Mantle

Mantle makes it easy to write a simple model layer for your Cocoa or Cocoa Touch application.

The Typical Model Object

What’s wrong with the way model objects are usually written in Objective-C?

Let’s use the GitHub API for demonstration. How
would one typically represent a GitHub
issue in
Objective-C?

typedef enum : NSUInteger {
    GHIssueStateOpen,
    GHIssueStateClosed
} GHIssueState;

@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;

- (id)initWithDictionary:(NSDictionary *)dictionary;

@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;
}

- (id)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;
}

- (id)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 decodeUnsignedIntegerForKey:@"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 encodeUnsignedInteger:self.state forKey:@"state"];
}

- (id)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];
}

@end

Whew, that’s a lot of boilerplate for something so simple! And, even then, there
are some problems that this example doesn’t address:

• There’s no way to update a GHIssue with new data from the server.
• There’s no way to turn a GHIssue back into JSON.
• GHIssueState shouldn’t be encoded as-is. If the enum changes in the future,
  existing archives might break.
• If the interface of GHIssue changes down the road, existing archives might
  break.

Why Not Use Core Data?

Core Data solves certain problems very well. If you need to execute complex
queries across your data, handle a huge object graph with lots of relationships,
or support undo and redo, Core Data is an excellent fit.

It does, however, come with a couple of pain points:

• There’s still a lot of boilerplate. Managed objects reduce some of the
  boilerplate seen above, but Core Data has plenty of its own. Correctly
  setting up a Core Data stack (with a persistent store and persistent store
  coordinator) and executing fetches can take many lines of code.
• It’s hard to get right. Even experienced developers can make mistakes
  when using Core Data, and the framework is not forgiving.

If you’re just trying to access some JSON objects, Core Data can be a lot of
work for little gain.

Nonetheless, if you’re using or want to use Core Data in your app already,
Mantle can still be a convenient translation layer between the API and your
managed model objects.

MTLModel

Enter
MTLModel.
This is what GHIssue looks like inheriting from 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

Notably absent from this version are implementations of <NSCoding>,
<NSCopying>, -isEqual:, and -hash. By inspecting the @property
declarations you have in your subclass, MTLModel can provide default
implementations for all these methods.

The problems with the original example all happen to be fixed as well:

There’s no way to update a GHIssue with new data from the server.

MTLModel has an extensible -mergeValuesForKeysFromModel: method, which makes
it easy to specify how new model data should be integrated.

There’s no way to turn a GHIssue back into JSON.

This is where reversible transformers really come in handy. +[MTLJSONAdapter JSONDictionaryFromModel:error:] can transform any model object conforming to
<MTLJSONSerializing> back into a JSON dictionary. +[MTLJSONAdapter JSONArrayFromModels:error:] is the same but turns an array of model objects into an JSON array of dictionaries.

If the interface of GHIssue changes down the road, existing archives might break.

MTLModel automatically saves the version of the model object that was used for
archival. When unarchiving, -decodeValueForKey:withCoder:modelVersion: will
be invoked if overridden, giving you a convenient hook to upgrade old data.

MTLJSONSerializing

In order to serialize your model objects from or into JSON, you need to
implement <MTLJSONSerializing> in your MTLModel subclass. This allows you to
use MTLJSONAdapter to convert your model objects from JSON and back:

NSError *error = nil;
XYUser *user = [MTLJSONAdapter modelOfClass:XYUser.class fromJSONDictionary:JSONDictionary error:&error];

NSError *error = nil;
NSDictionary *JSONDictionary = [MTLJSONAdapter JSONDictionaryFromModel:user error:&error];

+JSONKeyPathsByPropertyKey

The dictionary returned by this method specifies how your model object’s
properties map to the keys in the JSON representation, for example:

@interface XYUser : MTLModel

@property (readonly, nonatomic, copy) NSString *name;
@property (readonly, nonatomic, strong) NSDate *createdAt;

@property (readonly, nonatomic, assign, getter = isMeUser) BOOL meUser;
@property (readonly, nonatomic, strong) XYHelper *helper;

@end

@implementation XYUser

+ (NSDictionary *)JSONKeyPathsByPropertyKey {
    return @{
        @"name": @"name",
        @"createdAt": @"created_at"
    };
}

- (instancetype)initWithDictionary:(NSDictionary *)dictionaryValue error:(NSError **)error {
    self = [super initWithDictionary:dictionaryValue error:error];
    if (self == nil) return nil;

    _helper = [XYHelper helperWithName:self.name createdAt:self.createdAt];

    return self;
}

@end

In this example, the XYUser class declares four properties that Mantle
handles in different ways:

• name is mapped to a key of the same name in the JSON representation.
• createdAt is converted to its snake case equivalent.
• meUser is not serialized into JSON.
• helper is initialized exactly once after JSON deserialization.

