Delegate, Lambda expression, callback

Today, I took over the WPF version of the Traffic Management project. As the first person took the lead, I was responsible for a bunch of semi-mature code, and then I started the painful process of reading the code.

My predecessors have been a master who has been engaged in five years. The code is very advanced, and various commissions, events, lambda, and Callbacks are everywhere. To me, I am a little white man who only understands simple delegation and events, reading can be painful. The most painful thing is that there were few or even no comments, and I was speechless. Work has to be done. After all, it has to be mixed up, so I will read the code he wrote. Today, let's take a look at a method in one of his codes as follows.

 

The following method is used to execute a query.

 1 /// <summary> 2         /// 执行查询 3         /// </summary> 4         /// <param name="startTime"></param> 5         /// <param name="endTime"></param> 6         /// <param name="listGateChannel"></param> 7         /// <param name="condition"></param> 8         /// <param name="page"></param> 9         /// <param name="pageSize"></param>10         private void QueryFromServer(DateTime startTime, DateTime endTime, List<RmpGateChannel> listGateChannel, string condition, int page, int pageSize)11         {12             m_rmpServiceHelper.GetVehicleAccess(startTime, endTime, listGateChannel, condition, pageSize, (lstVehicle) =>13             {14                 if (lstVehicle == null || lstVehicle.Count == 0)15                 {16                     MessageHelper.Show("未查询到相关记录", MessageType.Ok, 1);17                     return;18                 }19                 _listVehicle.Clear();20                 foreach (VehicleInfoDisplay item in lstVehicle)21                 {22                     _listVehicle.Add(item);23                     _allDataItems.Add(item);24                     dataPager.IsShowMoreButton = true;25                 }26                 dataPager.TotalCount = _allDataItems.Count;27             });28 29         }

View code

This method is the last parameter above (also a method in the anonymous method)

 1 /// <summary> 2         /// 获取第一页通行记录 3         /// </summary> 4         /// <param name="startTime"></param> 5         /// <param name="endTime"></param> 6         /// <param name="lstChannel"></param> 7         /// <param name="condition"></param> 8         /// <param name="callBack"></param> 9         public void GetVehicleAccess(DateTime startTime, DateTime endTime, List<RmpGateChannel> lstChannel, string condition, int pageSize, Action<List<VehicleInfoDisplay>> callBack)10         {11             CommunicationHelper.Instance.SendMessage((client) =>12             {13                 string[] arrTypes = new string[lstChannel.Count];14                 int?[] arrIds = new int?[lstChannel.Count];15                 for (int i = 0; i < arrTypes.Length; i++)16                 {17                     arrTypes[i] = "HostChannel";18                 }19                 for (int i = 0; i < lstChannel.Count; i++)20                 {21                     arrIds[i] = lstChannel[i].InternalObjectId;22                 }23                 m_pageInfo = client.PagePlusVehicleAccessInfoLogEx2(startTime, endTime, arrTypes, arrIds, condition, "", 1, pageSize);24                 List<vehicleAccessInfoLogEx3> tempList = new List<vehicleAccessInfoLogEx3>();25                 if (m_pageInfo.recordTotalNum > 0 && m_pageInfo.vehicleAccessInfo.Length > 0)26                 {27                     tempList.AddRange(m_pageInfo.vehicleAccessInfo.OrderBy(o => o.accessTime));28                 }29                 List<VehicleInfoDisplay> displayList = new List<VehicleInfoDisplay>();30                 foreach (vehicleAccessInfoLogEx1 obj in tempList)31                 {32                     VehicleInfoDisplay display = VehicleAccessInfoToDisplay.GetVehicleInfo(obj);33                     displayList.Add(display);34                 }35                 if (callBack != null)36                 {37                     DispatcherHelper.RunSync(new Action(() =>38                     {39                         callBack(displayList);40                     }), null);41                 }42             });43 44         }

View code

Is it a little big after reading it. After various data queries and analysis, we found that the last parameter of the first method is a Lambda expression, that is, an anonymous method. When a method is passed as a parameter, we first think of it as a delegate, and then execute some operations in this anonymous method. At this time, we find that this is still a callback. Why? We didn't pass the value to this anonymous method, but we did use this value parameter. According to the callback definition, it becomes clear that the called function passes a method to the main function, the main function passes a value to the called function, and then executes the called function.

 

Delegate, Lambda expression, callback