setNeedsDisplay: Too long between redraw cycles - iphone

Question

I'm new to cocoa. I thought I could set the UITableViewCell background view to my own custom subclass in order to display a loading bar in the background of the cell, but it seems that calling setNeedsDisplay when I want to update the bar is not fast enough for my needs. Right now I'm updating it (by that I mean updating data in the UIView and then calling setNeedsDisplay) every .1 seconds and only seeing it actual update every 30-40 seconds. Is this normal? Is there any way to speed it up? If not are there other options for what I'm trying to do?
This is my code for draw rect (there's a helper methode above it) and updateBarAt, which is what is called to update it:
-(void) updateBarAt: (float) playHead {
self.playHead = playHead;
NSLog(#"%f", self.playHead);
[self setNeedsDisplay];
}
-(void) drawBackground: (CGRect) rect{
double totalTime = self.playFor + self.wait;
double waitPercentage = self.wait / totalTime;
double playForPercentage = (self.playFor - self.fadeIn - self.fadeOut) / totalTime;
double fadeInPercentage = self.fadeIn / totalTime;
double fadeOutPercentage = self.fadeOut / totalTime;
CGContextRef context = UIGraphicsGetCurrentContext();
CGRect blueRect = CGRectMake((waitPercentage+fadeInPercentage)*rect.size.width, 0, playForPercentage*rect.size.width, rect.size.height);
CGContextSetRGBFillColor(context, .639, .737, 1, 1.0);
CGContextFillRect(context, blueRect);
CGRect greenRect = CGRectMake(0, 0, waitPercentage*rect.size.width, rect.size.height);
CGContextSetRGBFillColor(context, .843, 1, .651, 1.0);
CGContextFillRect(context, greenRect);
CGRect purpleRect = CGRectMake(waitPercentage*rect.size.width+.1, 0, fadeInPercentage*rect.size.width, rect.size.height);
CGContextSetRGBFillColor(context, .7451, .639, 1.0, 1.0);
CGContextFillRect(context, purpleRect);
CGRect purpleR = CGRectMake((1.0 - fadeOutPercentage)*rect.size.width - .5, 0, fadeOutPercentage*rect.size.width + .5, rect.size.height);
CGContextFillRect(context, purpleR);
}
- (void)drawRect:(CGRect)rect { //just does blue and green, need to add purple. Research
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetRGBFillColor(context, 1.0, 1.0, 1.0, 1.0);
CGContextFillRect(context, rect);
if (self.playHead == -1)
return;
[super drawRect:rect];
double totalTime = self.playFor + self.wait;
double playedPercentage = self.playHead / totalTime;
double waitPercentage = self.wait / totalTime;
double playForPercentage = (self.playFor - self.fadeIn - self.fadeOut) / totalTime;
double fadeInPercentage = self.fadeIn / totalTime;
double fadeOutPercentage = self.fadeOut / totalTime;
NSLog(#"Played: %f Wait: %f Play: %f Fade In: %f Fade Out: %f", playedPercentage, waitPercentage, playForPercentage, fadeInPercentage, fadeOutPercentage);
if (playedPercentage <= waitPercentage) {
waitPercentage = playedPercentage;
playForPercentage = 0;
fadeInPercentage = 0;
fadeOutPercentage = 0;
} else if (playedPercentage <= waitPercentage+fadeInPercentage) {
playForPercentage = 0;
fadeInPercentage = playedPercentage - waitPercentage;
fadeOutPercentage = 0;
} else if (playedPercentage <= waitPercentage+fadeInPercentage + playForPercentage) {
playForPercentage = playedPercentage - waitPercentage - fadeInPercentage;
fadeOutPercentage = 0;
} else {
fadeOutPercentage = playedPercentage - waitPercentage - playForPercentage;
}
//[self drawBackground:rect];
CGRect blueRect = CGRectMake((waitPercentage+fadeInPercentage)*rect.size.width, 0, playForPercentage*rect.size.width, rect.size.height);
CGContextSetRGBFillColor(context, 0.0, 0.0, 1.0, 1.0);
CGContextFillRect(context, blueRect);
CGRect greenRect = CGRectMake(0, 0, waitPercentage*rect.size.width, rect.size.height);
CGContextSetRGBFillColor(context, 0.0, 1.0, 0.0, 1.0);
CGContextFillRect(context, greenRect);
CGRect purpleRect = CGRectMake(waitPercentage*rect.size.width+.1, 0, fadeInPercentage*rect.size.width, rect.size.height);
CGContextSetRGBFillColor(context, 0.33, 0.1, .55, 1.0);
CGContextFillRect(context, purpleRect);
CGRect purpleR = CGRectMake((1.0 - fadeOutPercentage)*rect.size.width - .5, 0, fadeOutPercentage*rect.size.width + .5, rect.size.height);
CGContextFillRect(context, purpleR);
}

Answer 1

Most UIKit use needs to be done on the main thread. Most likely your updateBarAt: method isn't being called on the main thread which means your call to setNeedsDisplay is being done on some background thread.
One possible solution would be this:
- (void)updateBarAt:(float)playHead {
self.playHead = playHead;
NSLog(#"%f", self.playHead);
dispatch_async(dispatch_get_main_queue(), ^{
[self setNeedsDisplay];
});
}
Another option would be to wrap the call to updateBarAt: in the dispatch_async call.