iOS 跳转到appstore

NSString *str = [NSString stringWithFormat:@"itms-apps://ax.itunes.apple.com/WebObjects/MZStore.woa/wa/viewContentsUserReviews?type=Purple+Software&id=xxxx" ];
        if( ([[[UIDevice currentDevice] systemVersion] doubleValue]>=7.0)){
            str = [NSString stringWithFormat:@"itms-apps://itunes.apple.com/app/xxxx"];
        }
        [[UIApplication sharedApplication] openURL:[NSURL URLWithString:str]];