I have a problem regarding the best approach to make an App that has to download and show pdf's, It is fed by a JSON that has links to 147 pdf files, sized between 1 and 2 MB.
Questions:
What is the best approach to download all the files to an iPad?
Shall I use AFNetworking 2.0?
Is NSFileManager the way to save all the files?
Problems I may encounter:
With an asynchronous download, if lost connection or no more space on the iPad, what are the counter mesures?
Are there tutorials or examples that deal with this situation?
Sorry for all the questions but I'm new to this.
Best Regards.
What is the best approach to download all the files to an iPad?
This is really broad as #rmaddy suggested. Specific questions are more easily answered. There are lots of ways you could download a file via an HTTP request to your device each with pros/cons depending on your situation.
Shall I use AFNetworking 2.0?
Sure. You'll get no argument from me. This is a widely used and solid API to interface with HTTP-based resources.
Is NSFileManager the way to save all the files?
Yes. NSFileManager is the class you use to read/write files from/to your app's sandbox.
With an asynchronous download, if lost connection or no more space on the iPad, what are the counter measures?
I'm not 100% certain so I can't speak to exactly what happens in this case. AFNetworking may provide some help by writing to a temporary file during a download, etc....
Are there tutorials or examples that deal with this situation?
I have a sample project on Github that shows a table of files that you can download. You can watch the progress of your downloads, pause each request, resume and cancel as well. When you're done you can view the file you downloaded. It uses AFNetworking and might be useful to you:
https://github.com/chefnobody/StreamingDownloadTest
When downloading large files, the main counsel would be to avoid trying to load these into memory as you download them. Instead, make sure you stream them directly to persistent storage. In terms of handling space-specific errors, just make sure you check NSError objects that are returned to you in completion handlers or the appropriate delegate methods.
If using AFNetworking, you can specify the outputStream of the AFURLConnectionOperation to reference a NSOutputStream that you create, referencing some path in your persistent storage.
See Memory pressure issue while downloading multiple files using AFNetworking for example.
Alternatively, you can use NSURLSession (whether via AFNetworking or you do it yourself) and instantiate a NSURLSessionDownloadTask, which does the same sort of thing.
Google "NSURLSessionDownloadTask example" and you'll find tons of references. The block-based rendition of downloadTaskWithURL is incredibly simple. To do background downloads is a little more complicated and requires delegate-based implementation (see Downloading Files and Handling iOS Background Activity sections of URL Loading System Programming Guide: Using NSURLSession or watch the WWDC 2013 video, What’s New in Foundation Networking.)
Either way, you avoid some of the memory consumption challenges associated with downloading large files.
Related
this is an open question aiming at understanding what is the best practice or the most common solution for a problem that I think might be common.
Let's say I have a list of URLs to download; the list is itself hosted on a server, so I start a NSURLConnection that downloads it. The code in connectionDidFinishLoading will use the list of URLs to instantiate one new NSURLConnection, asynchronously, per each URL; this in turn will trigger even more NSURLConnections, and so on - until there are no more URLs. See it as a tree of connections.
What is the best way to detect when all connections have finished?
I'm aiming the question to iOS7, but comments about other versions are welcome.
A couple of thoughts:
In terms of triggering the subsequent downloads after you retrieve the list from the server, just put the logic to perform those subsequent downloads inside the completion handler block (or completion delegate method) of the first request.
In terms of downloading a bunch of files, if targeting iOS 7 and later, you might consider using NSURLSession instead of NSURLConnection.
First, the downloading of files with a nice modest memory footprint is enabled by initiating "download" tasks (rather than "data" tasks).
Second, you can do the downloads using a background NSURLSessionConfiguration, which will let the downloads continue even if the user leaves the app. See the Downloading Content in the Background section of the App Programming Guide for iOS. There are a lot of i's that need dotting and t's that need crossing if you do this, but it's a great feature to consider implementing.
