UITables with Downloaded Images – Easy Asynchronous Code
Readers… do look through the comments if you plan to use this code, other people have posted improvements. Thanks for all the great feedback everyone. MJ
The app ‘Postcards’ from my iPhone developer training class is a utility app for quickly sending a customized postcard, and one thing that makes it super easy is that you can grab pictures from Flickr to include in the postcard design. Postcards makes simple HTTP calls Flickr’s REST API to download public domain images and displays them in a UITableView for the user to pick from. Cocoa Touch makes this all simple and easy to code, and my first development version used synchronous calls to get the images by using NSData dataWithContentsOfURL:
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NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString:urls]];
Making synchronous calls to remote web servers from the thread that’s running the apps GUI is of course a bad idea that results in a laggy UI and unsatisfied users. Using synchronous calls in UITableView cellForRowAtIndexPath to load all the images results in a problem six times worse (for 6 rows on the screen) and makes scrolling basically broken as the table won’t scroll until it has the next cell, which it can’t get while the app is waiting for an image to download. Then imagine that on the Edge network! Obviously we need something multi-threaded that can load the images in parallel in the background and update the UI as they finish downloading.
Multi-threaded programming is hard and should be avoided whenever possible, and in this case Cocoa’s beautiful design came to my rescue:
UIView heirachy + URL loading system + delegate design = multi-threaded image loading with no multi-threaded coding!
How can you have your cake and eat it too? Every iPhone app is a multi-threaded program, or at least its running in conjuction with the multi-threaded iPhone operating system. Use the right delegate methods in the right ways, and you can take advantage of extra threads of execution that the iPhone gives you for free without writting any multi-threaded code of your own, hence sidesteping the problem of threading bugs in your code. An iPhone app is one big event loop – your classes have methods that the event loop calls in response to stuff happening on the device and in your app. When you use the URL loading system’s asynchronous APIs, the iPhone uses a different thread than the one running your app’s event loop to load the contents of the URL, and it makes callbacks via your apps event loop when data has been downloaded.
connection = [[NSURLConnection alloc] initWithRequest:request delegate:self]; - (void)connection:(NSURLConnection *)theConnection didReceiveData:(NSData *)incrementalData
Note carefully, when data has arrived from the remote webserver, that other iPhone thread doing the downloading doesn’t make calls into your objects at the same time as your methods are running, it puts messages into your apps event loop. If it called your app directly then chances are your app would be running some UI code or something and you’d have to write thread safe code. Instead, the call that data is ready arrives as an event on the event loop. Events on the event loop run single threaded, one at a time. Using this we can get asynchrous image download from Flickr without writting thread safe code ourselves. Even better, Cocoa’s URL loading system will download those URLs in parallel! For free!
That’s all well and good, but how do you get a table view to update the UITableViewCell with the image after its already been returned? A UIImage is imutable (right?) so you can’t change its image later when the image data has downloaded. Turns out Apple made this super easy too. Instead of putting a UIImage in the UITableViewCell, you put your own UIView object, that is sized correctly for the image you want to display, into the content view of the UITableCell (as a subview). At first your view object it can be empty, or it can have a dummy image in it, or you can pop in one of those spinny ’something is happening’ views. Then when the image data is downloaded, create a UIImageView with the image and pop it in your view in the cell. Hey presto… it appears. While all this is happening the user can be scrolling and going back and forth with a fully functioning UI.
I put this all together in a class AsyncImageView, listed below. It’s use is simple
- alloc and initWithRect:
- add it to a view, eg in a table cell’s content view;
- send it the loadImageFromURL: message.
LoadImageFromURL will return right away, the image will load in the background, and will automatically appear in the view when its finished downloading. The code posted below is something I whipped up pretty quickly (and I didn’t leak check yet!), but hey – parallel, asynchronous image download and display in about 40 lines of code with no thread-safe worries? Works in smooth scrolling tables, even on the Edge network? I rate it a big win, and wanted to share the technique.
