Following up with the post: Can AFNetworking return data synchronously (inside a block)?
One of the comments in that post was:
The trick to asynchronous programming is to break the procedural,
synchronous assumption that data is there when you ask for it.
Instead, with async, when you ask for something, you give it a
callback to perform when the data finally is ready. In this case, you
would call the block in the success block of the JSON operation.
Rather than the method returning data, it's told what to do when the
data is finished downloaded.
Although I'm using GCD and asynchronous downloading on iOS, I do not really understand how to implement this "procedural break" when programming with async.
For example, assume I need to download some JSON data, which includes lots of data including an image URL. I will have to download the actual image afterwards.
Each cell in the table takes in the data from the JSON/images downloaded. How would I implement this procedural break in this case?
While your data has not arrived, your table view dataSource tells its table view that it has zero rows, and displays a spinner. When the callback is fired, you store the data somewhere, remove the spinner, and call [tableView reloadData]. Basically, that's all there is to it.
Related
I need to perform a scenario with the following steps:
To make a network call with some search parameters provided by the user
Then, to parse its JSON response and create model entities
Then, for each entity created and if it has an associated image URL, to call the corresponding service for downloading such image and to show it in a UICollectionView when download finishes.
I need to handle concurrent network calls when downloading the N images I'd need. User should have the possibility to cancel the current search flow and start a new one. In addition, I should take into account that the cell for a certain image has not been displayed yet or it has been dequeued. So, my question is: which would be the best approach for this?
Perform the first network call (no concurrency) and the JSON response parsing directly using URLSession and URLSessionDownloadTask, and using Operation and OperationQueue for the subsequent steps. Use both invalidateAndCancel() for the URLSession and cancelAllOperations() for the OperationQueue if the user wants to cancel the current search.
Perform the complete process using Operation and OperationQueue.
Any other and more appropriate approach?
The best approach would be to do either a sync or async call to make the first service call. Once completed, parse the json synchronously and then use lazy loading concept (async call) to load if any images are to be downloaded to display the images.
Perform the network call then make the model objects. return these to your VC through your API completion block.
Then reload your collection view and pass the url to each cell and have each cell worry about itself, use something like SDWebImage to show the images async in the cells..
I have an API code, which loads a data necessary for my application.
It's as simple as:
- (void) getDataForKey:(NSString*) key onSuccess:(id (^)())completionBlock
I cache data returned from server, so next calls of that functions should not do network request, until there is some data missing for given key, then I need to load it again from server side.
Everything was okey as long as I had one request per screen, but right now I have a case where I need to do that for every cell on one screen.
Problem is my caching doesn't work because before the response comes in from the first one, 5-6 more are created at the same time.
What could be a solution here to not create multiple network request and make other calls waiting for the first one ?
You can try to make a RequestManager class. Use dictionary to cache the requesting request.
If the next request is the same type as first one, don't make a new request but return the first one. If you choose this solution, you need to manager a completionBlock list then you will be able to send result to all requesters.
If the next request is the same type as first one, waiting in another thread until the first one done. Then make a new request, you API will read cache automatically. Your must make sure your codes are thread-safe.
Or you can use operation queues to do this. Some documents:
Apple: Operation Queues
Soheil Azarpour: How To Use NSOperations and NSOperationQueues
May be there will be so many time consuming solutions for this. I have a trick. Create a BOOL in AppDelegate, its default is FALSE. When you receive first response, then set it TRUE. So when you go to other screen and before making request just check value of your BOOL variable in if condition. If its TRUE means response received so go for it otherwise in else don't do anything.
Currently I am running 3 task concurrently using AFNetworking.But my problem is that I need to refresh tableview once all the above three task complete.But as in AFNetworking all the operation are async. So any operation can finish first. I am not getting a point where I need to refresh my tableview. I am planning to do some critical section type implementation.Is there can other way to accomplish the above.
Use the built-in batched request operation feature of AFNetworking. The completion handler for the batch would include the logic to refresh your data source once all of the operations have finished.
Maybe having some sort of 'active requests array' is a solution for you.
Every time you make a request, add it to the (mutable) array.
When the request finishes, remove it from the array.
Every time a request finishes, check the array length (count).
If your array has become empty again, all request have been finished and you can reload your tableview.
I have an object that needs to be initialised with data from network and doesn't really make sense without the downloaded data. But it seems to me that doing an asynchronous network call in its init method is not a good idea because the object will not be ready to user right away and it might cause confusion. Should I only use a basic init method that will alloc init all its properties to create an empty object, and have other (non-init) methods populate the data from the network which will be called explicitly by other objects (such as the view controller using this object)? What would be a good way to approach this?
I think that the solution comes from having the right order of code running:
1) Go to network and fetch data (show the user acivity indicator in this time)
2) Server return response -> fetch the response into your object
3) Show the data or use it
Not sure how to best explain this - If I use blocks to load images for UITableViewCells, how best can I ensure that when an image actually finishes loading - it is the correct image for the cell. Say I'm making an app with user icons - so cell 5 is for John and it starts loading John's image. Say that request gets hung up and by the time it returns, that cell has been reused for Bill at cell 23. We obviously don't want to load John's icon - we just want Bill's.
This is a simplified explanation. I know that we can check if a cell is visible in the block before setting the image - but say in a different scenario I have a method that makes a remote request when called. When the remote request is done, it calls a block defined in that method that updates the UI. If I call it and before it returns I call it again and only want the most recent call's block executed. Is there a good pattern for doing this?
Currently, I try to store a variable that has some state in the method so that when it returns I can check if that state is still valid. Just thought I'd see if there was a better way. Thanks.
Just checking if any updates on this.