问题描述
我将整个类保存为 Parse Back4App 服务器上的 json 对象。将对象解析为 json 很痛苦，因为我需要将嵌套类 Book 和主类 DatabaseSyncItem 中的许多属性转换为字符串。现在我不知道如何将这个 json 对象转换回 DatabaseSyncObject。我认为将嵌套的 json book 对象恢复到对象模型也是如此，也就是说它不会以这种方式工作。
我收到错误，但没有成功。如果我尝试从 main 中的 json 对象列表中访问特定索引，我可以找到某个地方，但这非常混乱。在这个情况下，你会怎么做？帮助我，我第一次使用 json。
尝试应用这些帖子但没有工作：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include "lz4.h"
int main()
{
time_t start,end;
int compress,decompress;
int num_f=0;
int i;
const char* const src;
int srcSize=16385;
int dstCapacity = LZ4_compressBound(srcSize);
char* compressed_data = malloc((size_t)dstCapacity);
FILE * fc;
num_f = splitFile("/home/ziruo/research/test.txt",16384);
start = time(NULL);
for( i = 1; i <= num_f; i++)
{
fc = fopen(("/home/ziruo/research/text.txt.%03d",i),"r");
fgets(src,16384,fc);
printf("%s",src);
LZ4_compress_default(src,compressed_data,srcSize,dstCapacity);
}
//LZ4LIB_API int LZ4_compress_default(const char* src,char* dst,int srcSize,int dstCapacity);
end = time(NULL);
compress=difftime(end,start);
printf("%d",compress);
//printf("%d\n",splitFile("/home/ziruo/research/test.txt",16384));
return 0;
解决方法
似乎这个答案比我最初预期的要复杂得多。需要使用json可序列化包和json注解包。
包是必要的，当您查看用于处理类型、null 和嵌套类的生成代码时，您就会明白为什么。以防其他人遇到这种情况。
import 'package:hive/hive.dart';
import 'package:json_annotation/json_annotation.dart';
part 'book_model.g.dart';
@JsonSerializable(includeIfNull: true)
@HiveType(typeId: 0)
class Book {
@HiveField(0)
String title;
@HiveField(1)
String author;
@HiveField(2)
DateTime publishingDate;
@HiveField(3)
DateTime dateAdded;
@HiveField(4)
DateTime lastModified;
Book({
required this.title,required this.author,required this.publishingDate,required this.dateAdded,required this.lastModified,});
factory Book.fromJson(Map<String,dynamic> json) => _$BookFromJson(json);
Map<String,dynamic> toJson() => _$BookToJson(this);
@override
String toString() {
return '''
title: $title
author: $author
publishingDate: $publishingDate
dateAdded: $dateAdded
lastModified $lastModified
''';
}
}
// GENERATED CODE - DO NOT MODIFY BY HAND
part of 'book_model.dart';
// **************************************************************************
// TypeAdapterGenerator
// **************************************************************************
class BookAdapter extends TypeAdapter<Book> {
@override
final int typeId = 0;
@override
Book read(BinaryReader reader) {
final numOfFields = reader.readByte();
final fields = <int,dynamic>{
for (int i = 0; i < numOfFields; i++) reader.readByte(): reader.read(),};
return Book(
title: fields[0] as String,author: fields[1] as String,publishingDate: fields[2] as DateTime,dateAdded: fields[3] as DateTime,lastModified: fields[4] as DateTime,);
}
@override
void write(BinaryWriter writer,Book obj) {
writer
..writeByte(5)
..writeByte(0)
..write(obj.title)
..writeByte(1)
..write(obj.author)
..writeByte(2)
..write(obj.publishingDate)
..writeByte(3)
..write(obj.dateAdded)
..writeByte(4)
..write(obj.lastModified);
}
@override
int get hashCode => typeId.hashCode;
@override
bool operator ==(Object other) =>
identical(this,other) ||
other is BookAdapter &&
runtimeType == other.runtimeType &&
typeId == other.typeId;
}
// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************
Book _$BookFromJson(Map<String,dynamic> json) {
return Book(
title: json['title'] as String,author: json['author'] as String,publishingDate: DateTime.parse(json['publishingDate'] as String),dateAdded: DateTime.parse(json['dateAdded'] as String),lastModified: DateTime.parse(json['lastModified'] as String),);
}
Map<String,dynamic> _$BookToJson(Book instance) => <String,dynamic>{
'title': instance.title,'author': instance.author,'publishingDate': instance.publishingDate.toIso8601String(),'dateAdded': instance.dateAdded.toIso8601String(),'lastModified': instance.lastModified.toIso8601String(),};
import 'package:json_annotation/json_annotation.dart';
import 'book_model.dart';
import 'package:hive/hive.dart';
part 'database_sync_model.g.dart';
@JsonSerializable(includeIfNull: true)
@HiveType(typeId: 1)
class DatabaseSyncItem {
@HiveField(0)
Book? previousBookValue;
@HiveField(1)
Book? updatedBookValue;
@HiveField(2)
DateTime dateAdded;
@HiveField(3)
DateTime lastModified;
@HiveField(4)
DatabaseAction entryAction;
DatabaseSyncItem({
this.previousBookValue,this.updatedBookValue,required this.entryAction,});
factory DatabaseSyncItem.fromJson(Map<String,dynamic> json) => _$DatabaseSyncItemFromJson(json);
Map<String,dynamic> toJson() => _$DatabaseSyncItemToJson(this);
@override
String toString() {
return '''
previousValue: $previousBookValue
updatedValue: $updatedBookValue
dateAdded: $dateAdded
lastModified: $lastModified
entryAction: $entryAction
''';
}
}
enum DatabaseAction {
create,update,delete,}
// GENERATED CODE - DO NOT MODIFY BY HAND
part of 'database_sync_model.dart';
// **************************************************************************
// TypeAdapterGenerator
// **************************************************************************
class DatabaseSyncItemAdapter extends TypeAdapter<DatabaseSyncItem> {
@override
final int typeId = 1;
@override
DatabaseSyncItem read(BinaryReader reader) {
final numOfFields = reader.readByte();
final fields = <int,};
return DatabaseSyncItem(
previousBookValue: fields[0] as Book?,updatedBookValue: fields[1] as Book?,dateAdded: fields[2] as DateTime,lastModified: fields[3] as DateTime,entryAction: fields[4] as DatabaseAction,DatabaseSyncItem obj) {
writer
..writeByte(5)
..writeByte(0)
..write(obj.previousBookValue)
..writeByte(1)
..write(obj.updatedBookValue)
..writeByte(2)
..write(obj.dateAdded)
..writeByte(3)
..write(obj.lastModified)
..writeByte(4)
..write(obj.entryAction);
}
@override
int get hashCode => typeId.hashCode;
@override
bool operator ==(Object other) =>
identical(this,other) ||
other is DatabaseSyncItemAdapter &&
runtimeType == other.runtimeType &&
typeId == other.typeId;
}
// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************
DatabaseSyncItem _$DatabaseSyncItemFromJson(Map<String,dynamic> json) {
return DatabaseSyncItem(
previousBookValue: json['previousBookValue'] == null
? null
: Book.fromJson(json['previousBookValue'] as Map<String,dynamic>),updatedBookValue: json['updatedBookValue'] == null
? null
: Book.fromJson(json['updatedBookValue'] as Map<String,entryAction: _$enumDecode(_$DatabaseActionEnumMap,json['entryAction']),dynamic> _$DatabaseSyncItemToJson(DatabaseSyncItem instance) =>
<String,dynamic>{
'previousBookValue': instance.previousBookValue,'updatedBookValue': instance.updatedBookValue,'entryAction': _$DatabaseActionEnumMap[instance.entryAction],};
K _$enumDecode<K,V>(
Map<K,V> enumValues,Object? source,{
K? unknownValue,}) {
if (source == null) {
throw ArgumentError(
'A value must be provided. Supported values: '
'${enumValues.values.join(',')}',);
}
return enumValues.entries.singleWhere(
(e) => e.value == source,orElse: () {
if (unknownValue == null) {
throw ArgumentError(
'`$source` is not one of the supported values: '
'${enumValues.values.join(',);
}
return MapEntry(unknownValue,enumValues.values.first);
},).key;
}
const _$DatabaseActionEnumMap = {
DatabaseAction.create: 'create',DatabaseAction.update: 'update',DatabaseAction.delete: 'delete',};