we are struggling to cancel request that already sent when we use collection view cell .. ( i am not talking about cases when we want to cancel request because we already sent the same request) ... i guess we need to use the methods: cancelRequestForRequestReceipt .. the problem is that it's not clear how to get this RequestReceipt.
Example:
We have a collection view with different images (each image have a different url) .. if i will scroll back and forward fast , the method af_cancelImageRequest will do the job and will not create 2 active request for the same url.. Super!, the problem is when we try to scroll only to 1 direction and we have 1000 images .. basically we want to be able to cancel the request that just sent, before the image will return, meaning we don't have the image and we don't AlamofireImage to continue with that request ... after the cell is disappear (cause by slow internet and fest scrolling )..
So, if i got it right , we can use cancelRequestForRequestReceipt ... the problem is that we can't find how to get this RequestReceipt ...
BTW: i saw the example code that AlamofireImage demo app,
override func prepareForReuse() {
super.prepareForReuse()
imageView.af_cancelImageRequest()
imageView.layer.removeAllAnimations()
imageView.image = nil
}
As i said, this code will cancel request for images ONLY if the request is already in the operation queue ..
Help :)
I think you're misunderstanding was AlamofireImage is doing in this example, but first let's understand what does the prepareForReuse method.
Every time the cell is going to be dequeued this method is called, this means that if for example as you said you scroll forward and backward fast this method will be called, if you scroll in one direction fast with n images this method will be called every time a cell disappears.
As you're calling the method over the UIImageView and exist a reference inside the UITableViewCell you don't need the RequestReceiptbecause the request is going to be canceled for you.
In the case when you make a fast forward and backward the correct way of handle it is not cancel it after a throttle preventing this kind of behaviour.
AlamofireImage it's a really great library but I think you can benefit much more using KingFisher, it handle for you the throtle in the case of fast forward and backward and of course the other case of the cell being dequeued and the request is no finished.
Nevertheless this libraries handle all the hard work for you and you don't need to reinvent the wheel my advice is you learn what's happens behind the scenes and how you can make it if these libraries doesn't exist yet.
I hope this help you.
Related
I have TableView with customs cell representing events. It looks very close to first and third image here.
As you can see (sorry for small resolution) on some events there are photos of friends that are going to participate.
Unfortunately information about friends is not loaded with other information about events.
So after I got list of events I can make request to load list of friends that are going to participate in each event.
Right now I use code like
class EventCell : UITableViewCell {
var eventForCell : Event? {
didSet {
eventTitleLabel.text = eventForCell.title
eventDateLabel.text = eventForCell.date
presentFriends(eventID : eventForCell.id)
}
}
func presentFriends(eventID : Int) {
//searching for friends for event with specific eventID
.......
.......
//for every friend found adding UIImageView in cell
for friend in friendsOnEvent {
let avatar = UIImageView(....)
self.addSubview(avatar)
}
}
}
This code works but photos are not presented in smooth way. Also if you scroll list fast they start to blink. Maybe it is even not necessary to load them if user scrolls fast list of events. So I have two questions:
How can I make smooth scrolling experience taking in consideration that presenting friends for every event can take time and sometimes it finishes after cell was scrolled away.
If I had loaded list of events and already presenting cells with them. How can I update those cells after I get information about friends that are going to participate?
When user is scrolling and I am creating async tasks to display some images in cell I think I should use weak reference to self and maybe check it not to equal nil so task would be canceled if cell is not visible now. How should it be done?
Update:
I found info about tableView(_:prefetchRowsAt:) method inside UITableViewPrefetchingDataSource protocol, should I use it for this case? Maybe someone has some experience with it?
1. (Re)creating a view objects during cellForRowAt is generally a bad practice. From the screenshot I assume that there is a limit to how many avatars are there on a single cell - I would recommend creating all the UIImageView objects in the cell initializer, and then in presentFriends just set images to them, and either hide the unused ones (isHidden = true) or set their alpha to 0 (of course, don't forget to unhide those that are used).
2. If you are using SDWebImage to load images, implement prepareForReuse and cancel current downloads to get a bit of performance boost during scrolling, and prevent undefined behaviour (when you try to set another image while the previous one was not yet downloaded). Based on this question, this one and this one I would expect something like:
override func prepareForReuse() {
super.prepareForReuse()
self.imageView.sd_cancelCurrentImageLoad()
}
Or you can use [this gist][4] for an inspiration.
P.S.: Also, you will have to count with some blinking, since the images are downloaded from web - there will always be some delay. By caching you can get instantly those that were already downloaded, but new ones will have delay - there is no way to prevent that (unless you preload all the images that can appear in tableView before presenting tableView).
P.S.2: You can try to implement prefetching using [UITableViewDataSourcePrefetching][6]. This could help you out with blinking caused by downloading the avatars from web. This would make things a bit more complicated, but if you really want to remove that blinking you will have to get your hands dirty.
