'Declaration
Public Function New()
'Usage
Dim instance As New EasyDAItemChangedEventArgs(Of TValue)()
public EasyDAItemChangedEventArgs<TValue>()
public:
EasyDAItemChangedEventArgs<TValue>();
'Declaration
Public Function New()
'Usage
Dim instance As New EasyDAItemChangedEventArgs(Of TValue)()
public EasyDAItemChangedEventArgs<TValue>()
public:
EasyDAItemChangedEventArgs<TValue>();
When there is a change in a value of an OPC item you have subscribed to, the EasyDAClient object generates an ItemChanged event. For subscription mechanism to be useful, you should hook one or more event handlers to this event.
To be more precise, the ItemChanged event is actually generated in other cases, too.
First of all, you always receive at least one ItemChanged event notification after you make a subscription; this notification either contains the initial data for the item, or an indication that data is not currently available. This behavior allows your application to rely on the component to provide at least some information for each subscribed item.
Secondly, the ItemChanged event is generated every time the component loses connection to the item, and when it reestablishes the connection. This way, your application is informed about any problems related to the item, and can process them accordingly if needed.
You will also receive the ItemChanged notification if the quality of the item changes (not just its actual data value).
The ItemChanged event notification contains an EasyDAItemChangedEventArgs argument. You will find all kind of relevant data in this object. Some properties in this object contain valid information no matter what kind of change the notification is about. These properties are inside the Arguments property.
For further processing, your code should always inspect the value of Exception property of the event arguments. If this property is set to a null reference, the notification carries an actual change in item’s data, and the Vtq property has the new value, timestamp and quality of the item, in form of DAVtq object. If the Exception property is not a null reference, there has been an error related to the item, and the Vtq property is not valid. In such case, the Exception property contains information about the problem.
The ItemChanged event handler is called on a thread determined by the EasyDAClient component. For details, please refer to “Multithreading and Synchronization” chapter under “Advanced Topics”.
In short, however, we can say that if you are writing e.g. Windows Forms application, the component takes care of calling the event handler on the user interface thread of the form, making it safe for your code to update controls on the form, or do other form-related actions, from the event handler.
// This example subscribes to changes of 2 items separately, and displays rich information available with each item changed // event notification. // // Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html . using System; using System.Diagnostics; using System.Threading; using OpcLabs.EasyOpc.DataAccess; using OpcLabs.EasyOpc.DataAccess.OperationModel; namespace DocExamples.DataAccess._EasyDAItemChangedEventArgs { class General { public static void Main1() { // Instantiate the client object. using (var client = new EasyDAClient()) { var eventHandler = new EasyDAItemChangedEventHandler(client_ItemChanged); client.ItemChanged += eventHandler; Console.WriteLine("Subscribing items..."); client.SubscribeItem("", "OPCLabs.KitServer.2", "Simulation.Random", 5 * 1000); client.SubscribeItem("", "OPCLabs.KitServer.2", "Trends.Ramp (1 min)", 5 * 1000); Console.WriteLine("Processing item changed events for 1 minute..."); Thread.Sleep(60 * 1000); } } static void client_ItemChanged(object sender, EasyDAItemChangedEventArgs e) { Console.WriteLine(); Console.WriteLine($"e.Arguments.State: {e.Arguments.State}"); Console.WriteLine($"e.Arguments.ServerDescriptor.MachineName: {e.Arguments.ServerDescriptor.MachineName}"); Console.WriteLine($"e.Arguments.ServerDescriptor.ServerClass: {e.Arguments.ServerDescriptor.ServerClass}"); Console.WriteLine($"e.Arguments.ItemDescriptor.ItemId: {e.Arguments.ItemDescriptor.ItemId}"); Console.WriteLine($"e.Arguments.ItemDescriptor.AccessPath: {e.Arguments.ItemDescriptor.AccessPath}"); Console.WriteLine($"e.Arguments.ItemDescriptor.RequestedDataType: {e.Arguments.ItemDescriptor.RequestedDataType}"); Console.WriteLine($"e.Arguments.GroupParameters.Locale: {e.Arguments.GroupParameters.Locale}"); Console.WriteLine($"e.Arguments.GroupParameters.RequestedUpdateRate: {e.Arguments.GroupParameters.RequestedUpdateRate}"); Console.WriteLine($"e.Arguments.GroupParameters.PercentDeadband: {e.Arguments.GroupParameters.PercentDeadband}"); if (e.Succeeded) { Debug.Assert(!