Question

NOTE: I am using ARC

I have a lazy image loading technique that works brilliantly when loading images from a web url. However, I am having problems loading images from core data. Here is the method I am using:

I first alloc init my mutable arrays, load my data from core data and then store it in an array. Then I have this code:

NSOperationQueue *queue = [NSOperationQueue new];
NSInvocationOperation *operation = [[NSInvocationOperation alloc]
                                    initWithTarget:self
                                    selector:@selector(loadImage)
                                    object:nil];
[queue addOperation:operation];

And then…

- (void)loadImage {
    for (int i = 0; i < [myArrayFromCoreData count]; i++) {
        AnObject *myObject = [myArrayFromCoreData objectAtIndex:i];
        UIImage* image;
        if ([myObject.image length] == 0) {
            image = [UIImage imageNamed:@"default.png"];
        }
        else {
            image = [[UIImage alloc] initWithData:myObject.image];
        }
        [self performSelectorOnMainThread:@selector(displayImage:) withObject:[NSArray arrayWithObjects:image,myObject.myObjectId, nil] waitUntilDone:NO];
    }
}

- (void)displayImage:(NSArray*)array {
    [loadedImages setObject:[array objectAtIndex:0] forKey:[array objectAtIndex:1]];
    [self.myTable reloadData];
}

Up to here there is no lag and an NSLog shows that all my images are added to the myObjectLoadedImages array.

The problem I am having is there is a lag when scrolling the table view and it is sometimes causing a crash. Here is the code I am using to display the loaded images:

UIImage* image = [loadedImages objectForKey:myObject.myObjectId];
if (image != NULL) {
    myImageView.image = image;
}
else {
    myImageView.image = [UIImage imageNamed:@"default.png"];
}

This is in the cellForRowAtIndexPath method so it is being called every time the cell is displayed. Is there something I am doing wrong here as my code is adapted from a class which is working fine loading images from a web url.

Answer 1

First thought – you’re not caching your UIImages, so they’re not loaded from memory.

Have you tried https://github.com/nicklockwood/AsyncImageView/ ?
In my opinion it’s the best library for loading async images to any view, either from documents and web. It supports cache, progress views and many, many things that simples the code in controllers.