I’ve developed an iPhone programming training class that I will be teaching soon in the SFBay Area (though my partners and I could probably bring it to you). It’s very hands on, and specially designed to help professional programmers new to Cocoa and Objective-C over the difficult initial learning curve. In the class we build the Postcards app from start to finish, including AsyncImageView. Email me for more info: markj at markj.net
@interface AsyncImageView : UIView { NSURLConnection* connection; NSMutableData* data; } @end @implementation AsyncImageView - (void)loadImageFromURL:(NSURL*)url { if (connection!=nil) { [connection release]; } if (data!=nil) { [data release]; } NSURLRequest* request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0]; connection = [[NSURLConnection alloc] initWithRequest:request delegate:self]; //TODO error handling, what if connection is nil? } - (void)connection:(NSURLConnection *)theConnection didReceiveData:(NSData *)incrementalData { if (data==nil) { data = [[NSMutableData alloc] initWithCapacity:2048]; } [data appendData:incrementalData]; } - (void)connectionDidFinishLoading:(NSURLConnection*)theConnection { [connection release]; connection=nil; if ([[self subviews] count]>0) { [[[self subviews] objectAtIndex:0] removeFromSuperview]; } UIImageView* imageView = [[[UIImageView alloc] initWithImage:[UIImage imageWithData:data]] autorelease]; imageView.contentMode = UIViewContentModeScaleAspectFit; imageView.autoresizingMask = ( UIViewAutoresizingFlexibleWidth || UIViewAutoresizingFlexibleHeight ); [self addSubview:imageView]; imageView.frame = self.bounds; [imageView setNeedsLayout]; [self setNeedsLayout]; [data release]; data=nil; } - (UIImage*) image { UIImageView* iv = [[self subviews] objectAtIndex:0]; return [iv image]; } - (void)dealloc { [connection cancel]; [connection release]; [data release]; [super dealloc]; } @end
And here is the usage in UITableViewCell. The AsyncImageView gets tagged with 999, and when it gets recycled, that 999 tagged view gets fished out and removed. So only the cell is being recycled, not the AsyncImageView object. When its removed from the cells content view it also gets released, causing dealloc, which in turn cancels the url download (if its outstanding).
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { static NSString *CellIdentifier = @"ImageCell"; UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier]; if (cell == nil) { cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:CellIdentifier] autorelease]; } else { AsyncImageView* oldImage = (AsyncImageView*) [cell.contentView viewWithTag:999]; [oldImage removeFromSuperview]; } CGRect frame; frame.size.width=75; frame.size.height=75; frame.origin.x=0; frame.origin.y=0; AsyncImageView* asyncImage = [[[AsyncImageView alloc] initWithFrame:frame] autorelease]; asyncImage.tag = 999; NSURL* url = [imageDownload thumbnailURLAtIndex:indexPath.row]; [asyncImage loadImageFromURL:url]; [cell.contentView addSubview:asyncImage]; return cell; }
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Mark, Hi, OK I tried a couple of your suggestions. The app is behaving the same way on my phone. I didn’t try 2.2.1 yet, I’ve got some 3.0 specific code so I’ll have to create a test app to try 2.2.1. I did set a break point in the didReceiveData method, incrementalData seems fine, I hover over it and it looks like a valid object (the popup reports x number of bytes), however, as soon as I try to use it I get an “EXC_BAD_ACCESS”.
One last oddity, the console log looks clean until I start setting break points, then I see several “unable to read unknown load command…”. They start logging right after the app starts. I ran a couple Apple sample apps and I’m seeing it there too.
Thanks
John
Mark, I got it, stupid mistake on my part. In my cellForRowAtIndexPath I was only creating an instance of my image when the cell was equal to nil. I moved my code and it working great (Snow Leopard, SDK 3.0), thanks again for the resource. John
thankyou sooo much for this blog post. Taken me a while to get head around this ‘fast scrolling’ thing ;-)
one thing that i’m still having difficulty with and its been alluded to above, that being the removing of images from dequeued cells.
I’m using a version of the yellow jackets code above (thanks also to Wayne for the integration of URLCache, saved me a major headache).
Can anyone give me a hand with where I need to remove the old imageview?
if cell !=nil then { asyncImageView = (AsyncImageView *) [cell.contentView viewWithTag:ASYNC_IMAGE_TAG];
}
is multiple asyncImageViews the way to go?
many thanks
Nik
This is a great tutorial. I extended it to UIButton so I could click on the image. Does anyone here know how to get touches events to work with UIImageView and not UIButton?
Happy coding,
Dov
Guys, this has been a lifesaver, seriously. I’m a total iPhone newb, but this has been a really great post and set of comments. I would like to bump on Nik’s post though, I don’t seem to be able to make the previous image clear before the new one tries to load. the [oldImage removeFromSuperview] does remove it, but it removes the image and none others load back again. Either I’m confused or there’s something I’m missing…
Thanks!
Wayne,
Love the code, made one small tweak. I changed ine 78 in AsyncImageView.m to this
spinny.center = CGPointMake(self.width / 2, self.height / 2);
The original code worked well as long as the original of the AsyncImageView was 0,0. If it wasn’t, the spinner would get pushed away from the center.
I’ve got it working in my app, but the images are stacked on top of my cell textLabels. Anyone know of an easy remedy for this?
