从类返回NSURLConnection数据

Question

I have am NSURLConnection to get a JSON value from my web server. The code is placed in a class which can be called from other UIViewControllers.

I am a little unsure on how to return the JSON data from the class however.

Here is what I have tried: +(NSJSONSerialization *) getTask:(id)task_id{

    NSJSONSerialization *json;

    NSLog(@"task id = %@", task_id);

    NSString *post = [NSString stringWithFormat:@"&task_id=%@", task_id];

    NSData *postData = [post dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES];
    NSString *postLength = [NSString stringWithFormat:@"%lu",(unsigned long)[postData length]];
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
    [request setURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://www.iampeterstuart.co.uk/todo/index.php/task/get"]]];
    [request setHTTPMethod:@"POST"];
    [request setValue:postLength forHTTPHeaderField:@"Content-Length"];
    [request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Current-Type"];
    [request setHTTPBody:postData];

    NSURLConnection *conn = [[NSURLConnection alloc]initWithRequest:request delegate:self startImmediately:YES];

    [conn start];

    return json;
}

// Log the response for debugging
+ (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
    NSLog(@"Resonse: %@", response);
}
+ (void)connection:(NSURLConnection *)connection didReceiveData:(NSData*)data {
    NSString *jsonString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    NSData *jsonData = [jsonString dataUsingEncoding:NSUTF8StringEncoding];
    NSJSONSerialization *json = [NSJSONSerialization JSONObjectWithData:jsonData options:0 error:nil];
}

// Declare any connection errors
+ (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    NSLog(@"Error: %@", error);
}

Each time the variable json is null?

Could somebody please advice?

Many thanks,

Peter

Answer 1

You are thinking incorrectly about the delegate's behavior.

(void)connection:(NSURLConnection *)connection didReceiveData:(NSData*)data will be called whenever new data is available (downloaded from the server), which is the raw bytes of the byte stream. You should have an instance of NSMutableData and append that received data to the mutable data when data is received. Finally, when the whole data (eg JSON string in your example) is downloaded, -(void)connectionDidFinishLoading:(NSURLConnection *)connection will be called. In that method, you should parse the JSON string:

NSString *jsonString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSData *jsonData = [jsonString dataUsingEncoding:NSUTF8StringEncoding];
NSJSONSerialization *json = [NSJSONSerialization JSONObjectWithData:jsonData options:0 error:nil];

...where data is the cumulative data received. You can create the data instance as below in the class implementation:

NSMutableData *cumulativeData;

In init or whatever method that is initially called on your delegate before you start loading, call:

cumulativeData = [NSMutableData data];

Then in your (void)connection:(NSURLConnection *)connection didReceiveData:(NSData*)data method, call:

[cumulativeData appendData:data];

That should append the received data into the cumulative data that you are "building".