- (void)drawRect:(CGRect)rect {    [super drawRect:rect];    //步骤一:得到当前绘制画布上下文，用于后续将内容绘制在画布上    CGContextRef context = UIGraphicsGetCurrentContext();    //步骤二：将坐标系上下翻转。对于底层的绘制引擎来说，屏幕左下角是（0，0）坐标。而对于上层的UIKit来说，左上角是（0，0）坐标。所以我们为了之后的坐标系描述按UIKit来显示，这里做了一个坐标系上下翻转操作。翻转之后，底层和上层的（0，0）坐标就是重合的了。    CGContextSetTextMatrix(context, CGAffineTransformIdentity);    CGContextTranslateCTM(context, 0, self.bounds.size.height);    CGContextScaleCTM(context, 1.0, -1.0);        //步骤三：创建绘制区域    CGMutablePathRef path = CGPathCreateMutable();    CGPathAddRect(path, NULL, self.bounds); //    CGPathAddEllipseInRect(path, NULL, self.bounds);        //步骤四    NSAttributedString *attString = [[NSAttributedString alloc] initWithString:@"ScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhangScottZhang"];    CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)attString);    CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, [attString length]), path, NULL);    //步骤五    CTFrameDraw(frame, context);    //步骤六    CFRelease(frame);    CFRelease(path);    CFRelease(framesetter);}