(e.Vtq is null)); Console.WriteLine($"e.Vtq.Value: {e.Vtq.Value}"); Console.WriteLine($"e.Vtq.Timestamp: {e.Vtq.Timestamp}"); Console.WriteLine($"e.Vtq.TimestampLocal: {e.Vtq.TimestampLocal}"); Console.WriteLine($"e.Vtq.Quality: {e.Vtq.Quality}"); } else { Debug.Assert(!(e.Exception is null)); Console.WriteLine($"e.Exception.Message: {e.Exception.Message}"); Console.WriteLine($"e.Exception.Source: {e.Exception.Source}"); } } // Example output: // //Processing item changed events for 1 minute... // //e.Arguments.State: //e.Arguments.ServerDescriptor.MachineName: //e.Arguments.ServerDescriptor.ServerClass: OPCLabs.KitServer.2 //e.Arguments.ItemDescriptor.ItemId: Simulation.Random //e.Arguments.ItemDescriptor.AccessPath: //e.Arguments.ItemDescriptor.RequestedDataType: Empty //e.Arguments.GroupParameters.Locale: 0 //e.Arguments.GroupParameters.RequestedUpdateRate: 5000 //e.Arguments.GroupParameters.PercentDeadband: 0 //e.Vtq.Value: 0.00125125888851588 //e.Vtq.Timestamp: 4/10/2020 4:35:25 PM //e.Vtq.TimestampLocal: 4/10/2020 6:35:25 PM //e.Vtq.Quality: GoodNonspecific (192) // //e.Arguments.State: //e.Arguments.ServerDescriptor.MachineName: //e.Arguments.ServerDescriptor.ServerClass: OPCLabs.KitServer.2 //e.Arguments.ItemDescriptor.ItemId: Trends.Ramp(1 min) //e.Arguments.ItemDescriptor.AccessPath: //e.Arguments.ItemDescriptor.RequestedDataType: Empty //e.Arguments.GroupParameters.Locale: 0 //e.Arguments.GroupParameters.RequestedUpdateRate: 5000 //e.Arguments.GroupParameters.PercentDeadband: 0 //e.Vtq.Value: 0.431881904602051 //e.Vtq.Timestamp: 4/10/2020 4:35:25 PM //e.Vtq.TimestampLocal: 4/10/2020 6:35:25 PM //e.Vtq.Quality: GoodNonspecific (192) // //... } }
# This example subscribes to changes of 2 items separately, and displays rich information available with each item # changed event notification. # # Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html . # OPC client and subscriber examples in Python on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-Python . # The QuickOPC package is needed. Install it using "pip install opclabs_quickopc". import opclabs_quickopc import time # Import .NET namespaces. from OpcLabs.EasyOpc.DataAccess import * # Item changed event handler. def itemChanged(sender, e): assert e is not None print() print('e.Arguments.State: ', e.Arguments.State, sep='') print('e.Arguments.ServerDescriptor.MachineName: ', e.Arguments.ServerDescriptor.MachineName, sep='') print('e.Arguments.ServerDescriptor.ServerClass: ', e.Arguments.ServerDescriptor.ServerClass, sep='') print('e.Arguments.ItemDescriptor.ItemId: ', e.Arguments.ItemDescriptor.ItemId, sep='') print('e.Arguments.ItemDescriptor.AccessPath: ', e.Arguments.ItemDescriptor.AccessPath, sep='') print('e.Arguments.ItemDescriptor.RequestedDataType: ', e.Arguments.ItemDescriptor.RequestedDataType, sep='') print('e.Arguments.GroupParameters.Locale: ', e.Arguments.GroupParameters.Locale, sep='') print('e.Arguments.GroupParameters.RequestedUpdateRate: ', e.Arguments.GroupParameters.RequestedUpdateRate, sep='') print('e.Arguments.GroupParameters.PercentDeadband: ', e.Arguments.GroupParameters.PercentDeadband, sep='') if e.Succeeded: assert e.Vtq is not None print('e.Vtq.Value: ', e.Vtq.Value, sep='') print('e.Vtq.Timestamp: ', e.Vtq.Timestamp, sep='') print('e.Vtq.TimestampLocal: ', e.Vtq.TimestampLocal, sep='') print('e.Vtq.Quality: ', e.Vtq.Quality, sep='') else: assert e.Exception is not None print('e.Exception.Message: ', e.Exception.Message, sep='') print('e.Exception.Source: ', e.Exception.Source, sep='') # Instantiate the client object. client = EasyDAClient() client.ItemChanged += itemChanged print('Subscribing item changes for 2 items...') IEasyDAClientExtension.SubscribeItem(client, '', 'OPCLabs.KitServer.2', 'Simulation.Random', 5*1000) IEasyDAClientExtension.SubscribeItem(client, '', 'OPCLabs.KitServer.2', 'Trends.Ramp (1 min)', 5*1000) print('Processing item change notifications for 1 minute...') time.sleep(60) print('Unsubscribing all items...') client.UnsubscribeAllItems() client.ItemChanged -= itemChanged print('Finished.')
