You’ll want to read this post from 2011: HJCache – we’ve released a comprehensive library for free that makes it easy to use dynamically loaded and cached images in your iOS apps, as per this intro article…
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:
.
.
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;
}
I ended up preloading the text using a for loop.
Does create a bit of lag initially, but better than lagging whilst scrolling.
If people know how to asynchronously load text, I’d love to hear about tim
timfazio@gmail.com
Hmm… if you mark a view with [view setNeedsDisplay] then it will redraw, and you can use UITableView’s cellForRowAtIndexPath to get the cell view by row number. So once you have your asychronously loaded text you can lookup the UITableViewCell, change the text, and setNeedsDisplay:true (setNeedsDisplay might happen automatically when you change the text). I haven’t tried this, but let us know in a comment if it works for you.
Cheers, Mark.
Mark, Thank you for sharing. I’m having an issue with the delegate method connection:didReceiveData:, when I try to access “incrementalData” I get “unable to read unknown load command 0×80000022″ in the debugger console log. When I comment out appendData, I don’t get the error, no image of course. I’ve also tried displaying [incrementalData length} in the log but that throws the error also.
I’ve ugraded to Snow Leopard, sdk 3.0, not sure if that matters.
Any ideas? Thanks John
Hi John, try these and let us know in the comments what you find out:
- set a breakpoint at the start of that method and inspect the state of the object just before the crash.
- try running on a real device vs the simulator
- change your compile settings in XCode, see if it makes a difference if you target 3.0 or 2.2.1.
I went back to Leopard the same day I installed Snow Leopard because the Snow Leopard XCode or simulator is running differently, I’m not sure what the difference is. Also, there are definitely a few subtle memory management differences between the device and the simulator, so buggy code behaves differently on each.
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!