Class to handle Server Sent Events in a similar way to browser EventSource

EventSource.h

#import <Foundation/Foundation.h>


@class EventSource;

@protocol EventSourceDelegate <NSObject>

@optional
- (void)eventSourceDidOpenConnection:(EventSource*)eventSource;
- (void)eventSource:(EventSource*)eventSource didFailWithError:(NSError*)error;

@required
- (void)eventSource:(EventSource *)eventSource didReceiveMessage:(NSString*)message
                                                         eventID:(NSString*)eventID
                                                            type:(NSString*)type;

@end

@interface EventSource : NSObject

- (id)initWithURL:(NSURL*)url delegate:(NSObject<EventSourceDelegate>*)delegate;
- (void)cancel;

@property (weak, nonatomic) NSObject <EventSourceDelegate> *delegate;
@property (nonatomic, readonly) NSURL *url;
@property (nonatomic, readonly) NSString *lastEventID;

@end

EventSource.m

#import "EventSource.h"


#define kErrorCodeBadResponseStatusCode 1
#define kErrorCodeBadResponseContentType 2


@interface EventSource () <NSURLConnectionDelegate, NSURLConnectionDataDelegate>

@property (nonatomic, strong) NSURL *url;
@property (nonatomic, strong) NSURLConnection *connection;
@property (nonatomic, strong) NSMutableString *currentMessage;

@end

@implementation EventSource

- (id)initWithURL:(NSURL*)url delegate:(NSObject<EventSourceDelegate>*)delegate
{
    self = [super init];
    if (self)
    {
        _url = url;
        _delegate = delegate;
        
        // start connection
        _currentMessage = [NSMutableString string];
        NSURLRequest *request = [NSURLRequest requestWithURL:url];
        _connection = [NSURLConnection connectionWithRequest:request delegate:self];
    }
    return self;
}

- (void)cancel
{
    [_connection cancel];
}

#pragma mark - NSURLConnectionDelegate methods

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    if ([_delegate respondsToSelector:@selector(eventSource:didFailWithError:)]) {
        [_delegate eventSource:self didFailWithError:error];
    }
}

#pragma mark - NSURLConnectionDataDelegate methods

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;
    
    // ensure we've got a successful response
    if ([[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(200, 100)] containsIndex:[httpResponse statusCode]])
    {
        // ensure HTTP content-type is text/event-stream
        if ([httpResponse.allHeaderFields[@"content-type"] isEqualToString:@"text/event-stream"])
        {
            if ([_delegate respondsToSelector:@selector(eventSourceDidOpenConnection:)]) {
                [_delegate eventSourceDidOpenConnection:self];
            }
        }
        else
        {
            if ([_delegate respondsToSelector:@selector(eventSource:didFailWithError:)])
            {
                NSError *error = [NSError errorWithDomain:NSCocoaErrorDomain
                                                     code:kErrorCodeBadResponseContentType
                                                 userInfo:@{ NSLocalizedDescriptionKey: NSLocalizedString(@"Server must respond with a content-type of 'text/event-stream'.", nil) }];
                
                [_delegate eventSource:self didFailWithError:error];
            }
            
            [_connection cancel];
        }
    }
    else
    {
        if ([_delegate respondsToSelector:@selector(eventSource:didFailWithError:)])
        {
            NSError *error = [NSError errorWithDomain:NSCocoaErrorDomain
                                                 code:kErrorCodeBadResponseStatusCode
                                             userInfo:@{ NSLocalizedDescriptionKey: NSLocalizedString(@"Server must respond with a status code in the 200-299 range.", nil) }];
            
            [_delegate eventSource:self didFailWithError:error];
        }
        
        [_connection cancel];
    }
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    NSString *receivedString = [[NSString alloc] initWithData:data
                                                     encoding:NSUTF8StringEncoding];
    [_currentMessage appendString:receivedString];
    
    NSArray *messages = [_currentMessage componentsSeparatedByString:@"\n\n"];
    
    for (int i = 0; i < messages.count - 1; i++)
    {
        [self processMessage:messages[i]];
    }
    
    _currentMessage = [NSMutableString stringWithString:[messages lastObject]];
}


#pragma mark - Privates

- (void)processMessage:(NSString*)message
{
    NSMutableString *eventMessage = [NSMutableString string];
    NSString *eventType = nil;
    NSString *eventID = nil;
    
    NSArray *lines = [message componentsSeparatedByString:@"\n"];
    
    for (NSString *line in lines)
    {
        if ([line hasPrefix:@":"])
        {
            // comment; do nothing
        }
        else if ([line hasPrefix:@"id:"])
        {
            // id
            NSString *value = [line substringFromIndex:3];
            value = [value stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
            eventID = value;
            _lastEventID = value;
        }
        else if ([line hasPrefix:@"event:"])
        {
            // event
            NSString *value = [line substringFromIndex:6];
            value = [value stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
            eventType = value;
        }
        else if ([line hasPrefix:@"data:"])
        {
            // data
            NSString *value = [line substringFromIndex:5];
            value = [value stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
            [eventMessage appendFormat:@"%@\n", value];
        }
    }
    
    [_delegate eventSource:self didReceiveMessage:eventMessage eventID:eventType type:eventType];
}

@end

You might want to replace https://gist.github.com/elsurudo/6039065#file-eventsource-m-L80 with

if ([[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(200, 100)] containsIndex:[httpResponse statusCode]]) {

to account for all possible HTTP success codes, instead of just 200.

You also might consider ensuring the HTTP content-type is text/event-stream before parsing.

Thanks for the tips! Will include those in the next revision. I just posted an update that gets rid of the unnecessary EventSourceMessage class (seemed un-Cocoa-like to do that, instead of just passing those fields to the delegate).

Also, I refactored to ensure that we can handle the case where the server might buffer several full messages, plus, say, only the first half of the last message. Should be more robust now; let me know what you think.

change [httpResponse.allHeaderFields[@"content-type"] isEqualToString:@"text/event-stream"] to [httpResponse.allHeaderFields[@"content-type"] hasPrefix:@"text/event-stream"] may be better.