Dealing with JSON on iPhone

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You can easily use the JSON (JavaScript Object Notation) data format in client-server communications when writing an iPhone app. This blog is not suggesting that JSON is a more superior format for data exchange than its counterparts such as XML. In fact, we have many projects that don’t use JSON. However, handling JSON is relatively straight forward in ObjectiveC.

Unfortunately, Apple iPhone SDK (as of this writing, the latest is iPhone 2.2.1) doesn’t come with a built-in JSON parser. But I found out a good one called json-framework. It is both a generator and a parser. As a generator, json-framework can create JSON data from an NSDictionary. As a parser, you can pass to json-framework an NSString that consists of JSON data and it will return a NSDictionary that encapsulates the parsed data.

Next, I’m going to show you several examples. Before you proceed, download the library and make sure you add it to your SDK path list (see INSTALL file that comes with it). If setup properly, you should be able to start using the library by importing its header file:

#import “JSON/JSON.h”

Consider the following code:

NSDictionary *requestData = [NSDictionary dictionaryWithObjectsAndKeys:

 @”grio”@”username”,

 @”hellogrio”@”password”,

  nil];

This instantiates a new dictionary which we’ll turn into a JSON string. To do so, you’ll need to use a function called JSONRepresentation. This function is added as a category to NSObject. It means that as long as there’s an import of JSON.h file, you can call the function on any NSObject object.

NSString* jsonString = [requestData JSONRepresentation];


And this is what you got when you print (NSLog(@”%@”, jsonString);):


{“username”:”grio”,”password”:”hellogrio”}

 

Parsing is just as simple. Consider the following JSON data:
{
“menu”: {
“id”: “file”,
“value”: “File”,
“popup”: {
“menuitem”: [
{
“value”: “New”,
“onclick”: “CreateNewDoc()”
},
{
“value”: “Open”,
“onclick”: “OpenDoc()”
},
{
“value”: “Close”,
“onclick”: “CloseDoc()”
}
]
}
}
}
Assume that this is the data that you received from a web service called and is currently stored in an NSString called jsonResult. To parse it, you need to create SBJSON object and call one of its initialization method, objectWithString.

SBJSON *json = [[SBJSON new] autorelease];

NSError *jsonError;

NSDictionary *parsedJSON = [json objectWithString:jsonResult error:&jsonError];


If parsing fails for reasons such as invalid construct of JSON format, jsonError variable will be filled with the error info. If it is successful, parsedJSON will contain keys whose values are either an NSString or NSDictionary. Let’s look at the inside of parsedJSON:

NSDictionary* menu = [parsedJSON objectForKey:@”menu”];

NSLog(@”Menu id: %@”, [menu objectForKey:@”id”]);

NSLog(@”Menu value: %@”, [menu objectForKey:@”value”]);


And here’s the output:

Menu id: file

Menu value: File

Observe the JSON data again. popup is an NSDictionary which has an array of menuitem.

NSDictionary* popup = [menu objectForKey:@”popup”];

NSArray* menuItems = [popup objectForKey:@”menuitem”];

NSEnumerator *enumerator = [menuItems objectEnumerator];

NSDictionary* item;

while (item = (NSDictionary*)[enumerator nextObject]) {

NSLog(@”menuitem:value = %@”, [item objectForKey:@”value”]);

}


And this is the output:

menuitem:value = New

menuitem:value = Open

menuitem:value = Close

It’s that simple. Now you’re ready to use JSON in your next iPhone project.

8 Comments

  1. I liked the post. It was clear and explanatory.
    Only problem in this is going through the code due to the color combination. The blue color data on the black background is not visible. If there is chance please consider to change it.

  2. Nice article :-)
    We made too a similar thing but in French :p and with a tiny comparaison between xml and json (here).
    About json, I think there is a newer release of the library on the google project (since 12 sept I think).
    Good week :-)

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