Question

I am using -drawRect for the first time in an attempt to alternatively speed up a UITableView. However, the drawRect method seems to be slowing the table down quite largely.

Please can you tell me how I can improve the drawRect method below in order to speed up the table?

Edit—

In the drawRect method, I am writing two NSStrings to the cell’s view, two UIImages and a drop shadow to both of the NSStrings and one of the UIImages.

One of the aforementioned images is downloaded asynchronously and then setNeedsDisplay is called to draw that UIImage to the screen. I believe that this could initially be the reason for the lag occurring.

- (void) drawRect:(CGRect) rect {

    CGContextRef context = UIGraphicsGetCurrentContext();

    [[UIColor clearColor] set];

    CGContextFillRect(context, rect);

    CGContextSaveGState(context);
    CGContextSetShadow(context, CGSizeMake(1,1),1);
    //draw text here

    if (shouldDrawImage == YES) {

        CGContextDrawImage(context, CGRectMake(10, 10, 40, 40), self.image.CGImage);

    }

    CGContextDrawImage(context, CGRectMake(self.frame.size.width - 16, 0, 16, self.frame.size.height), [UIImage imageNamed:@"right_bar_including_holes"].CGImage);

    NSString *authorName = [[self.info objectForKey:@"user"] objectForKey:@"full_name"];

    [RGB(219, 240, 73) set];

    CGSize maximumLabelSize = CGSizeMake(self.frame.size.width - 10 - 55 - 16,9999);
    CGSize authorsize = [authorName sizeWithFont:[UIFont boldSystemFontOfSize:15]
                                     constrainedToSize:maximumLabelSize 
                                         lineBreakMode:UILineBreakModeWordWrap]; 
    [authorName drawInRect:CGRectMake(60, 10, self.frame.size.width - 60, authorsize.height) withFont:[UIFont boldSystemFontOfSize:15]];

    [RGB(249,249,249) set];

    NSString *description = [self.info objectForKey:@"description"];
    CGSize descriptionSize = [description sizeWithFont:[UIFont systemFontOfSize:14] constrainedToSize:maximumLabelSize lineBreakMode:UILineBreakModeWordWrap];

    [description drawInRect:CGRectMake(60, authorsize.height + 15, descriptionSize.width, descriptionSize.height) withFont:[UIFont systemFontOfSize:14]];

    CGContextRestoreGState(context);

}

- (NSString *) reuseIdentifier {
    return NSStringFromClass([SlideCell class]);
}

- (void) updateCellInfo:(NSDictionary *)_info {
    [self setInfo:_info];

    UIImageView *iv = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 1, 1)];
    [iv setImageWithURLRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:[[self.info objectForKey:@"user"] objectForKey:@"avatar"]]] placeholderImage:nil success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *_image) {
        dispatch_async(dispatch_get_main_queue(), ^{
            shouldDrawImage = YES;
            self.image = [self roundedImage:_image];
            [iv release];
            [self setNeedsDisplay];
        });

    } failure:nil];

    [self setNeedsDisplay];

    [self setSelectionStyle:UITableViewCellSelectionStyleNone];
}

Answer 1

Yes – you run Instruments’ Time Profile on your app to tell you exactly how much time is spent, and where. It will tell you if it is the image, string, shadow, or something else.