The following code is blocking UI (can't tap another tab until images finish loading).
I have verified it is the UIImage *imageToShow = [[UIImage alloc] initWithData:data]; call that is the culprit by commenting that line out and keeping the download (which I know happens asynchronously and calls completion handler on main thread).
In the case where only the initWithData: line is commented, the UI response is fine. But how can that line be the line holding up the UI if it is clearly dispatched in background?
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
...
[objectStore getFileInContainer:#"public_images" filename:filename completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
UIImage *imageToShow = [[UIImage alloc] initWithData:data];
dispatch_async(dispatch_get_main_queue(), ^{
collectionImageView.image = imageToShow;
});
});
}];
...
}
UIImage doesn't read and decode the image until the first time it's actually used/drawn. To force this work to happen on a background thread, you have to use/draw the image on the background thread before doing the main thread -setImage: call. Many folks find this counter-intuitive. I explained this in considerable detail in another answer.
The part most likely to cause the UI to hang is the actual setting of the image to the image view, especially if the image is large, because this does (and must) occur on the main thread.
I would suggest resizing or scaling down the image first while you are still operating in the background thread. Then once you have the image resized, hop back on the main thread and assign it to the ``imageView.image```.
Here is a simple example of one way to resize a UIImage: Resize UIImage by keeping Aspect ratio and width
I am adding images from server. I am using NSMutableArray and custom UITableViewCell. Problem : When I run the project. UITableViewCell displaying same images. I think it refreshing cell. How can I fix that issue?
Below dispatch method I used,
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//this will start the image loading in bg
dispatch_async(concurrentQueue, ^{
NSError *nserror = nil;
NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString: [imagesArray2 objectAtIndex:indexPath.row]]options:NSDataReadingUncached error:&nserror];
//this will set the image when loading is finished
dispatch_async(dispatch_get_main_queue(), ^{
if (nserror)
{
[cell.imgview setImage:[UIImage imageNamed:#"placeholderimage.png"]];
}
else
{
[cell.imgview setImage:[UIImage imageWithData:imageData]];
}
[mytablview reloadData];
});
});
Thanks for any help.
Just reset cell imageView, before loading new image.
[cell.imgview setImage:[UIImage imageNamed:#"placeholderimage.png"]];
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(concurrentQueue, ^{
NSError *nserror = nil;
NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString: [imagesArray2 objectAtIndex:indexPath.row]]options:NSDataReadingUncached error:&nserror];
dispatch_async(dispatch_get_main_queue(), ^{
if (nserror) {
[cell.imgview setImage:[UIImage imageNamed:#"placeholderimage.png"]];
} else {
[cell.imgview setImage:[UIImage imageWithData:imageData]];
}
});
});
This doesn't really answer your question, but it may help you change the direction you're heading in.
When you finish loading the image, the cell may have already started loading a different image because UITableViews reuse their cells. The operation of asynchronously loading the image and setting it once it's done doesn't get cancelled and so you're going to get incorrect images popping up all over the shop, since there's no guarantee of which image loading finishes first.
Please see my comment regarding AFNetworking, which I highly recommend that you use, but if you go down a different path then you should be using some kind of NSOperation subclass for loading the images. That way you can cancel the operations when the cells are reused and you can then safely load a different image for that cell.
Save the url in the cell that will ultimately show the image. Then when you attempt to show the image after it arrives make sure the url in the cell matches the url of the image you are trying to show. If it doesn't match don't show it; this means the cell has been reused (due to scrolling or refresh) and no longer requires that image. No need to stop the loading of the image, just the display.
I have an UIView with an UIImageView subview. I need to load an image in the UIImageView without blocking the UI. The blocking call seems to be: UIImage imageNamed:. Here is what I thought solved this problem:
-(void)updateImageViewContent {
dispatch_async(
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
UIImage * img = [UIImage imageNamed:#"background.jpg"];
dispatch_sync(dispatch_get_main_queue(), ^{
[[self imageView] setImage:img];
});
});
}
The image is small (150x100).
However the UI is still blocked when loading the image. What am I missing ?
