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Mantle--国外程序员最常用的iOS模型&字典转换框架

Mantle简介

Mantle 是iOS和Mac平台下基于Objective-C编写的一个简单高效的模型层框架。

Mantle能做什么

Mantle可以轻松把JSON数据、字典（Dictionary）和模型（即Objective对象）之间的相互转换，支持自定义映射，并且内置实现了NSCoding和NSCoping，大大简化归档操作。

为什么要使用Mantle

传统的模型层方案遇到的问题

通常我们用Objective－C写的模型层遇到了什么问题？

我们可以用 Github API 来举例。现在假设我们想用Objective-C展现一个 Github Issue ，应该怎么做？

目前我们可以想到

直接解析JSON数据字典，然后展现给UI
将JSON数据转换为模型，在赋值给UI

关于1，弊端有很多，可以参考我的这篇文章：在iOS开发中使用字典转模型，现在假设我们选择了2，我们大致会定义下面的 GHIssue 模型：

GHIssue.h

 #import <Foundation/Foundation.h>   typedef enum : NSUInteger {      GHIssueStateOpen,      GHIssueStateClosed  } GHIssueState;    @class GHUser;  @interface GHIssue : NSObject <NSCoding, NSCopying>    @property (nonatomic, copy, readonly) NSURL *URL;  @property (nonatomic, copy, readonly) NSURL *HTMLURL;  @property (nonatomic, copy, readonly) NSNumber *number;  @property (nonatomic, assign, readonly) GHIssueState state;  @property (nonatomic, copy, readonly) NSString *reporterLogin;  @property (nonatomic, copy, readonly) NSDate *updatedAt;  @property (nonatomic, strong, readonly) GHUser *assignee;  @property (nonatomic, copy, readonly) NSDate *retrievedAt;    @property (nonatomic, copy) NSString *title;  @property (nonatomic, copy) NSString *body;    - (instancetype)initWithDictionary:(NSDictionary *)dictionary;    @end

GHIssue.m

 #import "GHIssue.h"  #import "GHUser.h"    @implementation GHIssue    + (NSDateFormatter *)dateFormatter {      NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];      dateFormatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];      dateFormatter.dateFormat = @"yyyy-MM-dd'T'HH:mm:ss'Z'";      return dateFormatter;  }    - (instancetype)initWithDictionary:(NSDictionary *)dictionary {      self = [self init];      if (self == nil) return nil;            _URL = [NSURL URLWithString:dictionary[@"url"]];      _HTMLURL = [NSURL URLWithString:dictionary[@"html_url"]];      _number = dictionary[@"number"];            if ([dictionary[@"state"] isEqualToString:@"open"]) {          _state = GHIssueStateOpen;      } else if ([dictionary[@"state"] isEqualToString:@"closed"]) {          _state = GHIssueStateClosed;      }            _title = [dictionary[@"title"] copy];      _retrievedAt = [NSDate date];      _body = [dictionary[@"body"] copy];      _reporterLogin = [dictionary[@"user"][@"login"] copy];      _assignee = [[GHUser alloc] initWithDictionary:dictionary[@"assignee"]];            _updatedAt = [self.class.dateFormatter dateFromString:dictionary[@"updated_at"]];            return self;  }    - (instancetype)initWithCoder:(NSCoder *)coder {      self = [self init];      if (self == nil) return nil;            _URL = [coder decodeObjectForKey:@"URL"];      _HTMLURL = [coder decodeObjectForKey:@"HTMLURL"];      _number = [coder decodeObjectForKey:@"number"];      _state = [coder decodeIntegerForKey:@"state"];      _title = [coder decodeObjectForKey:@"title"];      _retrievedAt = [NSDate date];      _body = [coder decodeObjectForKey:@"body"];      _reporterLogin = [coder decodeObjectForKey:@"reporterLogin"];      _assignee = [coder decodeObjectForKey:@"assignee"];      _updatedAt = [coder decodeObjectForKey:@"updatedAt"];            return self;  }    - (void)encodeWithCoder:(NSCoder *)coder {      if (self.URL != nil) [coder encodeObject:self.URL forKey:@"URL"];      if (self.HTMLURL != nil) [coder encodeObject:self.HTMLURL forKey:@"HTMLURL"];      if (self.number != nil) [coder encodeObject:self.number forKey:@"number"];      if (self.title != nil) [coder encodeObject:self.title forKey:@"title"];      if (self.body != nil) [coder encodeObject:self.body forKey:@"body"];      if (self.reporterLogin != nil) [coder encodeObject:self.reporterLogin forKey:@"reporterLogin"];      if (self.assignee != nil) [coder encodeObject:self.assignee forKey:@"assignee"];      if (self.updatedAt != nil) [coder encodeObject:self.updatedAt forKey:@"updatedAt"];            [coder encodeInteger:self.state forKey:@"state"];  }    - (instancetype)copyWithZone:(NSZone *)zone {      GHIssue *issue = [[self.class allocWithZone:zone] init];      issue->_URL = self.URL;      issue->_HTMLURL = self.HTMLURL;      issue->_number = self.number;      issue->_state = self.state;      issue->_reporterLogin = self.reporterLogin;      issue->_assignee = self.assignee;      issue->_updatedAt = self.updatedAt;            issue.title = self.title;      issue->_retrievedAt = [NSDate date];      issue.body = self.body;            return issue;  }    - (NSUInteger)hash {      return self.number.hash;  }    - (BOOL)isEqual:(GHIssue *)issue {      if (![issue isKindOfClass:GHIssue.class]) return NO;            return [self.number isEqual:issue.number] && [self.title isEqual:issue.title] && [self.body isEqual:issue.body];  }