See WWDC 2013 What's New in Foundation Networking for an introduction to NSURLSession. Or see the relevent chapter of the URL Loading System Programming Guide.
In terms of keeping track of whether you're done, as Wain suggests, you can just keep track of the number of requests issued and the number of requests completed/failed, and in your "task completion" logic, just compare these two numbers, and initiate the "all done" logic if the number of completions matches the number of requests. There are a bunch of ways of doing this, somewhat dependent upon the details of your implementation, but hopefully this illustrates the basic idea.
Instead of using GCD you should consider using NSOperationQueue. You should also limit the number of concurrent operations, certainly on mobile devices, to perhaps 4 so you don't flood the network with requests.
Now, the number of operations on the queue is the remaining count. You can add a block to the end of each operation to check the queue count and execute any completion logic.
As Rob says in his answer you might want to consider NSURLSession rather than doing this yourself. It has a number of advantages.
Other options are building your own download manager class, or using a ready-made third party framework like AFNetworking. I've only worked with AFNetworking a little bit but from what I've seen its elegant, powerful, and easy to use.
Our company wrote an async download manager class based on NSURLConnection for a project that predates both AFNetworking and NSURLSession. It's not that hard, but it isn't as flexible as either NSURLSession or AFNetworking.
I have created a simple testing app to learn how to use NSURLSession. This App has to download images from a webservice and present them into a UITableView.
I've already written the first part of the App that reads a list of images urls from the web service, now, I want to display this list.
My doubt is:
given that the list of images could be a really long list, is it ok to create a NSURLSessionDownloadTask for each image?
I thought to create the session in the cellForRowAtIndexPath function and store the NSURLSessions in a NSDictionary using as key the IndexPath of the cell (and probably relying on NSURLCache to avoid to download the same images more than once).
Other solutions:
I can see three more solutions:
Using GCD with dispatch_async
Subclassing NSOperation and essentially store an NSOperation for any image I need to download.
Using a third party library like AFNetwork... but since it is a learning purpose app I prefer to go completely with my code
.
If the multiple NSURLSession isn't a good solution, I'd choose one of those options.
What do you think about this approach?
NSURLSessionTask is fine for a large number of downloads. One advantage of it over some of the other methods you mentioned is that downloads can be cancelled or paused. It also correctly implements concurrency for network operations, which is more difficult than many cats on the internet will lead you to believe (if you don't believe me, view the eskimo's 2010 WWDC session and sample code. NSOperation for network connections is not trivial).
NSURLSessionTask and friends are designed for exactly the kinds of problems you are trying to solve, and it's very well tested.
For a tableview, start the task in tableView:willDisplayCell:forRowAtIndexPath: and cancel (or pause) a task in tableView:didEndDisplayingCell:forRowAtIndexPath:. That will limit the active downloads to the currently visible cells.
Suggestion:
I also came across a similar situation were I need to download about 2000 Image files and 100 Video files. For that purpose I implemented a custom download manager using NSOperationQueue and blocks.
I have added this library to GitHub, please feel free to check the implementation.
IMO whilst it is ok to create an NSURLSessionTask for each image a standard first in first out implementation will cause problems when scrolling through your cells. The reason for this is that downloads will be queued on your NSURLSession and tasks will be executed in the order they've been added to the queue, in other words in a FIFO manner. Imagine a scenario where you've scrolled through a vast number of cells and you have to wait for all downloads to complete in order. You would not only have to wait a long time, you would be making unnecessary network requests for image assets that may no longer be relevant to your user.
Nick Lockwood created a great NSOperationQueue subclass called NSOperationStack that reverses the order of operations so that the the last operation is executed first (LIFO). IMO for a large number of downloads a LIFO implementation is a must.
NSOsperationStack is available here
If you combine this with an implementation that uses cellForRowAtIndexPath to initiate and NSURLCache to store downloads, you should end up with a very streamlined and efficient solution.
I would use (or at least take a look at) SDWebImage's SDWebImageManager.
