Instantiating Custom Class from NSDictionary

Question

I have a feeling that this is stupid question, but I'll ask anyway...

I have a collection of NSDictionary objects whose key/value pairs correspond to a custom class I've created, call it MyClass. Is there an easy or "best practice" method for me to basically do something like MyClass * instance = [map NSDictionary properties to MyClass ];? I have a feeling I need to do something with NSCoding or NSKeyedUnarchiver, but rather than stumble through it on my own, I figure someone out there might be able to point me in the right direction.

Answer 1

The -setValuesForKeysWithDictionary: method, along with -dictionaryWithValuesForKeys:, is what you want to use.

Example:

// In your custom class
+ (id)customClassWithProperties:(NSDictionary *)properties {
   return [[[self alloc] initWithProperties:properties] autorelease];
}

- (id)initWithProperties:(NSDictionary *)properties {
   if (self = [self init]) {
      [self setValuesForKeysWithDictionary:properties];
   }
   return self;
}

// ...and to easily derive the dictionary
NSDictionary *properties = [anObject dictionaryWithValuesForKeys:[anObject allKeys]];

Answer 2

There is no allKeys on NSObject. You'll need to create an extra category on NSObject like below:

NSObject+PropertyArray.h

@interface NSObject (PropertyArray)
- (NSArray *) allKeys;
@end

NSObject+PropertyArray.m

#import <objc/runtime.h>

@implementation NSObject (PropertyArray)
- (NSArray *) allKeys {
    Class clazz = [self class];
    u_int count;

    objc_property_t* properties = class_copyPropertyList(clazz, &count);
    NSMutableArray* propertyArray = [NSMutableArray arrayWithCapacity:count];
    for (int i = 0; i < count ; i++) {
        const char* propertyName = property_getName(properties[i]);
        [propertyArray addObject:[NSString  stringWithCString:propertyName encoding:NSUTF8StringEncoding]];
    }
    free(properties);

   return [NSArray arrayWithArray:propertyArray];
}
@end

Example:

#import "NSObject+PropertyArray.h"

...

MyObject *obj = [[MyObject alloc] init];
obj.a = @"Hello A";  //setting some values to attributes
obj.b = @"Hello B";

//dictionaryWithValuesForKeys requires keys in NSArray. You can now
//construct such NSArray using `allKeys` from NSObject(PropertyArray) category
NSDictionary *objDict = [obj dictionaryWithValuesForKeys:[obj allKeys]];

//Resurrect MyObject from NSDictionary using setValuesForKeysWithDictionary
MyObject *objResur = [[MyObject alloc] init];
[objResur setValuesForKeysWithDictionary:objDict];

Answer 3

Assuming that your class conforms to the Key-Value Coding protocol, you could use the following: (defined as a category on NSDictionary for convenience):

// myNSDictionaryCategory.h:
@interface NSDictionary (myCategory)
- (void)mapPropertiesToObject:(id)instance
@end

// myNSDictionaryCategory.m:
- (void)mapPropertiesToObject:(id)instance
{
    for (NSString * propertyKey in [self allKeys])
    {
        [instance setValue:[self objectForKey:propertyKey]
                    forKey:propertyKey];
    }
}

And here's how you would use it:

#import "myNSDictionaryCategory.h"
//...
[someDictionary mapPropertiesToObject:someObject];

Answer 4

If your doing this sort of thing chances are your dealing with JSON and you should probably have a look at Mantle https://github.com/Mantle/Mantle

You will then get a convenient method dictionaryValue

[anObject dictionaryValue];

Answer 5

Just add category for NSObject for getting dictionaryRepresentation from your custom objects (in my case using in JSON serialization only):

//  NSObject+JSONSerialize.h
#import <Foundation/Foundation.h>

@interface NSObject(JSONSerialize)

- (NSDictionary *)dictionaryRepresentation;

@end

//  NSObject+JSONSerialize.m
#import "NSObject+JSONSerialize.h"
#import <objc/runtime.h>

@implementation NSObject(JSONSerialize)

+ (instancetype)instanceWithDictionary:(NSDictionary *)aDictionary {
    return [[self alloc] initWithDictionary:aDictionary];
}

- (instancetype)initWithDictionary:(NSDictionary *)aDictionary {
    aDictionary = [aDictionary clean];

    self.isReady = NO;

    for (NSString* propName in [self allPropertyNames]) {
        [self setValue:aDictionary[propName] forKey:propName];
    }

    //You can add there some custom properties with wrong names like "id"
    //[self setValue:aDictionary[@"id"] forKeyPath:@"objectID"];
    self.isReady = YES;

    return self;
}

- (NSDictionary *)dictionaryRepresentation {
    NSMutableDictionary *result = [NSMutableDictionary dictionary];
    NSArray *propertyNames = [self allPropertyNames];

    id object;
    for (NSString *key in propertyNames) {
        object = [self valueForKey:key];
        if (object) {
            [result setObject:object forKey:key];
        }
    }

    return result;
}

- (NSArray *)allPropertyNames {
    unsigned count;
    objc_property_t *properties = class_copyPropertyList([self class], &count);

    NSMutableArray *rv = [NSMutableArray array];

    unsigned i;
    for (i = 0; i < count; i++) {
        objc_property_t property = properties[i];
        NSString *name = [NSString stringWithUTF8String:property_getName(property)];
        [rv addObject:name];
    }
    //You can add there some custom properties with wrong names like "id"
    //[rv addObject:@"objectID"];
    //Example use inside initWithDictionary:
    //[self setValue:aDictionary[@"id"] forKeyPath:@"objectID"];

    free(properties);

    return rv;
}

@end

Also, you can see that my solution will not work with custom objects with nested objects or arrays. For Arrays - just change the lines of code in dictionaryRepresentation method:

    if (object) {
        if ([object isKindOfClass:[NSArray class]]) {
            @autoreleasepool {
                NSMutableArray *array = [NSMutableArray array];
                for (id item in (NSArray *)object) {
                    [array addObject:[item dictionaryRepresentation]];
                }

                [result setObject:array forKey:key];
            }
        } else {
            [result setObject:object forKey:key];
        }
    }