/**
 *  調整一個子控制元件到它父控制元件的最上層
 *
 *  @param superview 父控制元件
 *  @param subview   子控制元件
 */
- (void)exchangeSubviewOnAboveSuperview:(UIView *)superview withSubview:(UIView *)subview
{
    NSUInteger count = superview.subviews.count;
    for (int i=0;i<count; i++) {
        if ([superview.subviews[i] isKindOfClass:[subview class]]) {
            [superview exchangeSubviewAtIndex:i withSubviewAtIndex:count-1];
        }
    }
}