Xcode - How to draw a View using thread?

Question

I have graphic view which is drawn using -(void)drawRect:(CGRect)rect function, it is a little complex graphic which includes date as X axis and time as Y axis. So every time I draw it, it always takes 2,3 seconds to display the final graphic.The red point maybe red, green or yellow, its colour depends on the data read from data base.

My question is that how to draw this view using multithread or in a fast way. For example, draw the X axis and Y axis in one thread, and draw each day's graphic in separated thread(using 30 threads) or draw each hour for 30 days' graphics in separated thread(using 5 threads)? I have tried using the following function:

for(int i = 0; i < 31; i++){
NSString *threadStr = [NSString stringWithFormat:@"%i", i];
dispatch_queue_t queue1 = dispatch_queue_create([threadStr UTF8String], NULL);
dispatch_async(queue1, ^{
//Draw my graphics for one day
.....
});
}

But sometimes these threads read the database at the same time and some of them cannot get the data properly, this cause the final graphic is not complete. Could anyone give me some suggestion? Thank you very much.

I am trying to find out the reason why it delays 2, 3 seconds if I draw it using main thread. Is it because of reading SQLite3 Data base ?

The procedure of drawing this graphic is :

- (void)drawRect:(CGRect)rect
{
    1.draw X axis with date label;
    2.draw Y axis with time label;
    NSArray *timeArray = @[@"08:00", 11:00", 14:00", 17:00", 20:00"];
    for(NSString *timeStr in timeArray){
       for(int i = 0; i < 31; i++){
       NSString *DateTimeStr = dateStr + timeStr;
       int a = [DataManager getResult: DateTimeStr];
       UIColor *RoundColor;
       if(a == 1){
         RoundColor = [UIColor colorWithRed:229/255.0 green:0/255.0 blue:145/255.0 alpha:1.0];
       }
       else if(a == 2){
         RoundColor = [UIColor colorWithRed:0/255.0 green:164/255.0 blue:229/255.0 alpha:1.0];
       }
       else if(a == 3){
         RoundColor = [UIColor colorWithRed:229/255.0 green:221/255.0 blue:0/255.0 alpha:colorAlpha];;
       }

       const CGFloat *components = CGColorGetComponents(RoundColor.CGColor);
       CGContextSetRGBFillColor(context, components[0], components[1], components[2], components[3]);
       CGContextFillEllipseInRect(context, CGRectMake(roundXnow,roundYnow,roundR,roundR));
       }
    }
}

I am thinking if the delay occurs when I am reading the SQLite3 data base, because I know there are a lot of record in my data base.

Answer 1

Drawing here is a very light operation, most likely the timing issues you're facing are related to the calculations (or fetching of data) or whatever else you're doing before actually drawing the dot. So use instruments, time profiler in particular. It will show you what operations take longer (look at the pikes in the graphic that will be displayed while your app is running). This will give you the clue.

Besides as it's correctly mentioned here, all the UI should be performed in the main thread. But those pre-calculations and fetches that you probably have with your data - that's where you should look for multithreaded solutions if it's at all possible.