Brilliant. I’m currently porting earthalbum to the iPhone. I don’t usually comment on these types of threads, but this is brilliantly clear and useful code– thanks a ton.
Hey Guys.
Thank you for this amazing post. No more scrolling problems. I was hanging around with this problem for a while… Thanks a lot!!!!!!!!!!
Hi!
Can someone please make a sourefile of this great tutorial?
I can’t get it work. Thanks!
Hi Mark,
Great Article.. but I have one problem, images are not retained when I scroll down and then back up. It always reloads the images while scrolling. Is it possible to retain the images one they are loaded initially. Thanks in advance.
Hey, Umm… I’m trying to implement this in my application… having some problems… not seeing the image in my rss feeder / wall application.
I bring in the URL from the in an rss feed i setup.
Here is my table cell thingie… I’m seeing the full url in the NSLog i setup … so I know I’m grabbing that just fine… any comments?
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *MyIdentifier = @”MyIdentifier”;
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];
if (cell == nil) {
cell = [[[UITableViewCell alloc]
initWithFrame:CGRectZero reuseIdentifier:MyIdentifier]
autorelease];
} else {
AsyncImageView* oldImage = (AsyncImageView*)
[cell.contentView viewWithTag:999];
[oldImage removeFromSuperview];
}
// Set up the cell
int postIndex = [indexPath indexAtPosition: [indexPath length] – 1];
cell.textLabel.text = [[posts objectAtIndex: postIndex] objectForKey: @”title”];
NSString *urlString = [[posts objectAtIndex: postIndex] objectForKey: @”link”];
NSURL *url = [NSURL URLWithString:urlString];
NSLog(@”%@”, urlString);
CGRect frame;
frame.size.width=75; frame.size.height=75;
frame.origin.x=0; frame.origin.y=0;
AsyncImageView* asyncImage = [[[AsyncImageView alloc] initWithFrame:frame] autorelease];
asyncImage.tag = 999;
[asyncImage loadImageFromURL: url];
[cell.contentView addSubview:asyncImage];
/*UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0,0,45,45)]; // set the location and size of your imageview here.
imageView.image = [UIImage imageNamed:@"mzwLogo.gif"]; // set the image for the imageview
//imageView.image = loadImage.image;
[cell addSubview:imageView];*/
NSLog(@”just set up the cell”);
return cell;
}
Nice post, it removes the bug of scrolling table content.
Thanks
What happened to Wayne’s post and codel? I see people mention it but I don’t see the code. I’d love to add caching to this. And thanks for the async image loading, a big help to me too.
i got a problem with this solution, after scrolling down images gets disappear and again it loads image.
is there any solution for that..?
Thanks in advance
how can we retain those images which are loaded initially ?
Thanks for this useful and inspiring post!
I think I found a leak (fair enough as you explicitly warned about the lack of leak-check…)
NSURLRequest* request = [NSURLRequest requestWithURL:url ... timeoutInterval:60.0];
connection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
At this point, you should do a [request release].
Or alternatively, you could use: [NSURLRequest initWithURL: url ... timeoutInterval:60.0];
Does it make sense?
Hi Mark,
So many thanks for this class. it’s a true life saver. one small problem i’m having.
if somebody could email timfazio@gmail.com if they know what’s going on, that would save many late nights.
I have an app which searches for cafes and displays them. I am using the async image view class with the
now, the images display the loading spinny icon until i scroll to the bottom of the table and back to the top. only after scrolling back to the top does the image load. i can’t work out how to make them display as soon as the table is loaded, like they used to? all the references are correct, as the images do evenutally load after you’ve scrolled to the bottom of the table.
i’m wondering if it has something to do with dequeing and reusing cells or something, but i just can’t work it out– the class works fine in another table, the only difference being the other table is loaded from code not a nib. that said, it worked fine when it was loaded from a nib…
here’s the code reference for the table that’s not working
Code:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *CustomCellIdentifier = @”CustomCellIdentifier “;
CustomCell *cell = (CustomCell *)[tableView dequeueReusableCellWithIdentifier: CustomCellIdentifier];
AsyncImageView *asyncImageView = nil;
if (cell == nil) {
NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@”CustomCell” owner:self options:nil];
for (id oneObject in nib)
if ([oneObject isKindOfClass:[CustomCell class]])
cell = (CustomCell *)oneObject;
CGRect asynImageRect = CGRectMake(10, 10, 60, 60);
asyncImageView = [[AsyncImageView alloc] initWithFrame:asynImageRect];
asyncImageView.tag = 9000;
[cell.contentView addSubview:asyncImageView];
[asyncImageView release];
}
Cafe *bCafe = [appDelegate.cafes objectAtIndex:indexPath.row];
cell.txtMain.text = bCafe.cafename;
…. load other variables here ….
