Preloading Images in iOS

gistfile1.m

// ImagePreloadingViewController - An iOS view controller for trying out different
// UIImage preloading strategies. Results for 1 MB png on iPhone 4S:
//  - No preload: drawing the image right away (80 ms)
//  - Header preload: getting the width (10 ms) and then drawing (100 ms)
//  - Data preload: getting the data (110 ms) and then drawing (75 ms)
//  - Draw preload: drawing it once (100 ms) and then again (20 ms)
// In short: preload a UIImage by drawing it.
// License: BSD
// Author:  Leonard van Driel, 2012

@interface ImagePreloadingViewController : UIViewController
@end

@implementation ImagePreloadingViewController {
    UITextView *output;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    for (NSUInteger i = 0; i < 4; i++) {
        UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
        button.frame = CGRectMake(10, 10 + 50 * i, self.view.bounds.size.width - 20, 40);
        [button addTarget:self action:@selector(run:) forControlEvents:UIControlEventTouchUpInside];
        switch (i) {
            case 0: [button setTitle:@"No preload" forState:UIControlStateNormal]; break;
            case 1: [button setTitle:@"Header preload" forState:UIControlStateNormal]; break;
            case 2: [button setTitle:@"Data preload" forState:UIControlStateNormal]; break;
            case 3: [button setTitle:@"Draw preload" forState:UIControlStateNormal]; break;
        }
        button.tag = i;
        [self.view addSubview:button];
    }
    output = [[UITextView alloc] init];
    output.frame = CGRectMake(10, 210, self.view.bounds.size.width - 20, self.view.bounds.size.height - 270);
    [self.view addSubview:output];
}

#pragma mark - Timing

- (void)run:(UIButton *)button
{
    NSUInteger index = button.tag;
    [self preload:index];
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, .1 * NSEC_PER_SEC);
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        [self load:index];
    });
}

- (void)preload:(NSUInteger)index
{
    NSString *name = [NSString stringWithFormat:@"%i.png", index];
    double time = CFAbsoluteTimeGetCurrent();
    switch (index) {
        case 0: break;
        case 1: [self header:[UIImage imageNamed:name]]; break;
        case 2: [self data:[UIImage imageNamed:name]]; break;
        case 3: [self draw:[UIImage imageNamed:name]]; break;
    }
    output.text = [output.text stringByAppendingFormat:@"%i: %4.0f ms preload", index, (CFAbsoluteTimeGetCurrent() - time) * 1000];
}

- (void)load:(NSUInteger)index
{
    NSString *name = [NSString stringWithFormat:@"%i.png", index];
    double time = CFAbsoluteTimeGetCurrent();
    [self draw:[UIImage imageNamed:name]];
    output.text = [output.text stringByAppendingFormat:@" .. %4.0f ms draw\n", (CFAbsoluteTimeGetCurrent() - time) * 1000];
}

#pragma mark - Preload types

- (void)header:(UIImage *)image
{
    CGImageRef ref = image.CGImage;
    CGImageGetWidth(ref);
}

- (void)data:(UIImage *)image
{
    CGImageRef ref = image.CGImage;
    CGDataProviderRef provider = CGImageGetDataProvider(ref);
    CGDataProviderCopyData(provider);
}

- (void)draw:(UIImage *)image
{
    CGImageRef ref = image.CGImage;
    size_t width = CGImageGetWidth(ref);
    size_t height = CGImageGetHeight(ref);
    CGColorSpaceRef space = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(NULL, width, height, 8, width * 4, space, kCGImageAlphaPremultipliedFirst);
    CGColorSpaceRelease(space);
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), ref);
    CGContextRelease(context);
}

@end