使用到:
CGRectInset
CGRectoffset
UIEdgeInsetsInsetRect
这三个函数的使用情况

//CGRectInset  将原来的矩形放大或者缩小，正表示缩小，－表示放大。
CGRect rect= CGRectMake(20, 50, 100, 80);
 CGRect rect1=CGRectInset(rect, -10, 20);
 NSLog(@"%@",p(rect1));
//输出结果：2014-11-22 18:48:55.351 TestCGRectInset[8893:60b] {{10, 70}, {120, 40}}

//CGRectOffset 这个函数就是将原来矩形的坐标点变化一下，就是左上角点
CGRect rect= CGRectMake(20, 50, 100, 80);
CGRect rect1=CGRectOffset(rect, -10, 20);
NSLog(@"%@",p(rect1));
//输出结果：2014-11-22 18:51:58.217 TestCGRectInset[8913:60b] {{10, 70}, {100, 80}}

 //UIEdgeInsetsInsetRect 表示在原来的rect基础上根据边缘距离内切一个rect出来
CGRect rect= CGRectMake(20, 50, 100, 80);
UIEdgeInsets ed=UIEdgeInsetsMake(-3, -4, -5, -6);
CGRect  r=  UIEdgeInsetsInsetRect(rect, ed);
NSLog(@"%@",p(r));

绘制代码如下:

- (void)drawRect:(CGRect)rect
{
//    rect.size.height = 140;
//    rect.size.width  = 180;
//    rect请设置为屏幕宽高

    const CGFloat RECT_PADDING = 8.0;
    rect = CGRectInset(rect, (rect.size.width-180)/2, (rect.size.height-140)/2);

    const CGFloat ROUND_RECT_CORNER_RADIUS = 5.0;
    CGPathRef roundRectPath = NewPathWithRoundRect(rect, ROUND_RECT_CORNER_RADIUS);

    CGContextRef context = UIGraphicsGetCurrentContext();

    const CGFloat BACKGROUND_OPACITY = 0.85;
    CGContextSetRGBFillColor(context, 0, 0, 0, BACKGROUND_OPACITY);
    CGContextAddPath(context, roundRectPath);
    CGContextFillPath(context);

    const CGFloat STROKE_OPACITY = 0.25;
    CGContextSetRGBStrokeColor(context, 1, 1, 1, STROKE_OPACITY);
    CGContextAddPath(context, roundRectPath);
    CGContextStrokePath(context);

    CGPathRelease(roundRectPath);
}

CGPathRef NewPathWithRoundRect(CGRect rect, CGFloat cornerRadius)
{
    //
    // Create the boundary path
    //
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, NULL,
                      rect.origin.x,
                      rect.origin.y + rect.size.height - cornerRadius);

    // Top left corner
    CGPathAddArcToPoint(path, NULL,
                        rect.origin.x,
                        rect.origin.y,
                        rect.origin.x + rect.size.width,
                        rect.origin.y,
                        cornerRadius);

    // Top right corner
    CGPathAddArcToPoint(path, NULL,
                        rect.origin.x + rect.size.width,
                        rect.origin.y,
                        rect.origin.x + rect.size.width,
                        rect.origin.y + rect.size.height,
                        cornerRadius);

    // Bottom right corner
    CGPathAddArcToPoint(path, NULL,
                        rect.origin.x + rect.size.width,
                        rect.origin.y + rect.size.height,
                        rect.origin.x,
                        rect.origin.y + rect.size.height,
                        cornerRadius);

    // Bottom left corner
    CGPathAddArcToPoint(path, NULL,
                        rect.origin.x,
                        rect.origin.y + rect.size.height,
                        rect.origin.x,
                        rect.origin.y,
                        cornerRadius);

    // Close the path at the rounded rect
    CGPathCloseSubpath(path);

    return path;
}

如图: