Objective C: For loop is finished before image is finished - ios

Question

I have an arrayOfLinks that contains a lot of url links. I need to get images from these links. I am using the following code to do so.
- (void)getImages {
NSArray *links = arrayOfLinks;
for (NSString *link in links ) {
[self.picImage sd_setImageWithURL:[NSURL URLWithString:link] placeholderImage:nil options:SDWebImageHighPriority completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
NSLog(#"pic image set finished");
}];
}
[self finishSettingImages];
NSLog(#"for loop finished");
}
This almost works, but the problem is the for loop finishes before the images are set. I want to call the method finishSettingImages after each image is set. What would be the best way to do this?
Edit: I want the method 'finishSettingImages' called after ALL the images are set. Sorry for not clarifying this.

Answer 1

Just use GCD groups.
NSArray *links = arrayOfLinks;
dispatch_group_t group = dispatch_group_create();
for (NSString *link in links) {
dispatch_group_enter(group);
[self.picImage sd_setImageWithURL:[NSURL URLWithString:link] placeholderImage:nil options:SDWebImageHighPriority completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
dispatch_group_leave(group);
}];
}
// you can use dispatch_get_main_queue() or background here
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
// all is done
});
If you only need to get images, you can use SDWebImageManager:
NSArray *links = arrayOfLinks;
__block NSMutableArray *images = [NSMutableArray new];
dispatch_group_t group = dispatch_group_create();
for (NSString *link in links) {
dispatch_group_enter(group);
SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager downloadImageWithURL:[NSURL URLWithString:link]
options:SDWebImageRetryFailed
progress:^(NSInteger receivedSize, NSInteger expectedSize) {}
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
if (image) {
[images addObject:image];
}
dispatch_group_leave(group);
}];
}
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
// all is done
for (UIImage *image in images) {
//do something with images
}
});
Code example for Swift 3/4:
var arrayOfLinks = ... // you string links are here
let dispatchGroup = DispatchGroup()
for link in arrayOfLinks {
if let url = URL(string: link) {
dispatchGroup.enter()
self.imageView.sd_setImage(with: url) { (image, error, cacheType, url) in
dispatchGroup.leave()
}
}
}
dispatchGroup.notify(queue: .main) {
print("all is done ")
}

Answer 2

EDIT: this answer is bad, as Josh in the comments pointed out, I failed to see the race condition with the index == total check
Try this:
- (void)getImages
{
NSArray *links = arrayOfLinks;
NSUInteger total = [links count];
__block NSUInteger index = 0;
__weak id *weakSelf = self; //if your picImage retains the completion block as a property,
//then you will want to capture a weak reference to self, so you don't have a retain cycle
for (NSString *link in links ) {
[self.picImage sd_setImageWithURL:[NSURL URLWithString:link] placeholderImage:nil options:SDWebImageHighPriority completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
NSLog(#"pic image set finished");
index += 1;
if( index == total )
{
//again if you don't store this block as a property, you can just reference self instead of weakSelf
[weakSelf finishSettingImages];
}
}];
}
NSLog(#"for loop finished");
}
the __block attribute makes the block use a pointer to the variable so that it can modify the value across executions of the block. That's how we can increment the index variable. total is not __block so the current state of total when you define the block is what's going to be used every time the block executes. So even if you set total to 100 from 10 within the block, the next time the block executes it will still see total as 10.