Question

I’m developing an iOS 4 app for iPad with latest SDK and XCode 4.2.

I have a problem with a JSON web service. I’m using a timer to check every 3 seconds if some data has changed. The problem is that data changes, but I can’t see that changes on my app.

Data received asynchronously is always the same. It is like NSURLConnection has a cache or something like that, and it always returning the first data it takes.

Here is my code:

ControlPanelJSON.h

#import <Foundation/Foundation.h>
#import "SBJson.h"
#import "WebServiceDelegate.h"

@interface ControlPanelJSON : NSObject
{
    SBJsonParser* parser;
    NSURLRequest* request;
    NSMutableData* receivedData;

    BOOL isEncuestaVisible;
    BOOL isInfoVisible;
    BOOL isTwitterVisible;

    id<WebServiceDelegate> delegate;
}
@property (nonatomic, readonly) BOOL isEncuestaVisible;
@property (nonatomic, readonly) BOOL isInfoVisible;
@property (nonatomic, readonly) BOOL isTwitterVisible;

- (id) initWithWebServiceURLString:(NSString*)webServiceURL
                          delegate:(id<WebServiceDelegate>)del;

- (void)callWebService;

@end

ControlPanelJSON.m

#import "ControlPanelJSON.h"

#define kRootKey @"s"
#define kControlKey @"c"
#define kEstadoKey @"e"

#define kEncuestaTitle @"[ zzz ]"
#define kInfoTitle @"[ yyy ]"
#define kTwitterTitle @"[ xxx ]"

@implementation ControlPanelJSON

@synthesize isEncuestaVisible;
@synthesize isInfoVisible;
@synthesize isTwitterVisible;

- (id) initWithWebServiceURLString:(NSString*)webServiceURL
                          delegate:(id<WebServiceDelegate>)del
{
    if (self = [super init])
    {
        delegate = [del retain];
        request = [[NSURLRequest alloc] initWithURL: [NSURL URLWithString: webServiceURL]
                                        cachePolicy: NSURLCacheStorageNotAllowed
                                    timeoutInterval: 60.0];
        parser = [[SBJsonParser alloc] init];
    }
    return self;
}

- (void) dealloc
{
    [parser release];
    [request release];
    [receivedData release];

    [super dealloc];
}

- (void)callWebService
{
    NSURLConnection *theConnection=[[NSURLConnection alloc] initWithRequest:request delegate:self];
    if (theConnection)
    {
        // Create the NSMutableData to hold the received data.
        // receivedData is an instance variable declared elsewhere.
        receivedData = [[NSMutableData data] retain];
    }
    else
    {
        [delegate errorReceivedWithMessage:NSLocalizedString(@"CONNERROR", nil)];
    }
}

#pragma mark -
#pragma mark - NSURLConnectionDelegate

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    // This method is called when the server has determined that it
    // has enough information to create the NSURLResponse.

    // It can be called multiple times, for example in the case of a
    // redirect, so each time we reset the data.

    // Cuando se recibe este mensaje se debe descartar todo lo recibido anteriormente.
    [receivedData setLength:0];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    // Append the new data to receivedData.
    // receivedData is an instance variable declared elsewhere.
    [receivedData appendData:data];
}

- (void)connection:(NSURLConnection *)connection
  didFailWithError:(NSError *)error
{
    // release the connection, and the data object
    [connection release];
    // receivedData is declared as a method instance elsewhere
    [receivedData release];

    // inform the user
    [delegate errorReceivedWithMessage:
     [NSString stringWithFormat:@"Error - %@ %@",
      [error localizedDescription],
      [[error userInfo] objectForKey:NSURLErrorFailingURLStringErrorKey]]];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{   
    NSString *json_string = [[NSString alloc] initWithData:receivedData encoding:NSUTF8StringEncoding];
    NSDictionary* datos = [parser objectWithString:json_string error:nil]; // TODO: Otro error que manejar

    [connection release];
    //[receivedData release];
    //[parser release];
    [json_string release];

    NSArray* data = [datos objectForKey:kRootKey];
    for (int i = 0; i < data.count; i++)
    {
        NSDictionary* object = [data objectAtIndex:i];
        NSString* controlName = [object objectForKey:kControlKey];
        NSString* controlEstado = [object objectForKey:kEstadoKey];

        if ([controlName isEqualToString: kEncuestaTitle])
        {
            isEncuestaVisible = ([controlEstado isEqualToString: @"1"]);
            continue;
        }
        else if ([controlName isEqualToString: kInfoTitle])
        {
            isInfoVisible = ([controlEstado isEqualToString: @"1"]);
            continue;
        }
        else if ([controlName isEqualToString: kTwitterTitle])
        {
            isTwitterVisible = ([controlEstado isEqualToString: @"1"]);
            continue;
        }
    }

    [delegate dataReceived];
}

@end

After this code line:

NSString *json_string = [[NSString alloc] initWithData:receivedData encoding:NSUTF8StringEncoding];

I added a NSLog, and I always getting the same json_string

{“ctrls”: [ { “control”: “[ xxx ]”,”estado”: “0” },{ “control”: “[ yyy ]”,”estado”: “0” },{ “control”: “[ zzz ]”,”estado”: “0” } ] }

Any clue?

Answer 1

Try changing the cache policy when you create your NSURLRequest to:

NSURLRequestReloadIgnoringLocalAndRemoteCacheData