ios7 跳轉到appstore評分

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