if (!asyncImageView)
asyncImageView = (AsyncImageView*)[cell viewWithTag:9000];
NSString *cafeImage = bCafe.image;
cafeImage = [cafeImage stringByReplacingOccurrencesOfString:@"." withString:@".cropped1."];
NSString *cafeImageString = [NSString stringWithFormat:@"http://www.mysite.com/images/%@", cafeImage];
NSURL *cafeImageStringURL = [NSURL URLWithString:cafeImageString];
[asyncImageView loadImageFromURL:cafeImageStringURL];
return cell;
}
and for the working table…
Code:
-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
NSLog(@”TABLING”);
static NSString *CellTableIdentifier = @”CellTableIdentifier “;
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:
CellTableIdentifier];
AsyncImageView *asyncImageView = nil;
if (cell == nil) {
cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier: CellTableIdentifier] autorelease];
//set avatar image
CGRect asynImageRect = CGRectMake(20, 20, 60, 60);
asyncImageView = [[AsyncImageView alloc] initWithFrame:asynImageRect];
asyncImageView.tag = 9000;
[cell.contentView addSubview:asyncImageView];
[asyncImageView release];
… set up other variables here …
}
int currentCell = indexPath.row + rowAdder;
…. load other variables here ….
//fetch user avatar from web
if (!asyncImageView)
asyncImageView = (AsyncImageView*)[cell viewWithTag:9000];
Review *gReview = [appDelegate.reviews objectAtIndex:currentCell];
NSString *avatarImage = gReview.avatar;
avatarImage = [avatarImage stringByReplacingOccurrencesOfString:@"." withString:@".cropped1."];
if ([avatarImage isEqualToString:@"empty"]) {
NSString *placeholderString = @”http://www.mysite.com/images/avatar.jpg”;
NSURL *placeholderUrl = [NSURL URLWithString:placeholderString];
[asyncImageView loadImageFromURL:placeholderUrl];
}
else {
NSString *avatarImageString = [NSString stringWithFormat:@"http://www.mysite.com/avatars/%@", avatarImage];
NSURL *avatarImageStringURL = [NSURL URLWithString:avatarImageString];
[asyncImageView loadImageFromURL:avatarImageStringURL];
}
return cell;
}
i hope i’m blind and have just forgotten something very simple…
any ideas? it’s driving me nuts!
When loading images i am not getting full image for the first time i.e., i am getting image with some gray color. Why this happens. when i log, i am getting Corrupt JPEG data: premature end of data segment.
Can anyone help me out…………..
Thanks and Regards,
Swathi M
Hi!! Many people are looking for the same improvement: caching images already loaded for cells already scrolled. Scrolling up and down causes the reloading of previous loaded images. Someone talks about Wayne improvement which solves this issue, but it’s no more on this post.
Please help us!
Thanks in advance ;)
Anyone has any ideas on how to extend this to set the image in an UIButton?
Has anyone been able to add a spinner in the aboce AsynImageView class, I am struggling with this.
For info, I al using the async donwload for loading two picture in my code the following way:
CGRect frame;
frame.size.width=300; frame.size.height=100;
frame.origin.x=0; frame.origin.y=0;
AsyncImageView* asyncImage = [[[AsyncImageView alloc] initWithFrame:frame] autorelease];
NSURL *url_img = [NSURL URLWithString:newencodedImgUrl];
[asyncImage loadImageFromURL:url_img];
[imgGraphView addSubview:asyncImage];
AsyncImageView* asyncImage2 = [[[AsyncImageView alloc] initWithFrame:frame] autorelease];
NSURL *url_img2 = [NSURL URLWithString:encodedImgUrl];
[asyncImage2 loadImageFromURL:url_img2];
[pieGraphView addSubview:asyncImage2];
An I have tried to add the spinner in AsynImageView like this:
In loadImageFromUrl:
piespinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhite];
piespinner.center = CGPointMake(self.center.x, self.center.y);
[self addSubview:piespinner];
[self bringSubviewToFront:piespinner];
And in the connnectiondidfinishLoading:
[piespinner stopAnimating];
[piespinner release];
If anyone can help that’ll be much appreciated
Sorry for the spam, finally got it working. The code if anyone is interrested in loadImageFromUrl
piespinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhite];
piespinner.center = CGPointMake(self.center.x, self.center.y);
[self addSubview:piespinner];
[piespinner startAnimating];
MarkJ, great, great script, thanks a million.
