How do I use this block method? - objective-c

Question

I am a little bit confused about objective c programming with blocks.
for example Here is a method:
in the .h
- (void)downloadDataWithURLString:(NSString *)urlString
completionHandler:(void(^) (NSArray * response, NSError *error))completionHandler;
in the .m:
- (void)downloadedDataURLString:(NSString *)urlString
completionHandler:(void (^)(NSArray *, NSError *))completionHandler {
// some things get done here. But what!?
}
My main questions is.... how do I implement this completion handler? What variables would be returned with the array and error? it is one area for the code but how do I tell it what to do when it is completed?

Answer 1

It's up to the caller to supply code to be run by the method (the body of the block). It's up to the implementor to invoke that code.
To start with a simple example, say the caller just wanted you to form an array with the urlString and call back, then you would do this:
- (void)downloadedDataURLString:(NSString *)urlString
completionHandler:(void (^)(NSArray *, NSError *))completionHandler {
NSArray *callBackWithThis = #[urlString, #"Look ma, no hands"];
completionHandler(callBackWithThis, nil);
}
The caller would do this:
- (void)someMethodInTheSameClass {
// make an array
[self downloadedDataURLString:#"put me in an array"
completionHandler:^(NSArray *array, NSError *error) {
NSLog(#"called back with %#", array);
}];
}
The caller will log a two item array with #"put me in an array" and #"Look ma, no hands". In a more realistic example, say somebody asked you to call them back when you're finished downloading something:
- (void)downloadedDataURLString:(NSString *)urlString
completionHandler:(void (^)(NSArray *, NSError *))completionHandler {
// imagine your caller wants you to do a GET from a web api
// stripped down, that would look like this
// build a request
NSURL *url = [NSURL URLWithString:urlString];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
// run it asynch
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
if (!error) {
// imagine that the api answers a JSON array. parse it
NSError *parseError;
id parse = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableContainers error:&parseError];
// here's the part you care about: the completionHandler can be called like a function. the code the caller supplies will be run
if (!parseError) {
completionHandler(parse, nil);
} else {
NSLog(#"json parse error, error is %#", parseError);
completionHandler(nil, parseError);
}
} else {
NSLog(#"error making request %#", error);
completionHandler(nil, error);
}
}];
// remember, this launches the request and returns right away
// you are calling the block later, after the request has finished
}

Answer 2

While I can't be entirely sure without seeing any more details about the method, or its exact implementation I suspect this: this methods creates a new background thread, retrieves the data from the server and converts the JSON/XML to an NSArray response. If an error occurred, the error object contains a pointer to the NSError. After doing that, the completion handler is called on the main thread. The completion handler is the block in which you can specify which code should be executed after attempting to retrieve the data.
Here is some sample code on how to call this method to get you started:
[self downloadDataWithURLString:#"http://www.google.com"
completionHandler:^(NSArray *response, NSError *error) {
if (! error) {
// Do something awesome with the 'response' array
} else {
NSLog(#"An error occured while downloading data: %#", error);
}
}];