First of all, as you can see from the documentation, prefetchRowsAt does not give you a cell object - thus you will have to prefetch images to your model object instead of simply using sd_setImage on the UIImageView object at a given cell. Maybe the aforementioned gist would help you out with downloading images to model.
Now also as the documentation states:
Loading Data Asynchronously
The tableView(_:prefetchRowsAt:) method is not necessarily called for every cell in the table view. Your implementation of tableView(_:cellForRowAt:) must therefore be able to cope with the following potential situations:
Data has been loaded via the prefetch request, and is ready to be displayed.
Data is currently being prefetched, but is not yet available.
Data has not yet been requested.
Simply said, when you are in cellForRowAt, you cannot rely on prefetchRowsAt being called - it might have been called, and it might not have been called (or maybe the call is in progress).
Documentation continues to suggest using Operation as a backing tool for downloading the resources - prefetchRowsAt could create Operation objects that would be responsible for downloading avatars for a given row. cellForRowAt can then ask the model for the Operation objects for the given row - if there are not any, it means that prefetchRowsAt was not called for that row, and you have to create Operation objects at that point. Otherwise you can use the operations created by prefetchRowsAt.
Anyway, if you decide that it is worth it, you can use this tutorial as an inspiration for implementing prefetching.
You can use UITableViewDataSourcePrefetching as you mentioned.
It's a protocol that calls your prefetch data source when some cells are going to be displayed but are not on the screen yet.
This way you can prepare all the resources that takes time to load before they are presented.
You just have to implement:
func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath])
and fetch the data related to the cells from all of the indexpaths.
Beware it's only available since iOS10.
I personally use AlamofireImage with a cache so images that have already been downloaded aren't fetched twice, there's plenty of alternatives but it's a good practice to use cached images on this kind of scenario.
I am processing several large RSS feeds and displaying results in a TableView. The processing starts after the user clicks on the relevant tab. All works well, but as it takes a couple of seconds to process, the NIB and Table don't load until the processing finishes and it looks like the iPhone has seized up. I have added an Activity indicator to the NIB, but because it doesn't load until the table is ready to display, it appears too late to be of any use.
Does anyone have any ideas how to display a message to a user while the table builds/loads? I have tried loading a UIView first and adding the Table as a subview but, again, both seem to load only after the table is ready.
Guidance appreciated.
It's kind of hard to guess what's going on from your description but it looks like your calls aren't asynchronous. Here's what you should be doing in your code:
Make all calls asynchronous. You said your phone is seizing up. Makes it sound like your requests and responses are happening on the main thread. There are many libraries you could use to handle asynchronous calls. ASIHTTPRequest for one example....
Don't wait for the data to come in before displaying the tableView. It's a design principle that you load as much of the UI as possible so that the user has something to look at while your data loads up in the background. What you should be doing is initializing an NSMutableArray to hold the data. Initially this array will contain no objects. This is the array that you use in your data source methods: Use array size for numberOfRowsInSection and use the array objects in cellForRowAtIndexPath. Once your RSS feed XML comes in and is parsed, store that in your arrays and call [tableView reloadData]. This way you don't leave your users looking at a blank screen. (Another good practice is when the array size is zero, show one cell in your tableview that says "data is loading" or something).
When you first initialize and load up your table and then fire off those RSS feed requests, that's where you show an activity indicator view on the tableView. Stop animating the indicator when the RSS data comes in and your tableView reloads.
These are the standard steps you should follow while showing non local data in a tableview. Makes for a smooth user experience.
Like I said before, it seems from your question that your calls are not asynchronous. If I'm wrong, correct me and let's take it from there...
When the return button on the keyboard for a textfield is tapped I want to add a UIView, then connect to a website with NSURlConnection sendsynchronousrequest and I have the code in that order
But when I run in the simulator (I can't run on device) the connection is run first then the subview is added (ie the opposite of the order of the code)
Why is this and how can stop it, because I want the view to added, then the connection done and then the view removed.
The subview is being added, but views are drawn by the runloop. By making a synchronous request on the main thread, you are blocking the runloop, so the view won't be drawn until after the request completes. Do the request asynchronously, either by using the async API or by doing a synchronous request in a background thread.
Many actions happen on the run loop, rather than in the order you code. If you really want to code the way you have, then performSelector:withObject:afterDelay: with a delay of 0 might work to trigger your NSURlConnection (you will need to move that code to a method).
As JK suggests, an asynchronous request might solve it anyway, and improve the UI. I'm a great fan of ASIHPPTRequest library, which makes async trivial.
I'm using the three20 project for my iPhone app. I've narrowed my problem down and I'm now just trying to re-create the 'Web Images in Table' example that comes with the project. I've copied the code exactly as in the project, with the exception that I do not use the TTNavigator (which the example does) but I am adding my TTTableViewController manually to a tabBar.
The problem is as follows; the images in the table should load automatically from the web, like in the example. But they only load after I scroll the table up and down.
