iphone没有安装支付宝时跳转到appstore下载页面

        //获取安全支付单例并调用安全支付接口
        AlixPay * alixpay = [AlixPay shared];
        int ret = [alixpay pay:orderString applicationScheme:appScheme];
        
        if (ret == kSPErrorAlipayClientNotInstalled) {
            UIAlertView * alertView = [[UIAlertView alloc] initWithTitle:@"提示"
                                                                 message:@"您还没有安装支付宝快捷支付，请先安装。"
                                                                delegate:self
                                                       cancelButtonTitle:@"确定"
                                                       otherButtonTitles:nil];
            [alertView setTag:123];
            [alertView show];
        }
        else if (ret == kSPErrorSignError) {
            NSLog(@"签名错误！");
        }

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex {
	if (alertView.tag == 123) {
		NSString * URLString = @"http://itunes.apple.com/cn/app/id535715926?mt=8";
		[[UIApplication sharedApplication] openURL:[NSURL URLWithString:URLString]];
	}
}