如何在固定宽度上放置一块NSString？

NSString *string = @"Hello, World!";
CGRect rect = CGRectMake(0, 0, 200, 50);
UIFont *font = [UIFont systemFontOfSize:18];
NSMutableParagraphStyle *paragraphStyle = [[NSMutableParagraphStyle alloc] init];
paragraphStyle.alignment = NSTextAlignmentCenter;
NSDictionary *attributes = @{NSFontAttributeName: font, NSParagraphStyleAttributeName: paragraphStyle};
[string drawInRect:rect withAttributes:attributes];

UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 200, 50)];
label.text = @"Hello, World!";
label.font = [UIFont systemFontOfSize:18];
label.textAlignment = NSTextAlignmentCenter;

NSString *string = @"Hello, World!";
CGRect rect = CGRectMake(0, 0, 200, 50);
UIFont *font = [UIFont systemFontOfSize:18];
NSMutableParagraphStyle *paragraphStyle = [[NSMutableParagraphStyle alloc] init];
paragraphStyle.alignment = NSTextAlignmentCenter;
NSDictionary *attributes = @{NSFontAttributeName: font, NSParagraphStyleAttributeName: paragraphStyle};
NSAttributedString *attributedString = [[NSAttributedString alloc] initWithString:string attributes:attributes];
CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)attributedString);
CGMutablePathRef path = CGPathCreateMutable();
CGPathAddRect(path, NULL, rect);
CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, 0), path, NULL);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSaveGState(context);
CGContextTranslateCTM(context, 0, rect.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
CTFrameDraw(frame, context);
CGContextRestoreGState(context);
CGPathRelease(path);
CFRelease(frame);
CFRelease(framesetter);