Here is a small code sample that exhibits this behaviour:
Create a new class based on UIImageView, set its user interaction to YES, add two instances in a UIView, and implement its touchesBegan method like this:
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
if (self.tag == 1) {
self.backgroundColor= [UIColor redColor];
}
else {
dispatch_async(dispatch_get_main_queue(), ^{
[self setImage:[UIImage imageNamed:#"woodenTile.jpg"]];
});
[UIView animateWithDuration:0.25 animations:
^(){[self setFrame:CGRectInset(self.frame, 50, 50)];}];
}
}
Assign the tag 1 to one of these imageViews.
What happens exactly when you tap the two views almost simultaneously, starting with the view that loads an image? Does the UI get blocked because it's waiting for [self setImage:[UIImage imageNamed:#"woodenTile.jpg"]]; to return ? If so, how may I do this asynchronously ?
Here is a project on github with ipmcc code
Use a long press then drag to draw a rectangle around the black squares. As I understand his answer, in theory the white selection rectangle should not be blocked the first time the image is loaded, but it actually is.
Two images are included in the project (one small: woodenTile.jpg and one larger: bois.jpg). The result is the same with both.
Image format
I don't really understand how this is related to the problem I still have with the UI being blocked while the image is loaded for the first time, but PNG images decode without blocking the UI, while JPG images do block the UI.
Chronology of the events
The blocking of the UI begins here..
.. and ends here.
AFNetworking solution
NSURL * url = [ [NSBundle mainBundle]URLForResource:#"bois" withExtension:#"jpg"];
NSURLRequest * request = [NSURLRequest requestWithURL:url];
[self.imageView setImageWithURLRequest:request
placeholderImage:[UIImage imageNamed:#"placeholder.png"]
success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
NSLog(#"success: %#", NSStringFromCGSize([image size]));
} failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
NSLog(#"failure: %#", response);
}];
// this code works. Used to test that url is valid. But it's blocking the UI as expected.
if (false)
if (url) {
[self.imageView setImage: [UIImage imageWithData:[NSData dataWithContentsOfURL:url]]]; }
Most of the time, it logs: success: {512, 512}
It also occasionnaly logs: success: {0, 0}
And sometimes: failure: <NSURLResponse: 0x146c0000> { URL: file:///var/mobile/Appl...
But the image is never changed.
The problem is that UIImage doesn't actually read and decode the image until the first time it's actually used/drawn. To force this work to happen on a background thread, you have to use/draw the image on the background thread before doing the main thread -setImage:. This worked for me:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
UIImage * img = [UIImage imageNamed:#"background.jpg"];
// Make a trivial (1x1) graphics context, and draw the image into it
UIGraphicsBeginImageContext(CGSizeMake(1,1));
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextDrawImage(context, CGRectMake(0, 0, 1, 1), [img CGImage]);
UIGraphicsEndImageContext();
// Now the image will have been loaded and decoded and is ready to rock for the main thread
dispatch_sync(dispatch_get_main_queue(), ^{
[[self imageView] setImage: img];
});
});
EDIT: The UI isn't blocking. You've specifically set it up to use UILongPressGestureRecognizer which waits, by default, a half a second before doing anything. The main thread is still processing events, but nothing is going to happen until that GR times out. If you do this:
longpress.minimumPressDuration = 0.01;
...you'll notice that it gets a lot snappier. The image loading is not the problem here.
EDIT 2: I've looked at the code, as posted to github, running on an iPad 2, and I simply do not get the hiccup you're describing. In fact, it's quite smooth. Here's a screenshot from running the code in the CoreAnimation instrument:
As you can see on the top graph, the FPS goes right up to ~60FPS and stays there throughout the gesture. On the bottom graph, you can see the blip at about 16s which is where the image is first loaded, but you can see that there's not a drop in the frame rate. Just from visual inspection, I see the selection layer intersect, and there's a small, but observable delay between the first intersection and the appearance of the image. As far as I can tell, the background loading code is doing its job as expected.
I wish I could help you more, but I'm just not seeing the problem.
You can use AFNetworking library , in which by importing the category
"UIImageView+AFNetworking.m"
and by using the method as follows :
[YourImageView setImageWithURL:[NSURL URLWithString:#"http://image_to_download_from_serrver.jpg"]
placeholderImage:[UIImage imageNamed:#"static_local_image.png"]
success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
//ON success perform
}
failure:NULL];
hope this helps .