Regarding the caching images problem, we solved that using this:
cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:nil] autorelease];
Good Luck!
Hello There this has been a great resource for me I want to thank mark a lot. For the viewers that want to know where wayne’s post is click on the link to older comments it is below the last comment on the page right before the word Trackbacks. I have implemented the yellowjacket code into my app.The problem is my app loads a lot of images in like over 1,000. Therefore allocating a lot of memory.
My question is for those of you using the yellojacket code. I would like to know how to empty the cache of the image objects. Within my view did disappear method of the table view’s delegate. Or in my app delegate’s did receive memory warning method.
The implementation of this would be greatly appreciated as I have slamming my head against the wall for about three days know trying to do this. I know it is something small that i’m just not doing properly. Thank you
Excellent tutorial. Thank you so much. I tried a bunch of different ways to get this to work before finding this site.
I’ve ported the code from Objective-C to MonoTouch (C#). Would you mind if I blogged about it, giving full reference to this site of course?
Ok people i finally found the way to empty the cache from the yellow jackets code. This code works for me. Hope it helps you out.
Make sure to make the reference to these methods in the header files of the class specified.
First you will need to add a method to the ImageCache Class to empty the NSDictionary in that class something like this.
-(void)emptyCache{
[myDictionary removeAllObjects];
NSLog(@”all objects removed Cache has been empty”);
}
Second You will add an identical method in the AsyncImageView class.
like this
-(void)emptyImages{
[imageCache emptyCache]; // tells the image class to implement the empty cache method
}
And lastly in your TableViewController Class. in the dealloc method add this line of code between the brackets.
[asyncImage emptyImages];
I implement this when my table view gets deallocated although you might want to els ware
I hope this helps the code has not been fully tested for leaks but if I am missing something feel free to modify.
Great Post. Thanks to everyone that contributed. I’m trying out the YellowJacket code and added what I hope is an improvement that solves the caching issue that a few users mentioned with earlier images showing up before new ones are loaded.
In AsyncImageView.m, line 71, I’ve provided an alternative image if a cached image does not exist for the current cell.
//————existing code—————
if (imageCache == nil) // lazily create image cache
imageCache = [[ImageCache alloc] initWithMaxSize:2*1024*1024]; // 2 MB Image cache
[urlString release];
urlString = [[url absoluteString] copy];
UIImage *cachedImage = [imageCache imageForKey:urlString];
if (cachedImage != nil) {
if ([[self subviews] count] > 0) {
[[[self subviews] objectAtIndex:0] removeFromSuperview];
}
UIImageView *imageView = [[[UIImageView alloc] initWithImage:cachedImage] autorelease];
imageView.contentMode = UIViewContentModeScaleAspectFit;
imageView.autoresizingMask =
UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
[self addSubview:imageView];
imageView.frame = self.bounds;
[imageView setNeedsLayout]; // is this necessary if superview gets setNeedsLayout?
[self setNeedsLayout];
return;
//———– Line 71 ————
} else {
// Use a default placeholder when no cached image is found
UIImageView *imageView = [[[UIImageView alloc] initWithImage:[UIImage
imageNamed:@"placeholder.png"]] autorelease];
imageView.contentMode = UIViewContentModeScaleAspectFit;
imageView.autoresizingMask =
UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
[self addSubview:imageView];
imageView.frame = self.bounds;
[imageView setNeedsLayout];
[self setNeedsLayout];
}
Be sure not to include a “return;” at the end of this else block or all your thumbs will be your default placeholder. You want the code to continue to run after you insert the placeholder.
Let me know if you find any problems!
hi,
i try to test on the loadImageFromURL, and don’t know how to get the image which should be return.
should you write out the usage of it in detail ?
Thank you for your help
Kan
Hi there.
Now this is truly a good piece of code. ANYWAY, HERE IS A TRICKY ONE: Can anyone figure out how to get device orientation change to work? If I setup something like that :
CGRect frame;
frame.size.width=320; frame.size.height=480;
frame.origin.x=0; frame.origin.y=0;
AsyncImageView* asyncImage = [[[AsyncImageView alloc] initWithFrame:frame] autorelease];
and I am in portrait mode an image which is larger than 320×480 will be autoresizedtofit. good so far. but then when I turn the device into landscape mode the image stays in size (does not resize itself to autowidth) and it sits at the very left. its like the whole frame with its 320×480 is rotated and does not resize itself to the landscape mode.
Anybody a solution to that?
Is anyone having crashes of the code with iPhone SDK 3.2 Beta?
Hi !
I have been trying to sort the cells in my UITable view in alphabetical order.
can anybody help me.
thanx in advance.