Image not showing on preloaded UIImageView - ios

Question

When app launches I add UIImageViews to screen with no image. Then I load image one by one using NSThread and set it for those views. The UiImageViews remain blank until after all images have been loaded and the thread finishes. Why doesn't the image show as soon as I set it for the UIImageView, why does it wait for NSThread to end?
To add image views :
for(i = value; i < val; i++){
UIButton *newbutton = [UIButton buttonWithType:UIButtonTypeCustom];
[newbutton setBackgroundColor:[UIColor whiteColor]];
UIImageView *newback = [[UIImageView alloc] init];
newback.contentMode = UIViewContentModeScaleAspectFit;
[newback setFrame:CGRectMake(0, 0, 140, 140)];
[newbutton addSubview:newback];
height = MIN(300, newSize.height);
[newbutton setFrame:CGRectMake(currentX, currentY, width, height)];
newbutton.layer.cornerRadius = 10;
newbutton.clipsToBounds = YES;
[newbutton addTarget:self action:#selector(processButtonClick:) forControlEvents:UIControlEventTouchUpInside];
}
To add images in a thread I call a function which does following :
for(i = value; i < val; i++){
UIImage *newimage = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:#“http://test.com/a.jpg”, hash]]]];
UIButton *newbutton = (UIButton*)[self.subviews objectAtIndex:i];
[((UIImageView*)[newbutton.subviews objectAtIndex:0]) newimage];
}

Answer 1

Generally speaking, UIKit is not thread-safe. Fetch data from URL in a background thread and set the image in the main thread.
EDIT:
In your main thread do something like
dispatch_async(someDispatchQ, ^{
[self functionToGetData];
});
and inside functionToGetData do
for(i = value; i < val; i++){
NSData *data = //get Data
UIImage *newImage = //make image from data
dispatch_async(dispatch_get_main_queue(), ^{
[imageView setImage:newImage];
});
}
This will ensure that you are using a background thread to fetch data/make image and using the main thread to set the image. This also eliminates the need to use a timer to constantly poll the background thread since the background thread automatically dispatches image-setting to the main thread as soon as an image is received.

Answer 2

use-
import class ImageDownloader.h & .m
link-> https://github.com/psychs/imagestore/tree/master/Classes/Libraries/ImageStore
//Now use this method to download
-(void)downloadImageWithURL:(NSString *)url{
if(self.downloder)return; //If already downloading please stay away
url=[NSString stringWithFormat:#"%#%0.0fx%0.0f/%#/%#",IMAGE_ENDPOINT,self.imgViewExhibitors.frame.size.width,self.imgViewExhibitors.frame.size.height,#"fit",[ImagePath forURLString:url]];
self.downloder=[[ImageDownloader alloc] init];
self.downloder.delegate=self;
self.downloder.indicator=self.indicator;
[self.downloder downloadImageWithURL:url];
}
-(void)imageDownloaded:(UIImage *)image forIndexPath:(NSIndexPath *)indexPath{
if(image){
//Put the downloaded image in dictionary to update while scrolling
//[self.speakerData setObject:image forKey:#"image"];
self.imgViewExhibitors.image=image;
}
}

Answer 3

A simple fix would be
for(i = value; i < val; i++){
UIImage *newimage = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:#“http://test.com/a.jpg”, hash]]]];
UIButton *newbutton = (UIButton*)[self.subviews objectAtIndex:i];
// this ensures that the UI update (setting the image) is done on the main thread.
// This is important for the UI to update properly.
dispatch_async(dispatch_get_main_queue(), ^{
[((UIImageView*)[newbutton.subviews objectAtIndex:0]) setImage:newimage];
}
}