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);

}