' This example subscribes to changes of 2 items separately, and displays rich information available with each item changed ' event notification. ' ' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html . Imports System.Threading Imports OpcLabs.EasyOpc.DataAccess Imports OpcLabs.EasyOpc.DataAccess.OperationModel Namespace DataAccess._EasyDAItemChangedEventArgs Partial Friend Class General Shared Sub Main1() ' Instantiate the client object. Using client = New EasyDAClient() Dim eventHandler = New EasyDAItemChangedEventHandler(AddressOf client_ItemChanged) AddHandler client.ItemChanged, eventHandler Console.WriteLine("Subscribing items...") client.SubscribeItem("", "OPCLabs.KitServer.2", "Simulation.Random", 5 * 1000) client.SubscribeItem("", "OPCLabs.KitServer.2", "Trends.Ramp (1 min)", 5 * 1000) Console.WriteLine("Processing item changed events for 1 minute...") Thread.Sleep(60 * 1000) End Using End Sub Private Shared Sub client_ItemChanged(sender As Object, e As EasyDAItemChangedEventArgs) Console.WriteLine() Console.WriteLine($"e.Arguments.State: {e.Arguments.State}") Console.WriteLine($"e.Arguments.ServerDescriptor.MachineName: {e.Arguments.ServerDescriptor.MachineName}") Console.WriteLine($"e.Arguments.ServerDescriptor.ServerClass: {e.Arguments.ServerDescriptor.ServerClass}") Console.WriteLine($"e.Arguments.ItemDescriptor.ItemId: {e.Arguments.ItemDescriptor.ItemId}") Console.WriteLine($"e.Arguments.ItemDescriptor.AccessPath: {e.Arguments.ItemDescriptor.AccessPath}") Console.WriteLine($"e.Arguments.ItemDescriptor.RequestedDataType: {e.Arguments.ItemDescriptor.RequestedDataType}") Console.WriteLine($"e.Arguments.GroupParameters.Locale: {e.Arguments.GroupParameters.Locale}") Console.WriteLine($"e.Arguments.GroupParameters.RequestedUpdateRate: {e.Arguments.GroupParameters.RequestedUpdateRate}") Console.WriteLine($"e.Arguments.GroupParameters.PercentDeadband: {e.Arguments.GroupParameters.PercentDeadband}") If e.Succeeded Then Debug.Assert(e.Vtq IsNot Nothing) Console.WriteLine($"e.Vtq.Value: {e.Vtq.Value}") Console.WriteLine($"e.Vtq.Timestamp: {e.Vtq.Timestamp}") Console.WriteLine($"e.Vtq.TimestampLocal: {e.Vtq.TimestampLocal}") Console.WriteLine($"e.Vtq.Quality: {e.Vtq.Quality}") Else Debug.Assert(e.Exception IsNot Nothing) Console.WriteLine($"e.Exception.Message: {e.Exception.Message}") Console.WriteLine($"e.Exception.Source: {e.Exception.Source}") End If End Sub ' Example output ' 'Processing item changed events for 1 minute... ' 'e.Arguments.State: 'e.Arguments.ServerDescriptor.MachineName 'e.Arguments.ServerDescriptor.ServerClass OPCLabs.KitServer.2 'e.Arguments.ItemDescriptor.ItemId: Simulation.Random 'e.Arguments.ItemDescriptor.AccessPath: 'e.Arguments.ItemDescriptor.RequestedDataType Empty 'e.Arguments.GroupParameters.Locale: 0 