I had a very similar issue with my application where I had to download lot of images and along with that my UI was continuously updating. Below is the simple tutorial link which resolved my issue:
NSOperations & NSOperationQueues Tutorial
this is the good way:
-(void)updateImageViewContent {
dispatch_async(dispatch_get_main_queue(), ^{
UIImage * img = [UIImage imageNamed:#"background.jpg"];
[[self imageView] setImage:img];
});
}
Why don't you use third party library like AsyncImageView? Using this, all you have to do is declare your AsyncImageView object and pass the url or image you want to load. An activity indicator will display during the image loading and nothing will block the UI.
-(void)touchesBegan: is called in the main thread. By calling dispatch_async(dispatch_get_main_queue) you just put the block in the queue. This block will be processed by GCD when the queue will be ready (i.e. system is over with processing your touches). That's why you can't see your woodenTile loaded and assigned to self.image until you release your finger and let GCD process all the blocks that have been queued in the main queue.
Replacing :
dispatch_async(dispatch_get_main_queue(), ^{
[self setImage:[UIImage imageNamed:#"woodenTile.jpg"]];
});
by :
[self setImage:[UIImage imageNamed:#"woodenTile.jpg"]];
should solve your issue… at least for the code that exhibits it.
Consider using SDWebImage: it not only downloads and caches the image in the background, but also loads and renders it.
I've used it with good results in a tableview that had large images that were slow to load even after downloaded.
https://github.com/nicklockwood/FXImageView
This is an image view which can handle background loading.
Usage
FXImageView *imageView = [[FXImageView alloc] initWithFrame:CGRectMake(0, 0, 100.0f, 150.0f)];
imageView.contentMode = UIViewContentModeScaleAspectFit;
imageView.asynchronous = YES;
//show placeholder
imageView.processedImage = [UIImage imageNamed:#"placeholder.png"];
//set image with URL. FXImageView will then download and process the image
[imageView setImageWithContentsOfURL:url];
To get an URL for your file you might find the following interesting:
Getting bundle file references / paths at app launch
When you are using AFNetwork in an application, you do not need to use any block for load image because AFNetwork provides solution for it. As below:
#import "UIImageView+AFNetworking.h"
And
Use **setImageWithURL** function of AFNetwork....
Thanks
One way i've implemented it is the Following: (Although i do not know if it's the best one)
At first i create a queue by using Serial Asynchronous or on Parallel Queue
queue = dispatch_queue_create("com.myapp.imageProcessingQueue", DISPATCH_QUEUE_SERIAL);**
or
queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,0);
**
Which ever you may find better for your needs.
Afterwards:
dispatch_async( queue, ^{
// Load UImage from URL
// by using ImageWithContentsOfUrl or
UIImage *imagename = [UIImage imageWithData:[NSData dataWithContentsOfURL:url]];
// Then to set the image it must be done on the main thread
dispatch_sync( dispatch_get_main_queue(), ^{
[page_cover setImage: imagename];
imagename = nil;
});
});
There is a set of methods introduced to UIImage in iOS 15 to decode images and create thumbnails asynchronously on background thread
func prepareForDisplay(completionHandler: (UIImage?) -> Void)
Decodes an image asynchronously and provides a new one for display in views and animations.
func prepareThumbnail(of: CGSize, completionHandler: (UIImage?) -> Void)
Creates a thumbnail image at the specified size asynchronously on a background thread.
You can also use a set of similar synchronous APIs, if you need more control over where you want the decoding to happen, e.g. specific queue:
func preparingForDisplay() -> UIImage?
func preparingThumbnail(of: CGSize) -> UIImage?
I'm trying to load an image from a local URL using the following:
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:fileURL]];
[self.imageView setImage:image];
NSLog(#"imageView set");
So I see in the console "imageView set" almost immediately, but it takes a very long time for it to be reflected in the UI (sometimes a few minutes!).
Any idea why this is happening?
This happened to me when I was setting the image in a background thread (in which I was downloading the image file). Just make sure your code is run in the main thread. The image will change immediately.
