Inherits from NSOperation
Declared in RKMappingOperation.h

Overview

Instances of RKMappingOperation perform transformation between object representations according to the rules expressed in RKObjectMapping objects. Mapping operations provide the foundation for the RestKit object mapping engine and perform the work of inspecting the attributes and relationships of a source object and determining how to map them into new representations on a destination object.

Metadata Mapping

The mapping operation provides support for mapping for a dictionary of metadata in addition to the source object. This metadata is made available by mapping key paths nested under a specially designated parent key that cannot exist in a source representation. By convention, metadata is typically nested under sub keys to effectively namespace usage between components. The object mapping engine itself reserves the ‘mapping’ key for its usage. Metadata is passed down through a hierarchy of mapping operations (i.e. as relationships are traversed), making a common set of ancillary information available for mapping for by any operation executed.

To understand how metadata works, consider the following example:

@interface RKMetadataExample : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, copy) NSURL *URL;
@property (nonatomic, copy) NSDate *mappedAt;
@end

RKMetadataExample *example = [RKMetadataExample new];
NSDictionary *representation = @{ @"name": @"Blake Watters" };
NSDictionary *metadata = @{ @"URL": [NSURL URLWithString:@"http://restkit.org"]http://restkit.org"] };

RKObjectMapping *objectMapping = [RKObjectMapping mappingForClass:[RKMetadataExample class]];
[objectMapping addAttributeMappingsFromDictionary:@{ @"name": @"name", @"@metadata.URL": @"URL" }];

RKMappingOperation *mappingOperation = [[RKMappingOperation alloc] initWithSourceObject:representation destinationObject:example mapping:objectMapping];
mappingOperation.metadata = metadata;

NSError *error = nil;
BOOL success = [mappingOperation execute:&error];

Note the use of the special keyPath @"@metadata.URL". The @metadata prefix indicates that the property is to be mapped from the metadata dictionary instead of from the source object representation. If any relationships were mapped, it would have access to this same metadata information as well.

In addition to any metadata provided to the mapping operation via the metadata property, the operation itself makes the following metadata key paths available for mapping:

  1. @metadata.mapping.collectionIndex – An NSNumber object specifying the index of the current object within a collection being mapped. This key is only available if the current representation exists within a collection.

Tasks

Initializing a Mapping Operation

Accessing Mapping Configuration

  •   sourceObject

    A dictionary of mappable elements containing simple values or nested object structures.

    property
  •   destinationObject

    The target object for this operation. Mappable values in the source object will be applied to the destination object using key-value coding.

    property
  •   mapping

    The mapping defining how values contained in the source object should be transformed to the destination object via key-value coding.

    property
  •   objectMapping

    The concrete object mapping for the operation.

    property
  •   metadata

    A dictionary of metadata available for mapping in addition to the source object.

    property

Configuring Delegate and Data Source

Accessing Mapping Details

  •   error

    The error, if any, that occurred during the execution of the mapping operation.

    property
  •   mappingInfo

    Returns a dictionary containing information about the mappings applied during the execution of the operation. The keys of the dictionary are key paths into the destinationObject for values that were mapped and the values are instances of RKMappingDetails that specify the object mapping and property mappings that were applied.

    property

Performing Mapping

  • – performMapping:

    Process all mappable values from the mappable dictionary and assign them to the target object according to the rules expressed in the object mapping definition

Properties

dataSource

The data source is responsible for providing the mapping operation with an appropriate target object for mapping when the destinationObject is nil.

@property (nonatomic, weak) id<RKMappingOperationDataSource> dataSource

Declared In

RKMappingOperation.h

delegate

The delegate to inform of interesting events during the mapping operation lifecycle.

@property (nonatomic, weak) id<RKMappingOperationDelegate> delegate

Declared In

RKMappingOperation.h

destinationObject

The target object for this operation. Mappable values in the source object will be applied to the destination object using key-value coding.

@property (nonatomic, strong, readonly) id destinationObject

Discussion

If initialized with a nil destination object, the mapping operation will attempt to find or create a destination object via the data source and will populate the value of the destinationObject property.

Declared In

RKMappingOperation.h

error

The error, if any, that occurred during the execution of the mapping operation.

@property (nonatomic, strong, readonly) NSError *error

Declared In

RKMappingOperation.h

mapping

The mapping defining how values contained in the source object should be transformed to the destination object via key-value coding.

@property (nonatomic, strong, readonly) RKMapping *mapping

Discussion

Will either be an instance of RKObjectMapping or RKDynamicMapping.

Declared In

RKMappingOperation.h

mappingInfo

Returns a dictionary containing information about the mappings applied during the execution of the operation. The keys of the dictionary are key paths into the destinationObject for values that were mapped and the values are instances of RKMappingDetails that specify the object mapping and property mappings that were applied.

@property (nonatomic, readonly) RKMappingInfo *mappingInfo

Discussion

Mapping info is aggregated for all child mapping operations executed for relationships.

Declared In

RKMappingOperation.h

metadata

A dictionary of metadata available for mapping in addition to the source object.

@property (nonatomic, copy) NSDictionary *metadata

Declared In

RKMappingOperation.h

objectMapping

The concrete object mapping for the operation.

@property (nonatomic, strong, readonly) RKObjectMapping *objectMapping

Discussion

If the value of mapping is an RKObjectMapping, returns the same value as mapping. If mapping is an RKDynamicMapping, then returns the concrete RKObjectMapping object selected for mapping sourceObject.

Declared In

RKMappingOperation.h

sourceObject

A dictionary of mappable elements containing simple values or nested object structures.

@property (nonatomic, strong, readonly) id sourceObject

Declared In

RKMappingOperation.h

Instance Methods

initWithSourceObject:destinationObject:mapping:

Initializes the receiver with a source object, a destination object and an object mapping with which to perform an object mapping.

- (id)initWithSourceObject:(id)sourceObject destinationObject:(id)destinationObject mapping:(RKMapping *)objectOrDynamicMapping

Parameters

sourceObject

The source object to be mapped. Cannot be nil.

destinationObject

The destination object the results are to be mapped onto. May be nil, in which case a new object target object will be obtained from the dataSource.

objectOrDynamicMapping

An instance of RKObjectMapping or RKDynamicMapping defining how the mapping is to be performed.

Return Value

The receiver, initialized with a source object, a destination object, and a mapping.

Declared In

RKMappingOperation.h

performMapping:

Process all mappable values from the mappable dictionary and assign them to the target object according to the rules expressed in the object mapping definition

- (BOOL)performMapping:(NSError **)error

Parameters

error

A pointer to an NSError reference to capture any error that occurs during the mapping. May be nil.

Return Value

A Boolean value indicating if the mapping operation was successful.

Declared In

RKMappingOperation.h