Besides downloading you can set priority and continue in the background options which I think you'll want to have.
I am currently developing a application which has to be able to show offline videos which need to be downloaded first.
The problem was that these videos can be bigger that the memory that I can allocate to my application. So parts that are downloaded have to be saved immediately instead of saved in a NSData object. I'm hearing conflicting stories on whether or not RESTKit should work, and ASIHTTPRequest seems to be deprecated.
I will follow the suggestion from this thread as it seems to be the best option.
NSURLConnection download large file (>40MB)
Consider using NSURLConnection to download the video file and write the data directly to a file (NSFileHandle).
One advantage of using this method is that the NSURLConnection didReceiveData delegate method is continuously called as data is received, so you can update a progress bar.
Check out AFNetworking for network managing. I am not sure if they have video downloading, but the framework works great for images and other types of downloads that I have down before.
Without explaining all the hasle with dealing with HTTP responses by chunks and streams I would recommend using AFDownloadRequestOperation. It supports resuming downloads and has callbacks for showing download progress. I love it and use it in most of my projects.
P.S. It uses AFNetworking, which is a great framework for making all kinds of HTTP requests.
I read something about multi thread access on DB but still not sure best way to do that for reading/writing in conjunction with async network download.
For instance I will have a page with images from the web, so I'm retrieving them by their URLs, using AFNetworking but I want to check first on my DB and write on it (or disk) the retrieved image for future use.
What can be the best way to do that without blocking the UI (like when scrolling)?
If I do that with a singleton that reads/writes it block the main thread.
Thanks for any tips on that.
AFNetworking is not the tool for that. Instead, you can take advantage of the built-in functionality of NSURLCache--specifically Peter Steinberger's fork of SDURLCache.
In -applicationDidFinishLaunchingWithOptions:, do NSURLCache +setSharedCache: with an instance of SDURLCache (with some amount of disk space allocated). All requests made through UIWebView (and AFNetworking, for that matter) will automatically be routed through NSURLCache before the request is made to check the cache. It's unobtrusive, drop-in, and follows cache directives correctly, and it should solve your problem quite nicely.
Problem statement:
I want to upload a large binary (such as an audio clip) from an iOS app to S3, and I'd like to make the app's handling of disconnects (or low connectivity) as robust as possible, preferably by uploading the binary as a series of chunks.
Unfortunately, neither the AWSiOS SDK, nor ASI's S3 framework seem to support to multi-part uploads, or indicate that they plan to add support. I realize that I can initiate a 'longish' upload using beginBackgroundTaskWithExpirationHandler: and that'll give me a window of time to complete the upload (currently 600 seconds, I believe), but what's to be done if I'm not in a situation to complete said upload within that timeframe?
Aside from worrying about completing tasks within that time frame, is their a 'best practice' for how an app should resume uploads, or even just break a larger upload into smaller chunks?
I've thought about writing a library to talk to S3's REST API specifically for multi-part uploads, but this seems like a problem other have either been solved, or realized needn't be solved (perhaps for being completely in appropriate for the platform).
Another (overly complicated) solution would be chunking the file on the device, uploading those to S3 (or elsewhere) and have them re-assembled on S3 via a server process. This seems even more unpalatable than rolling my own library for multi-part upload.
How are others handling this problem?
Apparently I was looking at some badly out of date documentation.
in AmazonS3Client see:
- (S3MultipartUpload * AmazonS3Client)initiateMultipartUploadWithKey:(NSString *)theKey withBucket:(NSString *)theBucket
Which will give you a S3MultipartUpload which will contain an uploadId.
You can then put together an S3UploadPartRequest using initWithMultipartUpload: (S3MultipartUpload *) multipartUpload and send that as you usually would.
S3UploadPartRequest contains an int property partNumber where you can specify the part # you're uploading.
you can write some code to do so, you can refer code from http://dextercoder.blogspot.in/2012/02/multipart-upload-to-amazon-s3-in-three.html. Core java code, steps can be used for iOS.