How to delay method until NSData finishes loading in NSUrlSession? - ios

Question

I am working on a project where I call one method from another. In the 2nd method I fetch data from a server using a NSURLSession. When the 2nd method returns the NSData to the first method, the data is converted into JSON and then returned to the viewcontroller that made the inital call on the first method. The problem I am having is that the first method is returning a null object because the NSData doesn't load fast enough. I'm not sure what to do about it.
Here is the code:
Method 1
-(NSDictionary*)returnJsonDictionaryFromUrl:(NSURL*)url {
NSData *data = [self makeHttpRequestWithUrl:url];
//NSLog(#"Data is: %#", data);
NSError *error;
NSDictionary *json = [NSJSONSerialization JSONObjectWithData:data
options:kNilOptions error:&error];
return json;
}
Method 2
-(NSData*)makeHttpRequestWithUrl:(NSURL*)url {
if (!_data) {
_data = [[NSData alloc]init];
}
NSURLSession *session = [NSURLSession sharedSession];
[[session dataTaskWithURL:url
completionHandler:^(NSData *data,
NSURLResponse *response,
NSError *error) {
_data = data;
}] resume];
return _data;
}
Thanks in advance for any suggestions!

Answer 1

So blocks by default are skipped over in execution and queued up (sometimes on other threads). This means when you're returning a variable that you had just set in a block, you should assume the block has not been executed and any variables you set inside it will not be set until much later.
The best way to handle this is to pass in the completion block to the method. So instead of having it return the NSData pointer, instead make it a void and simply pass in the completion block to that. This will be your request method:
-(void)makeHttpRequestWithUrl:(NSURL*)url completion:(void (^)(NSData *data, NSURLResponse *response, NSError *error))completion {
NSURLSession *session = [NSURLSession sharedSession];
[[session dataTaskWithURL:url
completionHandler:completion] resume];
}
And you'll call it like this:
[object makeHttpRequestWithUrl:url completion:^(NSData *data, NSURLResponse *response, NSError *error) {
// handle error
// use data
}

Answer 2

If you really must do a synchronous web request NSURLSession is not the right tool. For this we have NSURLConnection sendSynchronousRequest:returningResponse:error:. And if you don't even care about the HTTP response you also can use NSData dataWithContentOfURL:.
But you really shouldn't use those, especially on the main thread as this will cause your app to crash if the network request takes too long. Embrace the asynchronous nature of network requests and handle your data in the completion block as kpsharp suggests in his answer.