Asynchronously load UIImage to my data model?

Question

I have a data model that includes a property that is an array of UIImages and also a property that is an array of imageURLs (representing the same images).

Upon loading up a certain view, I populate a scrollview with images from the URLS using SDWebImage , which looks nice.

for (NSURL *url in self.project.imageURLs) {
    UIImageView *imageView = [[UIImageView alloc]init];
    [imageView setImageWithURL:[NSURL URLWithString:urlString] placeholderImage:[UIImage imageNamed:@"loading"] usingActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    [imageView setContentMode:UIViewContentModeScaleAspectFit];
    [self.scrollview addSubview:imageView];
    ...some other frame calculations...
}

My question is, how could I, at the same time, load these UIIamges into my data model (self.project.images) without locking up the UI. I'm assuming it's some kind of dispatch_async, but I can't figure out where to call this.

I have both properties because some of the images are coming from a web source and some from the local device/camera.

One possible solution is, when i am asynchronously loading up the data model initially with the urls, to go ahead and load up the UIImages at that time, but it seems like that is using a big chunk of memory that might not be needed. Since I am loading up to 20 projects, all with arrays of images.

Answer 1

In swift 4.2: use this way

  DispatchQueue.main.async {
        // loading data, adding to an array, setting an image to UIImageView
    }

Memory managing version:

  DispatchQueue.main.async {[weak weakSelf = self] in
        // loading data, adding to an array, setting an image to UIImageView
        // use weakSelf to avoid a memory leak: weakSelf.project.images ...
        // best way is to create a method in your ViewController – self.updateImage(), and call in this dispatch
    }

objC version:

__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
     // use weakSelf here to avoid memory leaking like this:
     // [weakSelf updateImageView: indexOfImage];
});