Question

The problem is that I manage scrollView with lots of tiles in it.
Each visible tile display image loaded from URL or (after first URL load) cached file in background. Invisible tiles recycles (set new frame and redraw).

Image load depends on tile position.

With long range scroll there is multiple redraw called for each tile: each tile loads (and display) different image several times before display the correct one.

So problem is to cancel all previously added operations for tile before add new.

I subclass NSInvocationOperation just to contain context object to detect operation attached to and before add new operation I canceling all operation for same tile:

 -(void)loadWithColler:(TileView *)coller {    
    if (queue == nil) {
        queue = [NSOperationQueue new];
    }

    NSInvocationOperationWithContext *loadImageOp = [NSInvocationOperationWithContext alloc];
    [loadImageOp initWithTarget:self selector:@selector(loadImage:) object:loadImageOp];
    [loadImageOp setContext:coller];

    [queue setSuspended:YES];
    NSArray *opers = [queue operations];
    for (NSInvocationOperationWithContext *nextOperation in opers) {

        if ([nextOperation context] == coller) {
            [nextOperation cancel];
        }

    }

    [queue addOperation:loadImageOp]; 
    [queue setSuspended:NO];    
    [loadImageOp release];
}

And in operation itself I check isCancelled:

    -(void)loadImage:(NSInvocationOperationWithContext *)operation {

        if (operation.isCancelled) return;

        TileView *coller = [operation context];

        /* TRY TO GET FILE FROM CACHE */    
        if (operation.isCancelled) return;

        if (data) {

            /* INIT WITH DATA IF LOADED */

        } else {
            /* LOAD FILE FROM URL AND CACHE IT */
        }

        if (operation.isCancelled) return;

        NSInvocationOperation *setImageOp = [[NSInvocationOperation alloc] initWithTarget:coller selector:@selector(setImage:) object:cachedImage];
        [[NSOperationQueue mainQueue] addOperation:setImageOp];
        [setImageOp release];

    }

But it is do nothing. Some times early returns works but tiles still load many images before the correct one.

So how could I success? And could this lots of unneeded operations cause delays on main thread when scrolling? (Because delays are exists and I do not know why…all load in background..)

Update:

With NSLog:
isCancelled while executing:
>
cancel loadImage method for:
>

So canceling work.

Now I save reference to last operation in TileView object and perform setImage operation only if invoked operation is equal to TileView operation.

Doesn’t make any difference…

Looks like there IS number of operations to load different images to one tile invoked one after another.

Any another suggestions?

For clearance:

There is singleton DataLoader (all code from it). And all tiles has call to it in drowRect:

[[DataLoader sharedDataLoader] loadWithColler:self];

Update:

NSInvocationOperation subclass:

@interface NSInvocationOperationWithContext : NSInvocationOperation {
    id context;
}

@property (nonatomic,retain,readwrite) id context;

@end


@implementation NSInvocationOperationWithContext

@synthesize context;


- (void)dealloc
{
    [context release];
    [super dealloc];
}
@end

Thanks a lot for any help!

SOLUTION:

From answer below: need to subclass from NSOperation

As I subclass NSOperation and put all loadImage: code into it “main” method (just move all code here and nothing else) and all work just perfect!

As about scroll delaying: it occurs cause loading images to UIImageView (it takes long time because of decompress and rasterize (as I understood).

So better way is to use CATiledLayer. It loads data in background and do it much faster.

Answer 1

The delays on main thread is due to a the mode of the runloop while you scroll. I suggest you to watch the WWDC2011 networking app sessions. I don’t know if it is fine to subclass an NSInvocationOperation that is a concrete subclass of NSOperation. I will subclass NSOperation instead. For my experience if you like to avoid sluggish scrolling, you should create NSOperation subclasses that load their main on a specific thread for networking operation (you must create it). There is a wonderful sample code from apple https://developer.apple.com/library/ios/#samplecode/MVCNetworking/Introduction/Intro.html