NSError *error = nil;
ZXMultiFormatWriter *writer = [ZXMultiFormatWriter writer];
ZXBitMatrix* result = [writer encode:@"A string to encode"
                              format:kBarcodeFormatQRCode
                               width:500
                              height:500
                               error:&error];
if (result) {
  CGImageRef image = [[ZXImage imageWithMatrix:result] cgimage];

  // This CGImageRef image can be placed in a UIImage, NSImage, or written to a file.
} else {
  NSString *errorMessage = [error localizedDescription];
}

CGImageRef imageToDecode;  // Given a CGImage in which we are looking for barcodes

ZXLuminanceSource *source = [[[ZXCGImageLuminanceSource alloc] initWithCGImage:imageToDecode] autorelease];
ZXBinaryBitmap *bitmap = [ZXBinaryBitmap binaryBitmapWithBinarizer:[ZXHybridBinarizer binarizerWithSource:source]];

NSError *error = nil;

// There are a number of hints we can give to the reader, including
// possible formats, allowed lengths, and the string encoding.
ZXDecodeHints *hints = [ZXDecodeHints hints];

ZXMultiFormatReader *reader = [ZXMultiFormatReader reader];
ZXResult *result = [reader decode:bitmap
                            hints:hints
                            error:&error];
if (result) {
  // The coded result as a string. The raw data can be accessed with
  // result.rawBytes and result.length.
  NSString *contents = result.text;

  // The barcode format, such as a QR code or UPC-A
  ZXBarcodeFormat format = result.barcodeFormat;
} else {
  // Use error to determine why we didn't get a result, such as a barcode
  // not being found, an invalid checksum, or a format inconsistency.
}