Use -[NSDictionary mtl_dictionaryByAddingEntriesFromDictionary:] if your
model’s superclass also implements MTLJSONSerializing to merge their mappings.

If you’d like to map all properties of a Model class to themselves, you can use
the +[NSDictionary mtl_identityPropertyMapWithModel:] helper method.

When deserializing JSON using
+[MTLJSONAdapter modelOfClass:fromJSONDictionary:error:], JSON keys that don’t
correspond to a property name or have an explicit mapping are ignored:

NSDictionary *JSONDictionary = @{
    @"name": @"john",
    @"created_at": @"2013/07/02 16:40:00 +0000",
    @"plan": @"lite"
};

XYUser *user = [MTLJSONAdapter modelOfClass:XYUser.class fromJSONDictionary:JSONDictionary error:&error];

Here, the plan would be ignored since it neither matches a property name of
XYUser nor is it otherwise mapped in +JSONKeyPathsByPropertyKey.

+JSONTransformerForKey:

Implement this optional method to convert a property from a different type when
deserializing from JSON.

+ (NSValueTransformer *)JSONTransformerForKey:(NSString *)key {
    if ([key isEqualToString:@"createdAt"]) {
        return [NSValueTransformer valueTransformerForName:XYDateValueTransformerName];
    }

    return nil;
}

key is the key that applies to your model object; not the original JSON key. Keep this in mind if you transform the key names using +JSONKeyPathsByPropertyKey.

For added convenience, if you implement +<key>JSONTransformer,
MTLJSONAdapter will use the result of that method instead. For example, dates
that are commonly represented as strings in JSON can be transformed to NSDates
like so:

    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];
    }];
}

If the transformer is reversible, it will also be used when serializing the
object into JSON.

+classForParsingJSONDictionary:

If you are implementing a class cluster, implement this optional method to
determine which subclass of your base class should be used when deserializing an
object from JSON.

@interface XYMessage : MTLModel

@end

@interface XYTextMessage: XYMessage

@property (readonly, nonatomic, copy) NSString *body;

@end

@interface XYPictureMessage : XYMessage

@property (readonly, nonatomic, strong) NSURL *imageURL;

@end

@implementation XYMessage

+ (Class)classForParsingJSONDictionary:(NSDictionary *)JSONDictionary {
    if (JSONDictionary[@"image_url"] != nil) {
        return XYPictureMessage.class;
    }

    if (JSONDictionary[@"body"] != nil) {
        return XYTextMessage.class;
    }

    NSAssert(NO, @"No matching class for the JSON dictionary '%@'.", JSONDictionary);
    return self;
}

@end

MTLJSONAdapter will then pick the class based on the JSON dictionary you pass
in:

NSDictionary *textMessage = @{
    @"id": @1,
    @"body": @"Hello World!"
};

NSDictionary *pictureMessage = @{
    @"id": @2,
    @"image_url": @"http://example.com/lolcat.gif"
};

XYTextMessage *messageA = [MTLJSONAdapter modelOfClass:XYMessage.class fromJSONDictionary:textMessage error:NULL];

XYPictureMessage *messageB = [MTLJSONAdapter modelOfClass:XYMessage.class fromJSONDictionary:pictureMessage error:NULL];

Persistence

Mantle doesn’t automatically persist your objects for you. However, MTLModel
does conform to <NSCoding>, so model objects can be archived to disk using
NSKeyedArchiver.

If you need something more powerful, or want to avoid keeping your whole model
in memory at once, Core Data may be a better choice.

System Requirements

Mantle supports the following platform deployment targets:

• macOS 10.10+
• iOS 9.0+
• tvOS 9.0+
• watchOS 2.0+

Importing Mantle

Manually

To add Mantle to your application:

1. Add the Mantle repository as a submodule of your application’s repository.
2. Run git submodule update --init --recursive from within the Mantle folder.
3. Drag and drop Mantle.xcodeproj into your application’s Xcode project.
4. On the “General” tab of your application target, add Mantle.framework to the “Embedded Binaries”.

If you’re instead developing Mantle on its own, use the Mantle.xcworkspace file.

Carthage

Simply add Mantle to your Cartfile:

github "Mantle/Mantle"

CocoaPods

Add Mantle to your Podfile under the build target they want it used in:

target 'MyAppOrFramework' do
  pod 'Mantle'
end

Then run a pod install within Terminal or the CocoaPods app.

Swift Package Manager

If you are writing an application, add Mantle to your project dependencies directly within Xcode.

If you are writing a package that requires Mantle as dependency, add it to the dependencies list in its Package.swift manifest, for example:

dependencies: [
    .package(url: "https://github.com/Mantle/Mantle.git", .upToNextMajor(from: "2.0.0"))
]

License

Mantle is released under the MIT license. See
LICENSE.md.

More Info

Have a question? Please open an issue!