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

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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",
"name":"Joe","updated_at":"2012-04-29T21:59:28Z"}},
{"message":{"created_at":"2012-04-29T21:58:50Z","id":2,
"message":"Learning about JSON", "message_date":"2012-04-
29","name":"Joe", "updated_at":"2012-04-29T21:59:38Z"}},
{"message":{"created_at":"2012-04-29T22:00:00Z","id":4,
"message":"Wow, JSON is easy.", "message_date":"2012-04-
29","name":"Kyle", "updated_at":"2012-04-29T22:00:00Z"}},
{"message":{"created_at":"2012-04-29T22:46:18Z","id":5,
"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
options:0
error:&parseError];

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]
initWithStyle:UITableViewCellStyleSubtitle
reuseIdentifier:@"MsgCell"] autorelease];

cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
NSDictionary *message =
(NSDictionary *)[[self.messageArray
objectAtIndex:indexPath.row]
objectForKey:@"message"];

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
numberOfRowsInSection:(NSInteger)section
{
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]
forKey:@"name"];

[messageDictionary setObject:[messageTextView text]
forKey:@"message"];

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]
forKey:@"message_date"];

[dateFormatter release];

NSDictionary *postDictionary =
[NSDictionary dictionaryWithObject:messageDictionary
forKey:@"message"];

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
dataWithJSONObject:postDictionary
options:NSJSONWritingPrettyPrinted
error:&jsonSerializationError];

if (!jsonSerializationError)
{
NSString *serJSON =
[[NSString alloc] initWithData:jsonData
encoding:NSUTF8StringEncoding];

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
requestWithURL:messageBoardURL
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:30.0];

[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
delegate:self];

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.

Summary

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.

Exercise

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.