// In a background thread...
UIImage *image = ... // Build or download the image
dispatch_async(dispatch_get_main_queue(), ^{
[self.imageView setImage:image]; // Run in main thread (UI thread)
});
You should load up instruments and see what exactly it is doing.
For starters, you should do what you can to avoid I/O on the drawing thread. Also, are there other I/O requests at the same time?
A UIImage does not necessarily need to be a singular bitmap representation -- it may be backed by a cache and/or loaded lazily. So just because an 'image' is 'set', does not mean that the optimal bitmap has been loaded into memory and prepared for rendering -- it may be deferred until render (draw) is requested.
Profiling OTOH will tell you (generally) why it is taking longer than expected.
Image loading from NSData takes more time.
Instead you can simply use the following code:
UIImage *image = [UIImage imageWithContentsOfFile:fileURL];
Try it.
So I tried the following:
dispatch_async(dispatch_get_global_queue(0, 0), ^{
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:fileURL]];
[self.imageView setImage:image];
NSLog(#"imageView set");
});
And it is quite a bit faster. Not sure why though.
I have an iOS app that has a UITableView with custom TableViewCells that contain a UIImageView. The image is loaded from a web service, so during the initial load, I display a "loading" image, and then use gcd to dispatch and get the image matching the data for that cell.
When I use a DISPATCH_QUEUE_PRIORITY_HIGH global queue to perform the image fetch, I sporadically get the wrong images loading in the tableview cells. If I use my own custom queue then the correct images get populated into the cells but the tableview performance is awful.
Here is the code...
// See if the icon is in the cache
if([self.photoCache objectForKey:[sample valueForKey:#"api_id"]]){
[[cell sampleIcon]setImage:[self.photoCache objectForKey:[sample valueForKey:#"api_id"]]];
}
else {
NSLog(#"Cache miss");
[cell.sampleIcon setImage:nil];
dispatch_queue_t cacheMissQueue = dispatch_queue_create("cacheMissQueue", NULL);
//dispatch_queue_t cacheMissQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(cacheMissQueue, ^{
if(sample.thumbnailFilename && sample.api_id){
NSData *thumbNailData = [[NSData alloc] initWithContentsOfFile:sample.thumbnailFilename];
UIImage *thumbNailImage = [[UIImage alloc]initWithData:thumbNailData];
if(thumbNailImage){
// Set the cell
dispatch_sync(dispatch_get_main_queue(), ^{
[[cell sampleIcon]setImage:thumbNailImage];
[cell setNeedsLayout];
});
// save it to cache for future references
NSLog(#"DEBUG: Saving to cache %# for sample %#",sample.thumbnailFilename,[sample objectID]);
[self.photoCache setObject:thumbNailImage forKey:sample.api_id];
}
}
});
dispatch_release(cacheMissQueue);
}
Watching the WWDC 2012 session #211 helped a lot and I changed the code from using GCD to NSOperationQueue and it solved the problem.
New code...
[[self imgQueue]addOperationWithBlock:^{
if(sample.thumbnailFilename && sample.api_id){
NSData *thumbNailData = [[NSData alloc] initWithContentsOfFile:sample.thumbnailFilename];
UIImage *thumbNailImage = [[UIImage alloc]initWithData:thumbNailData];
if(thumbNailImage){
// Set the cell
[[NSOperationQueue mainQueue]addOperationWithBlock:^{
[[cell sampleIcon]setImage:thumbNailImage];
[cell setNeedsLayout];
}];
// save it to cache for future references
[self.photoCache setObject:thumbNailImage forKey:sample.api_id];
}
}
}];
When you finally get an image, you need an association between the indexPath of the cell and the image. Since this is on a backgound thread, what I suggest you do is post a nofification using a block to the mainQueue that such and such an image is available. On the main thread only, you ask the tableView for the array of visible cells, and if the cell you have an image for is showing, you can then set the image directly at that time (your on the main thread, you know the cell is there and showing, and its not going to change for this runloop iteration.) If the cell is not showing, no problem, next time that cell comes into scope you will have the image waiting for it. I am doing this now in my app, its been out many months, and its all working well and getting good reviews on responsiveness (just as your app will if you do this!)