

如果您有一个build()尝试body立即使用的方法，您将尝试在网络调用完成之前构建 UI。如果是这种情况，您可以检查 if bodyis null，并在它是时显示加载微调器。

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