In the console it clearly says it is downloading the images, and you see the activity indicator spinning like forever.. And unless I scroll up and down once, the images will never appear.
Anyone? Thanks in advance.
P.S:
If I'm using this code in any random UIView, It also doesn't work (only shows a black square):
TTImageView* imageView = [[[TTImageView alloc] initWithFrame:CGRectMake(30, 30, 100, 100)] autorelease];
imageView.autoresizesToImage = YES;
imageView.URL = #"http://webpimp.nl/logo.png";
[self.view addSubview:imageView];
If I put this code in my AppDelegate (right onto the window), it does work .. strange?
POSSIBLE SOLUTION:
Although I stopped using TTImageView for this purpose, I do think I found out what the problem was; threading (hence accepting the answer of Deniz Mert Edincik). If I started the asynchronous download (because basically that is all the TTImageView is, an asynchronous download) from anywhere BUT the main thread, it would not start. If I started the download on the main thread, it would start immediately..
Sounds like a threading problem to me, are you creating TTImageView in the main runloop?
I find one interesting thing. When I use combination TTTableViewController, TTTableViewDataSource and TTModel I have same problem with loading TTImageView. My problem was, that my implementation of Model methods 'isLoading' and 'isLoaded' don't return proper values after initialization of model. That forces me to call reload on model manualy in 'viewDidAppear' method and that causes image loading problem. So I repair my 'isLoading' and 'isLoaded' methods to both return 'NO' after Model init, and everything is fine.
When an image finishes loading try sending a reloadData message to the table view. This forces the table to recalculate the size of the rows and redraw the table. Just be careful that you don't start downloading the image again in response to this message.
I've written something similar to this where an image view will load its own image from the web.
Im my experience, when the image had loaded successfully but was not shown in its view, it was a case that the cell needed to be told to redraw.
When you scroll the table view, the cells are set to redraw when the come onscreen, which is why they appear when you scroll.
When the image loads, tell the cell that it is sitting in to redraw by sending it the message setNeedsDisplay.
That way, when the image finishes downloading, the cell its sitting in (and only that cell) will redraw itself to show the new image.
It's possible that you might not need to redraw the entire cell and might be able to get away with simply redrawing the image view using the same method call. In my experience, my table cells view hierarchy was flattened, so I had to redraw the whole cell.
I don't have an answer for what you want to do, but I will say that this is considered a feature, and the expected behavior. You use TTImageView in UITableView when you want to do lazy loading of images. TTImageView will only load the images whose frames are visible on the screen. That way, the device uses its network resources to download images that the user has in front of them, rather than a bunch of images that the user isn't even trying to look at.
Consider having a long list that may contain a couple hundred thumbnail images (like a list of friends). I know from experience that if you kick off 100+ image requests on older devices, the memory usage will go through the roof, and your app will likely crash. TTImageView solves this problem.
This is a thread problem. You can load the images by including the line:
[TTURLRequestQueue mainQueue].suspended = NO;
in - (void)didLoadModel:(BOOL)firstTime.
I've set up an Objective-C category for an iPhone app's UIImageView class. The category's mission is to help load URL-based images asynchronously with memory/disk caching.
Now, in UIImageView+Cache.m I have access to an NSOperationQueue so I can kick off a loading thread. I create an NSOperation-derived object, initialized with the image URL and the target UIImageView, and a selector to perform on the target once the operation is complete. In the selector method, we set our freshly-loaded image (or, if not found, we set an alternate placeholder image), and we're done!
This works fine, until a UIImageView happens to be removed before the NSOperation completes. For instance, I have a previous/next segmented control in my UI that causes these UIImageViews to be removed and added anew (they're part of a larger "item" that is being viewed in the app), so it's very easy to tap these in rapid succession.
So if you decide to start tapping away before all the images are loaded - KABLAM! Unhappy thread has an invalid object and doesn't know it. :(
The closest thing I can find to help mitigate this is NSOperation's cancel and isCancelled methods, except you can't keep track of which operation object to cancel within a Category, because - if I understand correctly - Categories can't add IVARs to objects!
Maybe that means Categories aren't a good idea here? (Whines: "But I liiiiike Categories! Waaah!")
Advisement appreciated!
I probably wouldn't use a category for this situation. Categories are useful, but are usually unnecessary. I'd only use a category if you have a really good reason to. What exactly are you putting in the category?
I think you could implement the whole thing in the NSOperation subclass, which would be the best solution. Put a retain on the image view so it doesn't get deallocated before the image is downloaded, and cancel the download if the view is not visible anymore. If that's not possible, then subclass UIImageView instead of using a category.
I would say there is no harm in moving this into your own UIImageView subclass. Sure, you may like categories - but if they don't do the job then why hesitate in moving to a design that does?
Are you retaining the UIImageView by the NSOperation? Otherwise the imageView might be freed before the NSOperation completes, leading to kablooi central. You should do a retain and then, once you've done the setImage, do a release.