Mantle--国外程序员最常用的iOS模型&字典转换框架

2019-11-14 18:06:37

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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。