Working with and Parsing JSON - iOS Components and Frameworks: Understanding the Advanced Features of the iOS SDK (2014)

iOS Components and Frameworks: Understanding the Advanced Features of the iOS SDK (2014)

Chapter 7. Working with and Parsing JSON

JSON is a great way to send data back and forth between servers, websites, and iOS apps. It is lighter and easier to handle than XML, and with iOS’s built-in support for JSON, it is easy to integrate into an iOS project. Many popular websites, including Flickr, Twitter, and Google, offer APIs that provide results in JSON format, and many languages offer JSON support. This chapter demonstrates how to parse and present JSON from a sample message-board server in an app, and encode a new message entry in JSON to send to the server.


JavaScript Object Notation (JSON) is a lightweight format for sharing data. It is technically a part of the language JavaScript and provides a way to serialize JavaScript objects; however, practically, it is supported in a wide variety of programming languages, making it a great candidate for sharing data between different platforms. JSON also has the benefit of being human-readable.

JSON has a simple and intuitive syntax. At its most basic level, a JSON document can contain “objects,” which are essentially key-value dictionaries like Objective-C programmers are familiar with, or arrays. JSON can contain arrays of objects and arrays of values, and can nest arrays and objects. Values stored in JSON, either in arrays or associated with a key, can be other JSON objects, strings, numbers, or arrays, or true, false, or null.

Benefits of Using JSON

There are many reasons to use JSON in an iOS app:

Image Server Support: Communicating information to and from a remote server is a common use case for iOS apps. Since so many server languages have built-in support for JSON, it is a natural choice as a data format.

Image Lightweight: JSON has little formatting overhead when compared to XML and can present a significant savings in the amount of bandwidth needed to transmit data between a server and a device.

Image iOS Support: JSON is now fully supported as of iOS 5 with the addition of the NSJSONSerialization class. This class can conveniently provide an NSDictionary or NSArray (or even mutable varieties) from JSON data or can encode an NSDictionary or NSArray into JSON.

Image Presentation and Native Handling: The simplest method to get data from a server to an iOS device is just to use a UIWebView and display a web page; however, this approach has drawbacks in terms of performance and presentation. In many cases it is much better to just pull the data from the server, and present it on the device using native tools like UITableView. Performance can be much better, and presentation can be optimized to work on iOS screen sizes and take advantage of available retina displays.

JSON Resources

For more information on JSON, visit That site has a formal definition of JSON, with specific information on format and syntax.

Sample App Overview

The sample app for this chapter is Message Board, including a Ruby on Rails server and an iOS app.

The Ruby on Rails server consists of just one object: the message. It has been set up to support sending a list of messages in JSON, and to accept new messages in JSON format. The server also supports Web-based interactions.

The iOS app will pull messages from the server and display them in a standard table view and will be able to post new messages to the server in JSON format.

Accessing the Server

To view the Message Board Ruby on Rails server, visit The Messages home screen will be visible, as shown in Figure 7.1.


Figure 7.1 Messages home screen.

The messages server has been set up to handle creating and displaying messages on the Web and with JSON.

Getting JSON from the Server

To update the sample iOS app to handle JSON, the first thing to address is pulling the message list from the server and displaying it.

Building the Request

First set up the URL (in the MessageBoard-Prefix.pch file) so that the app can make calls to the right location:

#define kMessageBoardServerURLString


In the ICFViewController.m implementation, look at the viewWillAppear: method. This code will initiate the request to the server:

NSURL *msgURL = [NSURL URLWithString:kMessageBoardURLString];
NSURLRequest *msgRequest =
[NSURLRequest requestWithURL:msgURL

NSURLConnection *theConnection =
[[NSURLConnection alloc] initWithRequest:msgRequest

if (theConnection) {
NSMutableData *connData = [[NSMutableData alloc] init];
[self setConnectionData:connData];
} else {
NSLog(@"Connection failed...");
[self.activityView setHidden:YES];
[self.activityIndicator stopAnimating];


This creates and initiates a network request to the messages.json resource at the server URL. If the connection works, it sets up an NSData ivar to store the response data; otherwise, it just logs a failure. The network request will run asynchronously, and when data comes back it will hit the delegate methods set up in the class. The important thing to note is that nothing special is required here for JSON; this is a standard network call. The only difference is that the .json extension used in the URL tells the server the response should be in JSON format.


