How to get dynamicaly height of image in dispatch_async method in scrollview - objective-c

Question

I am using below code to get the images from server. i want to get dynamicaly height of image and add image in scrollview.
From below code when i get the height outside the dispatch_async method it shows zero.
How i can get the dynamically height of image with async image load.
- (void)viewDidLoad {
[self LoadViewPublicEvents];
}
-(void) LoadViewPublicEvents
{
for (int i=0;i<arrayPublicEvents.count;i++)
{
UIImageView *img_vw1=[[UIImageView alloc] init];
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:#"http://abc.us/uploads/event/%#",[[arrayPublicEvents objectAtIndex:i] valueForKey:#"image"]]]];
UIImage *images = [[UIImage alloc]initWithData:imageData];
dispatch_async(dispatch_get_main_queue(), ^{
img_vw1.image = images;
scaledHeight = images.size.height;
});
});
NSLog(#"%f",scaledHeight); // it print zero
img_vw1.backgroundColor=[UIColor clearColor];
img_vw1.frame=CGRectMake(0,y+5,screen_width,197);
[img_vw1 setContentMode:UIViewContentModeScaleAspectFit];
img_vw1.backgroundColor=[UIColor clearColor];
[self.scrll_vw addSubview:img_vw1];
}
}
Thanks in advance

Answer 1

Your code:
NSLog(#"%f",scaledHeight); // it print zero
img_vw1.backgroundColor=[UIColor clearColor];
img_vw1.frame=CGRectMake(0,y+5,screen_width,197);
[img_vw1 setContentMode:UIViewContentModeScaleAspectFit];
img_vw1.backgroundColor=[UIColor clearColor];
[self.scrll_vw addSubview:img_vw1];
Is being executed, before you loaded the image.
Thus you have to either wait (herefore you could use semaphores until the thread has finished) OR you place it inside of your block.
As you want to modify the UI it makes sense to place it into the main block:
UIImageView *img_vw1=[[UIImageView alloc] init];
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:#"http://abc.us/uploads/event/%#",[[arrayPublicEvents objectAtIndex:i] valueForKey:#"image"]]]];
UIImage *images = [[UIImage alloc]initWithData:imageData];
dispatch_async(dispatch_get_main_queue(), ^{
img_vw1.image = images;
scaledHeight = images.size.height;
NSLog(#"%f",scaledHeight); // it print zero
img_vw1.backgroundColor=[UIColor clearColor];
img_vw1.frame=CGRectMake(0,y+5,screen_width,197);
[img_vw1 setContentMode:UIViewContentModeScaleAspectFit];
img_vw1.backgroundColor=[UIColor clearColor];
[self.scrll_vw addSubview:img_vw1];
});
});
For more information, here is a link to Apple's documentation: https://developer.apple.com/library/content/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html