Multi threading objective-c

Question

I have to search database using keyword typed from key board from iphone. When ever i search the key board halts. and if i run in the background then i find an empty array. I want both key board so that key board doesnot halts and wait until array is full with data.I am using objective-c.

---

dispatch_semaphore_t task = dispatch_semaphore_create(0);

manager.completionQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

NSURLSessionDataTask *dataTask = [manager dataTaskWithRequest:request completionHandler:^(NSURLResponse *response, id responseObject, NSError *error) {
    if (error) {

    } else {
        //int i=0;
        //NSLog(@"%@",responseObject);

         header=responseObject;


        dispatch_semaphore_signal(task);

    }

}];

[dataTask resume];


dispatch_semaphore_wait(task, DISPATCH_TIME_FOREVER);
return header;

Answer 1

You've taken an inherently asynchronous method, dataTaskWithRequest, and have made it synchronous (i.e. it blocks the thread from which it was called). Get rid of all of that semaphore stuff. It's a really bad pattern, anyway.

I assume you did that because you wanted to return data from a network call. You shouldn't do that. You should use a completion handler pattern, e.g.

---

For example, let's imagine your method currently looks like:

- (id)performRequest:(NSURLRequest *)request {
    __block id header;

    manager.completionQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    NSURLSessionDataTask *dataTask = [manager dataTaskWithRequest:request completionHandler:^(NSURLResponse *response, id responseObject, NSError *error) {
        if (error) {

        } else {
            header = responseObject;
        }
        dispatch_semaphore_signal(semaphore);
    }];

    [dataTask resume];

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    return header;
}

You should change it to a void return type, and add a completionHandler parameter:

- (void)performRequest:(NSURLRequest *)request completionHandler:(void (^)(id _Nullable responseObject, NSError * _Nullable error))completionHandler {
    manager.completionQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

    NSURLSessionDataTask *dataTask = [manager dataTaskWithRequest:request completionHandler:^(NSURLResponse *response, id responseObject, NSError *error) {
        if (error) {
            completionHandler(nil, error);
        } else {
            completionHandler(responseObject, nil);
        }
    }];

    [dataTask resume];
}

And you'd call it like so:

[self performRequest:request completionHandler:^(id responseObject, NSError *error) {
    // use responseObject and error here
}];

// but not here