GHUser.h

 @interface GHUser : NSObject <NSCoding, NSCopying>   @property (nonatomic, copy) NSString *login;  @property (nonatomic, assign) NSUInteger id;  @property (nonatomic, copy) NSString *avatarUrl;  @property (nonatomic, copy) NSString *gravatarId;  @property (nonatomic, copy) NSString *url;  @property (nonatomic, copy) NSString *htmlUrl;  @property (nonatomic, copy) NSString *followersUrl;  @property (nonatomic, copy) NSString *followingUrl;  @property (nonatomic, copy) NSString *gistsUrl;  @property (nonatomic, copy) NSString *starredUrl;  @property (nonatomic, copy) NSString *subscriptionsUrl;  @property (nonatomic, copy) NSString *organizationsUrl;  @property (nonatomic, copy) NSString *reposUrl;  @property (nonatomic, copy) NSString *eventsUrl;  @property (nonatomic, copy) NSString *receivedEventsUrl;  @property (nonatomic, copy) NSString *type;  @property (nonatomic, assign) BOOL siteAdmin;    - (id)initWithDictionary:(NSDictionary *)dictionary;    @end

你会看到，如此简单的事情却有很多弊端。甚至，还有一些其他问题，这个例子里面没有展示出来。

无法使用服务器的新数据来更新这个 GHIssue
无法反过来将 GHIssue 转换成 JSON
对于 GHIssueState ，如果枚举改编了，现有的归档会崩溃
如果 GHIssue 接口改变了，现有的归档会崩溃。

使用MTLModel

如果使用MTLModel，我们可以这样，声明一个类继承自MTLModel

 typedef enum : NSUInteger {      GHIssueStateOpen,      GHIssueStateClosed  } GHIssueState;    @interface GHIssue : MTLModel <MTLJSONSerializing>    @property (nonatomic, copy, readonly) NSURL *URL;  @property (nonatomic, copy, readonly) NSURL *HTMLURL;  @property (nonatomic, copy, readonly) NSNumber *number;  @property (nonatomic, assign, readonly) GHIssueState state;  @property (nonatomic, copy, readonly) NSString *reporterLogin;  @property (nonatomic, strong, readonly) GHUser *assignee;  @property (nonatomic, copy, readonly) NSDate *updatedAt;    @property (nonatomic, copy) NSString *title;  @property (nonatomic, copy) NSString *body;    @property (nonatomic, copy, readonly) NSDate *retrievedAt;    @end  @implementation GHIssue    + (NSDateFormatter *)dateFormatter {      NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];      dateFormatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];      dateFormatter.dateFormat = @"yyyy-MM-dd'T'HH:mm:ss'Z'";      return dateFormatter;  }    + (NSDictionary *)JSONKeyPathsByPropertyKey {      return @{          @"URL": @"url",          @"HTMLURL": @"html_url",          @"number": @"number",          @"state": @"state",          @"reporterLogin": @"user.login",          @"assignee": @"assignee",          @"updatedAt": @"updated_at"      };  }    + (NSValueTransformer *)URLJSONTransformer {      return [NSValueTransformer valueTransformerForName:MTLURLValueTransformerName];  }    + (NSValueTransformer *)HTMLURLJSONTransformer {      return [NSValueTransformer valueTransformerForName:MTLURLValueTransformerName];  }    + (NSValueTransformer *)stateJSONTransformer {      return [NSValueTransformer mtl_valueMappingTransformerWithDictionary:@{          @"open": @(GHIssueStateOpen),          @"closed": @(GHIssueStateClosed)      }];  }    + (NSValueTransformer *)assigneeJSONTransformer {      return [MTLJSONAdapter dictionaryTransformerWithModelClass:GHUser.class];  }    + (NSValueTransformer *)updatedAtJSONTransformer {      return [MTLValueTransformer transformerUsingForwardBlock:^id(NSString *dateString, BOOL *success, NSError *__autoreleasing *error) {          return [self.dateFormatter dateFromString:dateString];      } reverseBlock:^id(NSDate *date, BOOL *success, NSError *__autoreleasing *error) {          return [self.dateFormatter stringFromDate:date];      }];  }    - (instancetype)initWithDictionary:(NSDictionary *)dictionaryValue error:(NSError **)error {      self = [super initWithDictionary:dictionaryValue error:error];      if (self == nil) return nil;        // Store a value that needs to be determined locally upon initialization.      _retrievedAt = [NSDate date];        return self;  }    @end

很明显，我们不需要再去实现 <NSCoding> , <NSCopying> , -isEqual: 和 -hash 。在你的子类里面生命属性，MTLModel可以提供这些方法的默认实现。

最初例子里面的问题，在这里都得到了很好的解决。

MTLModel提供了一个 - (void)mergeValueForKey:(NSString *)key fromModel:(id<MTLModel>)model{} ,可以与其他任何实现了MTLModel协议的模型对象集成。
+[MTLJSONAdapter JSONDictionaryFromModel:error:] 可以把任何遵循 MTLJSONSerializing>``协议的对象转换成JSON字典， +[MTLJSONAdapter JSONArrayFromModels:error:]```类似，不过转换的是一个数组。

MTLJSONAdapter 中的 fromJSONDictionary 和 JSONDictionaryFromModel 可以实现模型和JSON的相互转化。

JSONKeyPathsByPropertyKey 可以实现模型和JSON的自定义映射。

JSONTransformerForKey 可以对JSON和模型不同类型进行映射。

classForParsingJSONDictionary 如果你使用了类簇（关于类簇，请参考：类簇在iOS开发中的应用），classForParsingJSONDictionary可以让你选择使用哪一个类进行JSON反序列化。