# How to reference self in a block in Objective-C

When using ARC (automatic reference counting) in Objective-C, you need to be careful to avoid causing retain cycles. One place where a retain cycle can occur is where self is referenced in a block. To avoid a retain cycle, you can use the __unsafe_unretained modifier as such:

__unsafe_unretained id unretained_self = self;
reachability.reachableBlock = ^(Reachability *r) {
dispatch_async(dispatch_get_main_queue(), ^{
SurveyFormController *myself = unretained_self;
self.mapView.reachable = YES;
[myself configureView];
});
};


# String to Base64 String in Objective-C

Today, I needed to convert an ASCII-encoded NSString to a base64 NSString. I found a number of methods that convert from an NSString to a Base64 NSData, or from NSData to a Base64 string, but couldn't find one that did exactly what I needed. So here is a method that simply converts an NSString to a Base64 NSString:

+ (NSString *)base64String:(NSString *)str
{
NSData *theData = [str dataUsingEncoding: NSASCIIStringEncoding];
const uint8_t* input = (const uint8_t*)[theData bytes];
NSInteger length = [theData length];

static char table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

NSMutableData* data = [NSMutableData dataWithLength:((length + 2) / 3) * 4];
uint8_t* output = (uint8_t*)data.mutableBytes;

NSInteger i;
for (i=0; i < length; i += 3) {
NSInteger value = 0;
NSInteger j;
for (j = i; j < (i + 3); j++) {
value <<= 8;

if (j < length) {
value |= (0xFF & input[j]);
}
}

NSInteger theIndex ...

# How to place custom image in UITextField in iOS

Here is how to put an image in the right portion of a UITextField:

UITextField *textField = [[UITextField alloc] initWithFrame:CGRectMake(55, 10, 660, 30)];
textField.rightViewMode = UITextFieldViewModeWhileEditing;

UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 15, 15)];
imageView.image = [UIImage imageNamed:@"invalidFieldCircle"];
textField.rightView = imageView;


This also assumes that an image called "invalidFieldCircle.png" is in your project's "Resources" directory, which should be located just below the main project directory (you may have to create it).