Convert an iOS objective c object to a JSON string

Question

I have an objective C class like,

@interface message : NSObject {
 NSString *from;
 NSString *date;
 NSString *msg;
}

I have an NSMutableArray of instances of this message class. I want serialize all the instances in the NSMutableArray into a JSON file, using the new JSONSerialization APIs in iOS 5 SDK. How can I do this ?

Is creating a NSDictionary of each key, by iterating through each instance of the elements in the NSArray ? Can someone help with code of how to solve this ? I am not able to get good results in Google, as "JSON" skews the results to server-side calls and transfer of data instead of serialization. Thanks a lot.

EDIT:

NSError *writeError = nil; 
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:notifications options:NSJSONWritingPrettyPrinted error:&writeError];
NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding]; 
NSLog(@"JSON Output: %@", jsonString);

Damo · Accepted Answer · 2012-07-02 09:12:25Z

41

EDIT: I have made a dummy app that should be a good example for you.

I create a Message class from your code snippet;

//Message.h
@interface Message : NSObject {
    NSString *from_;
    NSString *date_;
    NSString *msg_;
}

@property (nonatomic, retain) NSString *from;
@property (nonatomic, retain) NSString *date;
@property (nonatomic, retain) NSString *msg;

-(NSDictionary *)dictionary;

@end

//Message.m
#import "Message.h"

@implementation Message

@synthesize from = from_;
@synthesize date = date_;
@synthesize msg = mesg_;

-(void) dealloc {
    self.from = nil;
    self.date = nil;
    self.msg = nil;
    [super dealloc];
}

-(NSDictionary *)dictionary {
    return [NSDictionary dictionaryWithObjectsAndKeys:self.from,@"from",self.date,    @"date",self.msg, @"msg", nil];
}

Then I set up an NSArray of two messages in the AppDelegate. The trick is that not only does the top level object (notifications in your case) need to be serializable but so do all the elements that notifications contains: Thats why I created the dictionary method in the Message class.

//AppDelegate.m
...
Message* message1 = [[Message alloc] init];
Message* message2 = [[Message alloc] init];

message1.from = @"a";
message1.date = @"b";
message1.msg = @"c";

message2.from = @"d";
message2.date = @"e";
message2.msg = @"f";

NSArray* notifications = [NSArray arrayWithObjects:message1.dictionary, message2.dictionary, nil];
[message1 release];
[message2 release];


NSError *writeError = nil; 
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:notifications options:NSJSONWritingPrettyPrinted error:&writeError];
NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding]; 
NSLog(@"JSON Output: %@", jsonString);

@end

The output when I run the application is thus:

2012-05-11 11:58:36.018 stack[3146:f803] JSON Output: [ { "msg" : "c", "from" : "a", "date" : "b" }, { "msg" : "f", "from" : "d", "date" : "e" } ]

ORIGINAL ANSWER:

Is this the documentation you are looking for?

edited Jul 2, 2012 at 9:12

answered May 11, 2012 at 9:05

Damo

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7 Comments

user439407 Over a year ago

The OP will first need to convert his object to a dictionary, but thats pretty straight forward(a static method will do the trick)

Sankar Over a year ago

Damo: that page does not explain about converting an array of custom class to JSON. That is what I am struggling to find.

Damo Over a year ago

If all your class ivars are NSString then it should just magically work.... caveat: I have not tried this.

Sankar Over a year ago

@Damo I get a runtime error: Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[message dictionaryRepresentation]: unrecognized selector sent to instance

Sankar Over a year ago

@Damo NSError *writeError = nil; NSData *jsonData = [NSJSONSerialization dataWithJSONObject:notifications options:NSJSONWritingPrettyPrinted error:&writeError]; NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding]; NSLog(@"JSON Output: %@", jsonString); notifications is an NSMutableArray of message objects

|

Alphapico · Accepted Answer · 2013-05-27 10:29:41Z

8

Now you can solve this problem easily using JSONModel. JSONModel is a library that generically serialize/deserialize your object based on Class. You can even use non-nsobject based for property like int, short and float. It can also cater nested-complex JSON. It handles error checking for you.

Deserialize example. in header file:

#import "JSONModel.h"

@interface Message : JSONModel 
@property (nonatomic, strong) NSString* from;
@property (nonatomic, strong) NSString* date;
@property (nonatomic, strong) NSString* message;
@end

in implementation file:

#import "JSONModelLib.h"
#import "yourPersonClass.h"

NSString *responseJSON = /*from somewhere*/;
Message *message = [[Message alloc] initWithString:responseJSON error:&err];
if (!err)
{
   NSLog(@"%@  %@  %@", message.from, message.date, message.message):
}

Serialize Example. In implementation file:

#import "JSONModelLib.h"
#import "yourPersonClass.h"

Message *message = [[Message alloc] init];
message.from = @"JSON beast";
message.date = @"2012";
message.message = @"This is the best method available so far";

NSLog(@"%@", [person toJSONString]);

edited May 27, 2013 at 10:29

answered May 27, 2013 at 9:54

Alphapico

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1 Comment

Mahmoud Adam Over a year ago

what if we want to serialize Array of messages using JSONModel?

Brad Parks · Accepted Answer · 2014-03-04 13:10:57Z

2

Note: This will only work with serializable objects. This answer was provided above in an edit to the question itself, but I always look for answers in the "answers" section myself ;-)

- (NSString*) convertObjectToJson:(NSObject*) object
{
    NSError *writeError = nil;

    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:object options:NSJSONWritingPrettyPrinted error:&writeError];
    NSString *result = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];

    return result;
}

edited Mar 4, 2014 at 13:10

answered Mar 4, 2014 at 1:05

Brad Parks

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2 Comments

kemdo Over a year ago

Hi. How to convert array of NSObject. Thanks you :)

Brad Parks Over a year ago

hey @kemdo - not sure - and I'm not doing ios dev anymore, so can't really say.... Good luck!

Burrows Wang · Accepted Answer · 2015-10-29 04:29:03Z

1

Here is a library i used in my projects BWJSONMatcher, which can help you easily match your json string up with your data model with no more than one line of code.

...
NSString *jsonString = @"{your-json-string}";
YourValueObject *dataModel = [YourValueObject fromJSONString:jsonString];

NSDictionary *jsonObject = @{your-json-object};
YourValueObject *dataModel = [YourValueObject fromJSONObject:jsonObject];
...
YourValueObject *dataModel = instance-of-your-value-object;
NSString *jsonString = [dataModel toJSONString];
NSDictionary *jsonObject = [dataModel toJSONObject];
...

answered Oct 29, 2015 at 4:29

Burrows Wang

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