Objective C，UIScrollview居中显示子视图

- (void)resetCanvasCenter{

    UIView *subView = self.subviews.firstObject;

    CGRect rect = self.bounds;

    CGSize size = self.contentSize;

    CGFloat offsetX = MAX((rect.size.width - size.width) * 0.5, 0.0);

    CGFloat offsetY = MAX((rect.size.height - size.height) * 0.5, 0.0);

    subView.center = CGPointMake(size.width * 0.5 + offsetX,size.height * 0.5 + offsetY);

}