Using the .json extension is not required for servers to return JSON format data; that is just how the sample server was set up. It is a common approach but is not required.

Inspecting the Response

To receive data from the network connection, implement two delegate methods:

- (void)connection:(NSURLConnection *)connection
didReceiveResponse:(NSURLResponse *)response
[self.connectionData setLength:0];

- (void)connection:(NSURLConnection *)connection
didReceiveData:(NSData *)data
[self.connectionData appendData:data];

The first method just sets the data length to zero when the response from the server is initiated. Note that, in some cases, this can happen more than once (if the server initiates a redirect, for example). The second method just appends any received data to a local data object.

When the network connection is complete, it will call this delegate method:

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
NSString *retString =
[NSString stringWithUTF8String:[connectionData bytes]];

NSLog(@"json returned: %@", retString);


The log message will display on the console the data received:

json returned: [{"message":{"created_at":"2012-04-29T21:59:28Z",
"id":3, "message":"JSON is fun!", "message_date":"2012-04-29",
"message":"Learning about JSON", "message_date":"2012-04-
29","name":"Joe", "updated_at":"2012-04-29T21:59:38Z"}},
"message":"Wow, JSON is easy.", "message_date":"2012-04-
29","name":"Kyle", "updated_at":"2012-04-29T22:00:00Z"}},
"message":"Trying a new message.", "message_date":"2012-04-
29","name":"Joe", "updated_at":"2012-04-29T22:46:18Z"}}]

Parsing JSON

Now that JSON has been received from the server, it is just a simple step to parse it. In the case of the sample app, an array of messages is expected, so parse the JSON into an NSArray:

- (void)connectionDidFinishLoading:(NSURLConnection *)connection

NSError *parseError = nil;
NSArray *jsonArray =
[NSJSONSerialization JSONObjectWithData:connectionData

if (!parseError) {
[self setMessageArray:jsonArray];
NSLog(@"json array is %@", jsonArray);
[messageTable reloadData];
} else {
NSString *err = [parseError localizedDescription];
NSLog(@"Encountered error parsing: %@", err);

[connection release], connection = nil;
[connectionData release], connectionData = nil;

[self.activityView setHidden:YES];
[self.activityIndicator stopAnimating];

NSJSONSerialization's method JSONObjectWithData:options:error: expects as parameters the data to be serialized, any desired options (for example, returning a mutable array instead of a regular array), and a reference to an NSError in case there are any parsing errors.

In this example, a local instance variable has been updated to the just-parsed array, and the table view has been told to reload data now that there is data to display.

Displaying the Data

Now that the JSON has been parsed into an NSArray, it can be displayed in a UITableView. The magic here is that there is no magic; the JSON received from the server is now just an array of NSDictionary instances. Each NSDictionary contains information for a message from the server, with attribute names and values. To display this in a table, just access the array and dictionaries as if they had been created locally.

- (UITableViewCell *)tableView:(UITableView *)tableView
cellForRowAtIndexPath:(NSIndexPath *)indexPath
UITableViewCell *cell =
[tableView dequeueReusableCellWithIdentifier:@"MsgCell"];

if (cell == nil) {
cell = [[[UITableViewCell alloc]
reuseIdentifier:@"MsgCell"] autorelease];

cell.selectionStyle = UITableViewCellSelectionStyleNone;
NSDictionary *message =
(NSDictionary *)[[self.messageArray

NSString *byLabel =
[NSString stringWithFormat:@"by %@ on %@",
[message objectForKey:@"name"],
[message objectForKey:@"message_date"]];

cell.textLabel.text = [message objectForKey:@"message"];
cell.detailTextLabel.text = byLabel;
return cell;

- (NSInteger)tableView:(UITableView *)tableView
return [[self messageArray] count];

The parsed JSON data will be visible in a standard table view, as shown in Figure 7.2.


Figure 7.2 Sample app message table view.

Posting a Message

The sample app includes ICFNewMessageViewController to post new messages to the server. There are two fields on that controller: one for a name and one for a message (see Figure 7.3). After the user enters that information and hits Save, it will be encoded in JSON and sent to the server.


Figure 7.3 Sample app new message view.

Encoding JSON

An important detail for sending JSON to a Ruby on Rails server is to encode the data so that it mirrors what the Rails server provides. When sending a new message to the server, it should have the same structure as an individual message received in the message list. To do this, a dictionary with the attribute names and values for the message is needed, and then a wrapper dictionary with the key “message” pointing to the attribute dictionary. This will exactly mirror what the server sends for a message. In the saveButtonTouched: method set up this dictionary, like so:

NSMutableDictionary *messageDictionary =
[NSMutableDictionary dictionaryWithCapacity:1];

[messageDictionary setObject:[nameTextField text]

[messageDictionary setObject:[messageTextView text]

NSDate *today = [NSDate date];

NSDateFormatter *dateFormatter =
[[NSDateFormatter alloc] init];

NSString *dateFmt = @"yyyy'-'MM'-'dd'T'HH':'mm':'ss'Z'";
[dateFormatter setDateFormat:dateFmt];
[messageDictionary setObject:[dateFormatter stringFromDate:today]

[dateFormatter release];

NSDictionary *postDictionary =
[NSDictionary dictionaryWithObject:messageDictionary

Note that NSJSONSerialization accepts only instances of NSDictionary, NSArray, NSString, NSNumber, or NSNull. For dates or other data types not directly supported by NSJSONSerialization, they will need to be converted to a supported format. For example, in this example the date was converted to a string in a format expected by the server. Now that there is a dictionary, it is a simple step to encode it in JSON:

NSError *jsonSerializationError = nil;
NSData *jsonData = [NSJSONSerialization

if (!jsonSerializationError)
NSString *serJSON =
[[NSString alloc] initWithData:jsonData

NSLog(@"serialized json: %@", serJSON);
} else
NSLog(@"JSON Encoding failed: %@",
[jsonSerializationError localizedDescription]);

NSJSONSerialization expects three parameters:

1. An NSDictionary or NSArray with the data to be encoded.

2. Serialization options (in our case, we specified NSJSONWritingPrettyPrinted so that it’s easy to read; otherwise, the JSON is produced with no whitespace for compactness).

3. A reference to an NSError.

If there are no errors encoding the JSON, it will look like this:

serialized json: {
"message" : {
"message" : "Six Test Messages",
"name" : "Joe",
"message_date" : "2012-04-01T14:31:11Z"

Sending JSON to the Server

After the JSON is encoded, there are a few important additional steps to send it to the server:

NSURL *messageBoardURL =
[NSURL URLWithString:kMessageBoardURLString];

NSMutableURLRequest *request = [NSMutableURLRequest

[request setHTTPMethod:@"POST"]; // 1

[request setValue:@"application/json"
forHTTPHeaderField:@"Accept"]; //2

[request setValue:@"application/json"
forHTTPHeaderField:@"Content-Type"]; //2

[request setValue:[NSString stringWithFormat:@"%d",
[jsonData length]] forHTTPHeaderField:@"Content-Length"]; //3

[request setHTTPBody: jsonData]; //4

NSURLConnection *jsonConnection =
[[NSURLConnection alloc] initWithRequest:request

if (jsonConnection)
NSMutableData *connData = [[NSMutableData alloc] init];
[self setConnectionData:connData];
[connData release];
} else
NSLog(@"Connection failed...");

The first step is to make sure that the request method is set to POST. This will ensure that the correct method on the server (create) gets called.

The second step is to set the "Accept" and "Content-Type" headers on the request to "application/json". This will inform the server that the app is sending JSON and it should treat the body of the request as JSON data.

The third step is to set the content length of the post. This is simple to determine; it is just the length of the JSON data being attached as the body of the request.

The fourth step (and arguably the most important) is to set the body of the request to be the encoded JSON data.

After all that is complete, the connection can be initiated, which will send the new message to the server asynchronously. When the connection finishes, dismiss the create view and refresh the message list.


This chapter introduced JSON (or JavaScript Object Notation). It explained how to request JSON data from a server in an iOS app, parse it, and display it in a table. The chapter described how to encode an NSDictionary or NSArray into JSON, and send it over the network to a server.


1. Add logic to the message list network call and the message post network call to check the HTTP response code. Handle the error if it is not 200 OK.