Plug to handle callback from Wechat’s payment gateway

If the callback is success and verified, the result value will be assigned to private :wechat_pay_result key of the Plug.Conn.t object.

A Phoenix Example

# lib/my_app/web/router.ex
scope "/wechat", as: :wechat do
  post "/pay/callback", WechatPay.Plug.Callback, [handler: MyApp.WechatPay.CallbackHandler]
end

# lib/my_app/wechat_pay/callback_handler.ex
defmodule MyApp.WechatPay.CallbackHandler do
  @behaviour WechatPay.Plug.Callback.Handler

  def handle_data(conn, data) do
    IO.inspect 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"
    # }
  end

  # optional
  def handle_error(conn, reason, data) do
    reason == "签名失败"
    data.return_code == "FAIL"
  end
end