CADisplaylink code fires even when display doesn't refresh - cadisplaylink

Question

Note: as of iOS7 this problem may only manifest in the simulator -- still testing.
I have an implementation of CADisplayLink, and I need the code to run if and only if the display actually refreshes
This doesn't happen.
Here's a very simple test program:
I start the display link running; in the first frame aLabel should display "WordFlash"; for the next 19 frames it should display "--------" and for the next 100 it should be blank; then the cycle should repeat.
Occasionally (say 1 in 8 times), and unpredictably, the screen won't refresh to display "WordFlash" though the code has indeed fired (as the counter has advanced). I need the counter to advance only if "WordFlash" has successfully displayed for exactly 1 frame.
Any ideas? I am totally stumped.
Note: This display-refresh-skipping seems to happen without correlation to the time it takes the device to execute the simple code (as in the NSLogged time-to-execute the code can be identical in two different cycles while only one cycle has successfully flashed the word.
#import "HomePwnerViewController.h"
#interface HomePwnerViewController ()
#end
#implementation HomePwnerViewController {
int counter;
UILabel *aLabel;
}
- (void)viewDidLoad
{
[super viewDidLoad];
aLabel = [[UILabel alloc] initWithFrame:self.view.frame];
[self.view addSubview:aLabel];
aLabel.alpha = 1;
aLabel.text = #"";
aLabel.textAlignment = NSTextAlignmentCenter;
[aLabel setFont:[UIFont fontWithName:#"Courier" size:50]];
[NSTimer scheduledTimerWithTimeInterval:1 target:self selector:#selector(testWordFlashMethod) userInfo:nil repeats:NO];
}
- (void) displaySensitiveProcedure: (CADisplayLink *) sender {
CFTimeInterval startTime = CACurrentMediaTime();
if (counter == 0)
aLabel.text = #"FlashWord";
else if (counter < 20)
aLabel.text = #"---------";
else
aLabel.text = #"";
CFTimeInterval endTime = CACurrentMediaTime();
if (counter == 0)
NSLog(#"frame time %f frame length %f ratio %f", endTime - startTime, sender.duration, (endTime - startTime)/sender.duration);
counter++;
if (counter == 120)
counter = 0;
}
- (void) testWordFlashMethod {
CADisplayLink *DL = [CADisplayLink displayLinkWithTarget:self selector:#selector(displaySensitiveProcedure:)];
DL.frameInterval = 1;
counter = 0;
[DL addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#end
Much obliged,
b

Answer 1

If I were to guess, I'd say that animating a UILabel on screen refresh is not the best way to do things. In my experience changing the text on them can take a bit of time to render. If you want to draw a dash across the screen, you might be better served by using Core Graphics API's, or a CALayer based drawing approach.

Answer 2

Your displaySensitiveProcedure is probably taking more than a single frame (16.7ms) to update the screen, which makes it skip a frame. If you consistently take more than that (say 20ms), you can have the DisplayLink fire less frequently to get smooth animation at a slower framerate - see the frameInterval property. I would suggest timing your callback by doing something like this:
- (void) displaySensitiveProcedure: (CADisplayLink *) sender {
CFTimeInterval startTime = CACurrentMediaTime();
... <code> ...
CFTimeInterval endTime = CACurrentMediaTime();
NSLog(#"frame time: %f", endTime - startTime);
}