Question

I run my animations in a UITAbleViewCell.
Each cell has its own animation and the cells are reusable.
I use [mView performSelectorInBackground:@selector(layoutSubview) withObject:nil];

There in the background thread I initiate the runLoop to perform tasks like this:

- (void)startAnimation 
{
    NSRunLoop *mLoop = [NSRunLoop currentRunLoop];
    self.animationTimer = [NSTimer scheduledTimerWithTimeInterval:animationInterval target:self selector:@selector(setNeedsDisplay) userInfo:nil repeats:YES];

    mRunLoop = YES;
    while (mRunLoop == YES && [mLoop runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:0.01]]);
}

and stop it:

- (void)stopAnimation 
{
    if (![NSThread isMainThread]) {
        [[NSThread currentThread] cancel];
    }

    mRunLoop = NO;
    self.animationTimer = nil;
    CFRunLoopStop(CFRunLoopGetCurrent());   
}

I run into problems when I fast scroll through table, because on the first cell initiation I begin the animation, so the first runLoop call occures which performs a setNeedDisplay and all the methods from it. But before finishing the first runLoop cycle the cell disappears from the view and is already available for reuse. So I begin clearing it, while the cycle is still performing operations and here I meet situations like

message sent to deallocated instance

So could you please give me some hints of how should I correctly stop performing the operations in that thread? I mean if I want to realese for example an object, which is performing some actions how to immediately stop’em?

Hope I gave enough info.
Thanks

UPDATE: No ideas at all?

Answer 1

I’ll take a completely different stab on it:

Get rid of the cell’s timers and background threads altogether!

Animation is not something where NSTimer is a good fit in the first place and having multiple timers won’t help much, either.

UITableView has a method visibleCells and a method indexPathsForVisibleRows. I’d suggest to use a single CADisplayLink — which is suited for animation, as it calls you back with the actual refresh rate of the display or a fraction thereof — in your tableview-controller and in the callback of that display-link iterate over the visible cells.

If you want to schedule the display-link on the run-loop of a secondary thread, feel free to do so, but I’d check if you can get away without extra threading first.

Some code:

@interface AnimatedTableViewController ()

@property (strong, nonatomic) CADisplayLink *cellAnimator;
- (void)__cellAnimatorFired:(CADisplayLink *)animator;

@end

@implementation AnimatedTableViewController

@synthesize cellAnimator = cellAnimator_;

- (void)setCellAnimator:(CADisplayLink *)animator
{
    if (animator == cellAnimator_)
        return;

    [cellAnimator_ invalidate];
    cellAnimator_ = animator;

    [cellAnimator_ addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSCommonRunLoopModes];
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];

    self.cellAnimator = [CADisplayLink displayLinkWithTarget:self selector:@selector(__cellAnimatorFired:)];
    ...
}

- (void)viewWillDisappear:(BOOL)animated
{
    self.cellAnimator = nil;
    ...

    [super viewWillDisappear:animated];
}

- (void)__cellAnimatorFired:(CADisplayLink *)animator
{
     NSArray *visibleCells = [self.tableView visibleCells];
     [visibleCells enumerateObjectsUsingBlock:^(UITableViewCell *cell, NSUInteger unused, BOOL *stop){
         [cell setNeedsDisplay];
     }];
}

...

@end