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Nested invocations provide asynchronous callbacks
• Communication between subsystems

-(IBAction)onClick:(NSButton *)sender 
{ 
dispatch_async(account->queue, 
^{

NSImageRep *image = renderAccountStatement(account);
dispatch_async(dispatch_get_main_queue(),^{

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Singleton Objects
Lazy initialization is common
@implementation FooController 

// #1
+ (FooController *)sharedInstance
{
static dispatch_once_t once; 
static FooController *instance; 
dispatch_once(&once, ^{

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- (UIImage *)scaleImage:(UIImage *)image toSize:(CGSize)newSize 
{
UIGraphicsBeginImageContextWithOptions(newSize, YES, 0.0);
CGRect imageRect = {{0.0, 0.0}, newSize}; 
[image drawInRect:imageRect]; 
UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext(); 
UIGraphicsEndImageContext(); 
return scaledImage;
}

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- (void)viewDidLoad {
  //find the UITextField view within searchBar (outlet to UISearchBar)
  //and assign self as delegate
  for (UIView *view in searchBar.subviews){
    if ([view isKindOfClass: [UITextField class]]) {
      UITextField *tf = (UITextField *)view;
      tf.delegate = self;
      break;
    }
  }

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AudioToolbox.framework
#import <AudioToolbox/AudioServices.h>

Now use this line of code everytime you want to make the phone vibrate:
AudioServicesPlaySystemSound(kSystemSoundID_Vibrate);

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- (void)drawRect:(CGRect)rect 
{
    
	CGContextRef context = UIGraphicsGetCurrentContext(); // get current context

//    CGFloat object_size = sizeStatus/2;
//    CGFloat sx, sy, fsize = object_size;
//    sx = rect.size.width - fsize;
//    sy = fsize;
//	CGContextMoveToPoint(context, sx, sy);

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    [self setWantsFullScreenLayout:YES];
    [[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleBlackTranslucent animated:YES];

    self.navigationController.navigationBar.translucent = YES;
    self.navigationController.navigationBar.backgroundColor = [UIColor clearColor];
    self.navigationController.navigationBar.barStyle = UIBarStyleBlackTranslucent;

    [[UIApplication sharedApplication] setStatusBarHidden:ControlsHidden];

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    NSArray *names = [UIFont familyNames];
    for (NSString *fontName in names) {
        NSLog(@"%@:", fontName);
        NSArray *fontnames = [UIFont fontNamesForFamilyName:fontName];
        for (NSString *fontNameinFamily in fontnames) {
            NSLog(@"-->%@", fontNameinFamily);
        }
    }

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@protocol SomeClassDelegate <NSObject>

- (void) SomeClassSelector:(ParamType*)param;

@end


@interface SomeClass : UIViewController {
id <SomeClassDelegate> delegate;
    

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@interface UIImage (TPAdditions)
- (UIImage*)imageScaledToSize:(CGSize)size;
@end
 
@implementation UIImage (TPAdditions)
- (UIImage*)imageScaledToSize:(CGSize)size {
    UIGraphicsBeginImageContext(size);
    [self drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();