Data transfer between NSOperations - objective-c

Question

I would like to obtain the following: I have two NSOperations in a NSOperationQueue. The firs is a download from a website (gets some json data) the next is parsing that data. This are dependent operations.
I don't understand how to link them together. If they are both allocated and in the queue, how do I transfer the json string to the operation that parses it? Is it a problem if this queue is inside another NSOperationQueue that executes an NSOperation that consists of the two mentioned previously?
All I could find is transfers of data to a delegate on the main thread (performSelectorOnMainThread), but I need all this operations to execute in the background.
Thanks.
Code:
NSDownload : NSOperation
- (instancetype)initWithURLString:(NSString *)urlString andDelegate:(id<JSONDataDelegate>)delegate
{
self = [super init];
if (self) {
_urlStr = urlString;
_delegate = delegate; /// this needs to be a NSOPeration
_receivedData = [NSMutableData dataWithCapacity:256];
}
return self;
}
#pragma mark - OVERRIDE
- (void)main
{
#autoreleasepool {
if (self.isCancelled) {
return;
}
NSURL *url = [NSURL URLWithString:self.urlStr];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
self.urlConnection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];
}
}
#pragma mark - NSURLConnectionDataDelegate
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
if (self.isCancelled) {
[connection cancel];
self.receivedData = nil;
return;
}
[self.receivedData appendData:data];
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
if (self.isCancelled) {
self.receivedData = nil;
return;
}
// return data to the delegate
NSDictionary *responseDict = #{JSON_REQUESTED_URL : self.urlStr,
JSON_RECEIVED_RESPONSE : self.receivedData};
[(NSObject *)self.delegate performSelectorOnMainThread:#selector(didReceiveJSONResponse:) withObject:responseDict waitUntilDone:NO]; // ok to uses performSelector as this data is not for use on the main thread ???
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
// return error to the delegate
[(NSObject *)self.delegate performSelectorOnMainThread:#selector(didFailToReceiveDataWithError:) withObject:error waitUntilDone:NO];
}

Answer 1

#user1028028:
Use the following approach.
1) Maintain the operation queue reference that you are using to add DownloadOperation.
2) In connectionDidFinishLoading method, create ParseOperation instance, set the json data and add it to operation queue. Maintain ParseOperation strong reference variable in DownloadOperation and handling of cancelling of parsing operation through DownloadOperation interface.
3) After completed parsing call the UI functionality in main thread.
I hope this helps.

Answer 2

As lucianomarisi notes, it would usually be best to just have the first operation generate the second operation. This is usually simpler to manage. Operation dependencies aren't really that common in my experience.
That said, it's of course possible to pass data between operations. For instance, you could create a datasource property on the second operation. That would be the object to ask for its data; that object would be the first operation. This approach may require locking, though.
You can also create a nextOp property on the first operation. When it completes, it would call setData: on the second operation before exiting. You probably wouldn't need locking for this, but you might. In most cases it would be better for the first operation to just schedule the nextOp at this point (which again looks like lucianomarisi's answer).
The point is that an operation is just an object. It can have any methods and properties you want on it. And you can pass one operation to another.
Keep in mind that since an operation runs in the background, there's no reason you need to use the asynchronous interface to NSURLConnection. The synchronous API (sendSynchronousRequest:returningResponse:error: is fine for this, and much simpler to code. You could even use a trivial NSBlockOperation. Alternately, you can use the asynchronous NSURLConnection interface, but then you really don't need an NSOperation.
I also notice:
_receivedData = [NSMutableData dataWithCapacity:256];
Is it really such a small piece of JSON data? It's hard to believe that this complexity is worth it to move such a small parsing operation to the background.
(As a side note, unless you know precisely the size of the memory, there's not usually much benefit to specifying a capacity manually. Even then it's not always clear that it's a benefit. I believe NSURLConnection is using dispatch data under the covers now, so you're actually requesting a memory block that will never be used. Of course Cocoa also won't allocate it because it optimizes that out... the point is that you might as well just use [NSMutableData data]. Cocoa is quite smart about these kinds of things; you generally can only get in the way of its optimizations.)

Answer 3

As Rob said, unless you have any particular reason to use operations use the synchronized call. Then perform the selector on MainThread or on any other thread you need. Unless you want to separate the retrieval and parsing in separate operations or thread (explicitly).
Here is the code I was using for json retrieval and parsing:
-(BOOL) loadWithURL:(NSString*) url params: (NSDictionary*) params andOutElements:(NSDictionary*) jElements
{
NSError *reqError = nil;
NSString* urlStr = #"";//#"http://";
urlStr = [urlStr stringByAppendingString:url];
NSURL* nsURL = [NSURL URLWithString:urlStr];
//Private API to bypass certificate ERROR Use only for DEBUG
//[NSURLRequest setAllowsAnyHTTPSCertificate:YES forHost:[nsURL host]];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL: nsURL
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:60.0];
[request setHTTPMethod:#"POST"];
NSString *postString = #"";
if(params!=nil) {
NSEnumerator* enumerator = params.keyEnumerator;
NSString* aKey = nil;
while ( (aKey = [enumerator nextObject]) != nil) {
NSString* value = [params objectForKey:aKey];
//Use our own encoded implementation instead of above Apple one due to failing to encode '&'
NSString* escapedUrlString =[value stringByAddingPercentEscapesUsingEncoding:NSASCIIStringEncoding];
//Required to Fix Apple bug with not encoding the '&' to %26
escapedUrlString = [escapedUrlString stringByReplacingOccurrencesOfString: #"&" withString:#"%26"];
//this is custom append method. Please implement it for you -> the result should be 'key=value' or '&keyNotFirst=value'
postString = [self appendCGIPairs:postString key:aKey value:escapedUrlString isFirst:false];
}
}
//************** Use custom enconding instead !!!! Error !!!!! **************
[request setHTTPBody:[postString dataUsingEncoding:NSUTF8StringEncoding]];
NSData *response = [NSURLConnection sendSynchronousRequest:request returningResponse:nil error:&reqError];
if(reqError!=nil) {
NSLog(#"SP Error %#", reqError);
return NO;
}
NSString *json_string = [[NSString alloc] initWithData:response encoding:NSUTF8StringEncoding];
//Handles Server Errors during execution of the web service that handles the call.
if([json_string hasPrefix:#"ERROR"] == YES){
NSLog(#"SP Error %#", lastError);
return NO;
}
//Very Careful!!!!!! Will stop for any reason.!!!!!!
//Handles errors from IIS Server that serves teh request.
NSRange range = [json_string rangeOfString:#"Runtime Error"];
if(range.location != NSNotFound) {
NSLog(#"SP Error %#", lastError);
return NO;
}
//Do the parsing
jElements = [[parser objectWithString:json_string error:nil] copy];
if([parser error] == nil) {
NSLog(#"Parsing completed");
} else {
jElements = nil;
NSLog(#"Json Parser error: %#", parser.error);
NSLog(#"Json string: %#", json_string);
return NO;
}
//Parsed JSON will be on jElements
return YES;
}