简单概括：

1、小程序发起购买，这时把商品id传给后台，检查商品状态是否存在或者库存是否充足，

2、生成订单（这里顺便生成小程序支付核心参数返回给小程序，包括prepay_id），

3、小程序用这些参数进行调用微信支付，支付成功，可以再把prepay_id重新掉接口进行（也可以存数据库等其他办法） 模板推送

确认订单

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请求后台 buyNow

public function buyNow($goods_id, $goods_num, $goods_sku_id)
    {
        // 商品结算信息
        $model = new OrderModel;
        $order = $model->getBuyNow($this->user, $goods_id, $goods_num, $goods_sku_id);
        if (!$this->request->isPost()) {
            return $this->renderSuccess($order);
        }
        if ($model->hasError()) {
            return $this->renderError($model->getError());
        }
        // 创建订单
        if ($model->add($this->user['user_id'], $order)) {
            // 发起微信支付
            return $this->renderSuccess([
                'payment' => $this->wxPay($model['order_no'], $this->user['open_id']
                    , $order['order_pay_price']),
                'order_id' => $model['order_id']
            ]);
        }
        $error = $model->getError() ?: '订单创建失败';
        return $this->renderError($error);
    }

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后台创建订单

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 public function add($user_id, $order)
    {
        if (empty($order['address'])) {
            $this->error = '请先选择收货地址';
            return false;
        }
        Db::startTrans();
        // 记录订单信息
        $this->save([
            'user_id' => $user_id,
            'wxapp_id' => self::$wxapp_id,
            'order_no' => $this->orderNo(),
            'total_price' => $order['order_total_price'],
            'pay_price' => $order['order_pay_price'],
            'express_price' => $order['express_price'],
        ]);
        // 订单商品列表
        $goodsList = [];
        // 更新商品库存 (下单减库存)
        $deductStockData = [];
        foreach ($order['goods_list'] as $goods) {
            /* @var Goods $goods */
            $goodsList[] = [
                'user_id' => $user_id,
                'wxapp_id' => self::$wxapp_id,
                'goods_id' => $goods['goods_id'],
                'goods_name' => $goods['goods_name'],
                'image_id' => $goods['image'][0]['image_id'],
                'deduct_stock_type' => $goods['deduct_stock_type'],
                'spec_type' => $goods['spec_type'],
                'spec_sku_id' => $goods['goods_sku']['spec_sku_id'],
                'goods_spec_id' => $goods['goods_sku']['goods_spec_id'],
                'goods_attr' => $goods['goods_sku']['goods_attr'],
                'content' => $goods['content'],
                'goods_no' => $goods['goods_sku']['goods_no'],
                'goods_price' => $goods['goods_sku']['goods_price'],
                'line_price' => $goods['goods_sku']['line_price'],
                'goods_weight' => $goods['goods_sku']['goods_weight'],
                'total_num' => $goods['total_num'],
                'total_price' => $goods['total_price'],
            ];
            // 下单减库存
            $goods['deduct_stock_type'] == 10 && $deductStockData[] = [
                'goods_spec_id' => $goods['goods_sku']['goods_spec_id'],
                'stock_num' => ['dec', $goods['total_num']]
            ];
        }
        // 保存订单商品信息
        $this->goods()->saveAll($goodsList);
        // 更新商品库存
        !empty($deductStockData) && (new GoodsSpec)->isUpdate()->saveAll($deductStockData);
        // 记录收货地址
        $this->address()->save([
            'user_id' => $user_id,
            'wxapp_id' => self::$wxapp_id,
            'name' => $order['address']['name'],
            'phone' => $order['address']['phone'],
            'province_id' => $order['address']['province_id'],
            'city_id' => $order['address']['city_id'],
            'region_id' => $order['address']['region_id'],
            'detail' => $order['address']['detail'],
        ]);
        Db::commit();
        return true;
    }

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创建后再生成prepay_id 返回小程序

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/**
     * 构建微信支付
     * @param $order_no
     * @param $open_id
     * @param $pay_price
     * @return array
     * @throws \app\common\exception\BaseException
     * @throws \think\exception\DbException
     */
    private function wxPay($order_no, $open_id, $pay_price)
    {
        $wxConfig = WxappModel::getWxappCache();
        $WxPay = new WxPay($wxConfig);
        return $WxPay->unifiedorder($order_no, $open_id, $pay_price);
    }

/**
     * 统一下单API
     * @param $order_no
     * @param $openid
     * @param $total_fee
     * @return array
     * @throws BaseException
     */
    public function unifiedorder($order_no, $openid, $total_fee)
    {
        // 当前时间
        $time = time();
        // 生成随机字符串
        $nonceStr = md5($time . $openid);
        // API参数
        $params = [
            'appid' => $this->config['app_id'],
            'attach' => 'test',
            'body' => $order_no,
            'mch_id' => $this->config['mchid'],
            'nonce_str' => $nonceStr,
            'notify_url' => base_url() . 'notice.php',  // 异步通知地址
            'openid' => $openid,
            'out_trade_no' => $order_no,
            'spbill_create_ip' => \request()->ip(),
            'total_fee' => $total_fee * 100, // 价格:单位分
            'trade_type' => 'JSAPI',
        ];
        // 生成签名
        $params['sign'] = $this->makeSign($params);
        // 请求API
        $url = 'https://api.mch.weixin.qq.com/pay/unifiedorder';
        $result = $this->postXmlCurl($this->toXml($params), $url);
        $prepay = $this->fromXml($result);
        // 请求失败
        if ($prepay['return_code'] === 'FAIL') {
            throw new BaseException(['msg' => $prepay['return_msg'], 'code' => -10]);
        }
        if ($prepay['result_code'] === 'FAIL') {
            throw new BaseException(['msg' => $prepay['return_msg'], 'code' => -10]);
        }
        // 生成 nonce_str 供前端使用
        $paySign = $this->makePaySign($params['nonce_str'], $prepay['prepay_id'], $time);
        return [
            'prepay_id' => $prepay['prepay_id'],
            'nonceStr' => $nonceStr,
            'timeStamp' => (string)$time,
            'paySign' => $paySign
        ];
    }

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模板消息在支付完成后将 prepay_id 请求模板发送

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模板消息见：

https://www.zxcsp.cn/article/77