I am downloading image on background thread and setting this image to UIImageView on main thread. But it is giving me some warning like
Using low GPU priority for background rendering
Also please check the below code for reference
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(void) {
int randomImgNum = [[contentDict valueForKey:#"imageIndex"]intValue];
UIImage *image = [UIImage imageNamed:[Utils getImageFromIndex:randomImgNum]];
UIImage *newBlurredImage = [image blurredImageWithRadius:5.0];
dispatch_sync(dispatch_get_main_queue(), ^(void) {
customVieww.imgCardView.image = image;
[self setBlurrImage:newBlurredImage];
});
});
Please let me know what could be the issue and how we can resolve it.
Also please let me know for any explanation if I miss anything.
Thank you in advance.
if you want to work on the main view you have to use the main queue
because it can cause a problem if you use a background queue to change the UIView
Update code:
dispatch_async(dispatch_get_main_queue(), ^{
int randomImgNum = [[contentDict valueForKey:#"imageIndex"]intValue];
UIImage *image = [UIImage imageNamed:[Utils getImageFromIndex:randomImgNum]];
UIImage *newBlurredImage = [image blurredImageWithRadius:5.0];
dispatch_sync(dispatch_get_main_queue(), ^(void) {
customVieww.imgCardView.image = image;
[self setBlurrImage:newBlurredImage];
});
});
check this :
According to apple documentation, iOS does not allow its GPU to any background application for obvious reason that its not on Foreground.
https://developer.apple.com/library/content/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/ImplementingaMultitasking-awareOpenGLESApplication/ImplementingaMultitasking-awareOpenGLESApplication.html
With iOS, I know that all updates to the UI must be done on the main thread. Are reads from the UI safe to do from outside of the main thread?
I have some computationally expensive logic that needs to use MKMapView's convertCoordinate method, which needs toPointToView: UIView. Is this safe to do this from outside of the main thread?
yes you can do frame calculation or whatever logic in back ground. But UI operations like add subview , animation must be in main thread.
CGPoint annPoint = [self.mapView convertCoordinate:coord toPointToView:self.mapView];
mapPic = [[UIImageView alloc] initWithImage:[UIImage imageNamed:#"pic.png"]];
mapPic.frame = CGRectMake(annPoint.x, annPoint.y, 32, 32);
[self.view addSubview:macPic];
it can be written as per your requirement:
- (void)doCalculation
{
//you can use any string instead "com.mycompany.myqueue"
dispatch_queue_t backgroundQueue = dispatch_queue_create("com.mycompany.myqueue", 0);
dispatch_async(backgroundQueue, ^{
CGPoint annPoint = [self.mapView convertCoordinate:coord toPointToView:self.mapView];
mapPic = [[UIImageView alloc] initWithImage:[UIImage imageNamed:#"pic.png"]];
mapPic.frame = CGRectMake(annPoint.x, annPoint.y, 32, 32);
dispatch_async(dispatch_get_main_queue(), ^{
[self.view addSubview:macPic];
});
});
}
hope it will help
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?
As pointed out by bbum here,
the doc says: "For the most part, UIKit classes should only be used from the main thread of an application This is especially true for derived classes UIResponder or involve the manipulation of user interface of your application in any way. ".
I thought I understood that the methods of drawings could not be called in a background thread, so that the creation could be done in the background, as the drawRect method is only called when the view is added. But maybe i am wrong.
In summary, does that this kind of code is risky?
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^{
NSString *fileName = [pathToModel stringByAppendingPathComponent:[[compDico valueForKey:#"fileName"] lastPathComponent]];
UIImageView *imageView = [[UIImageView alloc] initWithImage:[UIImage imageWithContentsOfFile:
fileName]];
UILabel *label=[[UILabel alloc]initWithFrame:CGRectMake(10, 62, 190, 20)];
[label setText:[[someArray objectAtIndex:i-1] someText]];
[label setNumberOfLines:0];
label.font=[UIFont fontWithName:#"arial" size:10.0f];
[label setBackgroundColor:[UIColor clearColor]];
// Create some other view here
// ...
dispatch_async(dispatch_get_main_queue(), ^{
[self.view addSubview:imageView];
[self.view addSubview:label];
//Add other view here
// ...
});
});
Thanks for in advance your responses!
Yes, this is risky. How risky it is only Apple developers can say.
If the documentation says "don't use it", just don't use it.
Note that many UI objects can (and do) use shared resources. If you use them in a background thread, you'll get a race condition on the shared resource and anything can happen.
Hi i'm new to iPhone application.In my application i used tableview.
when i click table cell it does to detail view which has UIScrollview. where i'm loading
images from the NSMutableArray. and in that NSMutableArray there are some URL images.
when i click table cell it takes more time to load all images.
please i need help to complete my application.Many people said to do asynchronously
but i don't know how to do.
thanks in advance. waiting for a solution .
You should probably use a library such as AFNetworking to load your images, it handles downloading and swapping in the image on the tableView.
Firstly I would like recommend you Paul Hegarty tutorials (in Lectures 9, 10 and mainly 11 (multithreading)) you can learn too much.
Here some sample code can help you quickly but superfluously:
dispatch_queue_t imageFetchQ = dispatch_queue_create("image fetcher", NULL);
dispatch_async(imageFetchQ, ^{
NSData *imageData = [[NSData alloc] initWithContentsOfURL:self.imageURL]; // could take a while
// UIImage is one of the few UIKit objects which is thread-safe, so we can do this here
UIImage *image = [[UIImage alloc] initWithData:imageData];
// check to make sure we are even still interested in this image (might have touched away)
if (self.imageURL == imageURL) {
// dispatch back to main queue to do UIKit work
dispatch_async(dispatch_get_main_queue(), ^{
if (image) {
self.scrollView.zoomScale = 1.0;
self.scrollView.contentSize = image.size;
self.imageView.image = image;
self.imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);
}
});
}
});