UISearchBar *searchBar=[[UISearchBar alloc]initWithFrame:frame];

    //这个设置背景透明可能无效

    searchBar.backgroundColor=[UIColor clearColor];

    //可采用如下方法去除搜索框背景

    for (UIView *view in searchBar.subviews) {

        // iOS7.0之前

        if ([view isKindOfClass:NSClassFromString(@"UISearchBarBackground")]) {

            [view removeFromSuperview];

            break;

        }

        // iOS7.0+

        if ([view isKindOfClass:NSClassFromString(@"UIView")] && view.subviews.count > 0) {

            [[view.subviews objectAtIndex:0] removeFromSuperview];

            break;

        }

    }