Feel lag when scroll and sametime load full screen image from asset in background thread

Question

Here is the thing: I have a scroll view, it did the lazy load for full screen image of user's photo:

[self.assetsLibrary assetForURL:[NSURL URLWithString:[[self.assets objectAtIndex:index] objectForKey:@"asset_url"]]
    resultBlock:^(ALAsset *asset) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            CGImageRef cgImage = asset.defaultRepresentation.fullScreenImage;
            UIImage *image = [UIImage imageWithCGImage:cgImage];
            dispatch_async(dispatch_get_main_queue(), ^{
                imageView.image = image;
            });
        });
                    }
failureBlock:^(NSError *error) {
    NSLog(@"error");
}];

I know it is expensive to load full screen image, so I put it in to the background thread, but it is still lag when I do the scroll. And still lag even I change it like this:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                            CGImageRef cgImage = asset.defaultRepresentation.fullScreenImage;
                            UIImage *image = [UIImage imageWithCGImage:cgImage];
                            imageView.image = image;
                            dispatch_async(dispatch_get_main_queue(), ^{
                            });
                        });

Obviously, nothing to do in the main queue, but it still lag until I comment the line:

// CGImageRef cgImage = asset.defaultRepresentation.fullScreenImage;

So I am so confused, is there something wrong when I used GCD? Somebody can help me to explain it? Any thing will be helpful.

Thank you, guys.

UPDATE

To @Fogmeister : The size of the photo is the full screen size, actuel imageView size is around half. Even I comment the line: "imageView.image = image;" it is still lag. Which means it is not from the resizing. I know where the time is being taken, here: "asset.defaultRepresentation.fullScreenImage;". When I comment it, everything fine, there is no more lag. So, which I don't understand is, I've already put it in the background thread...

Answer 1

Ok, finally I solved problem:

Instead of getting image directly by

asset.defaultRepresentation.fullScreenImage

I use the method from Apple's Exemple PhotosByLocation (code below) to get the image in BG thread. That works great, no more lag when scroll. But I am still confused, I don't know exactly why. So I appreciate if someone can explain it to me.

- (UIImage *)fullSizeImageForAssetRepresentation:(ALAssetRepresentation *)assetRepresentation {

UIImage *result = nil;
NSData *data = nil;

uint8_t *buffer = (uint8_t *)malloc(sizeof(uint8_t)*[assetRepresentation size]);
if (buffer != NULL) {
    NSError *error = nil;
    NSUInteger bytesRead = [assetRepresentation getBytes:buffer fromOffset:0 length:[assetRepresentation size] error:&error];
    data = [NSData dataWithBytes:buffer length:bytesRead];

    free(buffer);
}

if ([data length]) {
    CGImageSourceRef sourceRef = CGImageSourceCreateWithData((__bridge CFDataRef)data, nil);

    NSMutableDictionary *options = [NSMutableDictionary dictionary];

    [options setObject:(id)kCFBooleanTrue forKey:(id)kCGImageSourceShouldAllowFloat];
    [options setObject:(id)kCFBooleanTrue forKey:(id)kCGImageSourceCreateThumbnailFromImageAlways];
    [options setObject:(id)[NSNumber numberWithFloat:640.0f] forKey:(id)kCGImageSourceThumbnailMaxPixelSize];
    //[options setObject:(id)kCFBooleanTrue forKey:(id)kCGImageSourceCreateThumbnailWithTransform];

    CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(sourceRef, 0, (__bridge CFDictionaryRef)options);

    if (imageRef) {
        result = [UIImage imageWithCGImage:imageRef scale:[assetRepresentation scale] orientation:(UIImageOrientation)[assetRepresentation orientation]];
        CGImageRelease(imageRef);
    }

    if (sourceRef) CFRelease(sourceRef);
}

return result;
}

Answer 2

You're solution taken from Apple's PhotosByLocation is actually grabbing the biggest resolution image, not the fullscreen image. IOW, it's essentially the same as calling fullResolutionImage instead of fullScreenImage. How that fixes your problem, I'm not sure. I'm struggling with the same performance issue. If I use fullScreenImage, I get lags in my scrolling. But switching to fullResolutionImage gets rid of the lags. fullResolutionImage takes about twice as long as fullScreenImage, but since this is always in the background, it shouldn't really matter how much time it takes. I suspect that fullScreenImage is returning an image that needs some sort of additional processing once it gets rendered to the screen in the main thread - hence the lag.

Answer 3

Do you know the actual size of the photo? What is very expensive is scrolling images that are being resized to fit the screen.

Seeing as you're already loading in a BG thread it might be worth resizing the image to the size you are displaying it at before sticking it on the screen.

You can see where the time is being taken by using the CoreAnimation tool in Instruments by profiling the app from Xcode. It will even tell you which line of code is causing the slow down and missed animation frames.

Answer 4

From the apple documentation:

DISPATCH_QUEUE_PRIORITY_DEFAULT
Items dispatched to the queue run at the default priority; the queue is scheduled for execution after all high priority queues have been scheduled, but before any low priority queues have been scheduled.

DISPATCH_QUEUE_PRIORITY_BACKGROUND
Items dispatched to the queue run at background priority; the queue is scheduled for execution after all high priority queues have been scheduled and the system runs items on a thread whose priority is set for background status. Such a thread has the lowest priority and any disk I/O is throttled to minimize the impact on the system.

You're running it in a separate thread, but that's not necessarily a thread "in the background." A background thread loading something in my experience will be completely blocked by doing a UI update such as scrolling a UIScrollView. Have you tried using DISPATCH_QUEUE_PRIORITY_BACKGROUND?