Concurrent processing on main queue during UITableView scroll

Question

I have a stopwatch screen in my app.

There is a main label indicating passing time every 100 milliseconds (updates constantly).

Just below that label, in the same ViewController I have a UITableView.

Whenever I'm scrolling the UITableView, the stopwatch labels stops updating.

I've tried using Grand Central Dispatch as follows, but it doesn't work and I didn't expect it would, since it's still two operations running on the same queue.

dispatch_async(dispatch_get_main_queue(),^{
MyTimerObject *t = (MyTimerObject*)notification.object;
    lblMainTimer.text = t.MainTimerStringValue;});

So how should I approach this problem?

This all happens within the receiveNotification method of NSNotification, which make me wonder if it's not NSNotification that locks up during the table scroll event and not the label update...

Answer 1

Is your stopwatch updating with NSTimer or CADisplayLink? The issue associated with failure to update while scrolling is generally an incorrect choice of run loop modes.

For example, this will pause while tableview is scrolling.

NSTimer *timer = [NSTimer timerWithTimeInterval:0.1 target:self selector:@selector(handleTimer:) userInfo:nil repeats:TRUE];
[[NSRunLoop mainRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];

Whereas this will not:

[[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];

Bottom line, don't use NSDefaultRunLoopMode, but rather use NSRunLoopCommonModes, and you may find it continues to run even when scrolling the table view.