'e.Arguments.GroupParameters.RequestedUpdateRate: 5000 'e.Arguments.GroupParameters.PercentDeadband: 0 'e.Vtq.Value: 0.00125125888851588 'e.Vtq.Timestamp: 4/10/2020 4:35:25 PM 'e.Vtq.TimestampLocal: 4/10/2020 6:35:25 PM 'e.Vtq.Quality: GoodNonspecific (192) ' 'e.Arguments.State: 'e.Arguments.ServerDescriptor.MachineName 'e.Arguments.ServerDescriptor.ServerClass OPCLabs.KitServer.2 'e.Arguments.ItemDescriptor.ItemId: Trends.Ramp(1 min) 'e.Arguments.ItemDescriptor.AccessPath: 'e.Arguments.ItemDescriptor.RequestedDataType Empty 'e.Arguments.GroupParameters.Locale: 0 'e.Arguments.GroupParameters.RequestedUpdateRate: 5000 'e.Arguments.GroupParameters.PercentDeadband: 0 'e.Vtq.Value: 0.431881904602051 'e.Vtq.Timestamp: 4/10/2020 4:35:25 PM 'e.Vtq.TimestampLocal: 4/10/2020 6:35:25 PM 'e.Vtq.Quality: GoodNonspecific (192) ' '... End Class End Namespace
Rem This example subscribes to changes of 2 items separately, and displays rich information available with each item changed Rem event notification. Rem Rem Find all latest examples here : https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html . Option Explicit Dim Client: Set Client = CreateObject("OpcLabs.EasyOpc.DataAccess.EasyDAClient") WScript.ConnectObject Client, "Client_" Client.SubscribeItem "", "OPCLabs.KitServer.2", "Simulation.Random", 5*1000 Client.SubscribeItem "", "OPCLabs.KitServer.2", "Trends.Ramp (1 min)", 5*1000 WScript.Echo "Processing item changed events for 1 minute..." WScript.Sleep 60*1000 Sub Client_ItemChanged(Sender, e) On Error Resume Next WScript.Echo WScript.Echo "e.Arguments.State: " & e.Arguments.State WScript.Echo "e.Arguments.ServerDescriptor.MachineName: " & e.Arguments.ServerDescriptor.MachineName WScript.Echo "e.Arguments.ServerDescriptor.ServerClass: " & e.Arguments.ServerDescriptor.ServerClass WScript.Echo "e.Arguments.ItemDescriptor.ItemId: " & e.Arguments.ItemDescriptor.ItemId WScript.Echo "e.Arguments.ItemDescriptor.AccessPath: " & e.Arguments.ItemDescriptor.AccessPath WScript.Echo "e.Arguments.ItemDescriptor.RequestedDataType: " & e.Arguments.ItemDescriptor.RequestedDataType WScript.Echo "e.Arguments.GroupParameters.Locale: " & e.Arguments.GroupParameters.Locale WScript.Echo "e.Arguments.GroupParameters.RequestedUpdateRate: " & e.Arguments.GroupParameters.RequestedUpdateRate WScript.Echo "e.Arguments.GroupParameters.PercentDeadband: " & e.Arguments.GroupParameters.PercentDeadband WScript.Echo "e.Exception.Message: " & e.Exception.Message WScript.Echo "e.Exception.Source: " & e.Exception.Source WScript.Echo "e.Exception.ErrorCode: " & e.Exception.ErrorCode WScript.Echo "e.Vtq.Value: " & e.Vtq.Value WScript.Echo "e.Vtq.Timestamp: " & e.Vtq.Timestamp WScript.Echo "e.Vtq.TimestampLocal: " & e.Vtq.TimestampLocal WScript.Echo "e.Vtq.Quality: " & e.Vtq.Quality End Sub