Mantle--國外程序員最常用的iOS模型&字典轉換框架

2019-11-14 18:06:37

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Mantle簡介

Mantle是iOS和Mac平臺下基于Objective-C編寫的一個簡單高效的模型層框架。

Mantle能做什么

Mantle可以輕松把JSON數據、字典（Dictionary）和模型（即Objective對象）之間的相互轉換，支持自定義映射，并且內置實現了NSCoding和NSCoping，大大簡化歸檔操作。

為什么要使用Mantle

傳統的模型層方案遇到的問題

通常我們用Objective－C寫的模型層遇到了什么問題？

我們可以用Github API來舉例。現在假設我們想用Objective-C展現一個Github Issue，應該怎么做？

目前我們可以想到

直接解析JSON數據字典，然后展現給UI
將JSON數據轉換為模型，在賦值給UI

關于1，弊端有很多，可以參考我的這篇文章：在iOS開發中使用字典轉模型，現在假設我們選擇了2，我們大致會定義下面的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

你會看到，如此簡單的事情卻有很多弊端。甚至，還有一些其他問題，這個例子里面沒有展示出來。

無法使用服務器的新數據來更新這個 GHIssue
無法反過來將 GHIssue 轉換成 JSON
對于GHIssueState，如果枚舉改編了，現有的歸檔會崩潰
如果 GHIssue 接口改變了，現有的歸檔會崩潰。

使用MTLModel

如果使用MTLModel，我們可以這樣，聲明一個類繼承自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

很明顯，我們不需要再去實現<NSCoding>, <NSCopying>, -isEqual:和-hash。在你的子類里面生命屬性，MTLModel可以提供這些方法的默認實現。

最初例子里面的問題，在這里都得到了很好的解決。

MTLModel提供了一個- (void)mergeValueForKey:(NSString *)key fromModel:(id<MTLModel>)model{},可以與其他任何實現了MTLModel協議的模型對象集成。
+[MTLJSONAdapter JSONDictionaryFromModel:error:]可以把任何遵循MTLJSONSerializing>``協議的對象轉換成JSON字典，+[MTLJSONAdapter JSONArrayFromModels:error:]```類似，不過轉換的是一個數組。

MTLJSONAdapter中的fromJSONDictionary和JSONDictionaryFromModel可以實現模型和JSON的相互轉化。

JSONKeyPathsByPropertyKey可以實現模型和JSON的自定義映射。

JSONTransformerForKey可以對JSON和模型不同類型進行映射。

classForParsingJSONDictionary 如果你使用了類簇（關于類簇，請參考：類簇在iOS開發中的應用），classForParsingJSONDictionary可以讓你選擇使用哪一個類進行JSON反序列化。