The Handler behaviour is to handle the result of notification from Wechat's Payment Gateway.

use this module will help you implementing your own handler:

defmodule MyApp.WechatHandler do
  use WechatPay.Plug.Handler

  @impl WechatPay.Plug.Handler
  def handle_data(conn, data) do
    # the sign is already verified by `WechatPay.Utils.Signature.verify/3`.
    data = %{
      appid: "wx2421b1c4370ec43b",
      attach: "支付测试",
      bank_type: "CFT",
      fee_type: "CNY",
      is_subscribe: "Y",
      mch_id: "10000100",
      nonce_str: "5d2b6c2a8db53831f7eda20af46e531c",
      openid: "oUpF8uMEb4qRXf22hE3X68TekukE",
      out_trade_no: "1409811653",
      result_code: "SUCCESS",
      return_code: "SUCCESS",
      sign: "594B6D97F089D24B55156CE09A5FF412",
      sub_mch_id: "10000100",
      time_end: "20140903131540",
      total_fee: "1",
      trade_type: "JSAPI",
      transaction_id: "1004400740201409030005092168"
    }

    # return `:ok` to tell wechat server that you have successfully handled this notification.
    :ok
  end

  # This is optional
  @impl WechatPay.Plug.Handler
  def handle_error(conn, error, data) do
    Logger.error(inspect(error))
  end
end

If the request data is valid, MyApp.WechatHandler.handle_data/2 will be called with the a Plug.Conn struct and parsed data, this function should returns :ok to tell Wechat's Server the data processing is success, or {:error, any} to make the processing fail, so Wechat will retry seconds later.

Also, you can optional implement the handle_error/3 if you want to track the error.