Instantiating Custom Class from NSDictionary - iphone

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];
}
}