Aplicaciones Windows que usan devoluciones de llamada
En la mayoría de los escenarios de procesamiento asincrónico, querrá iniciar una operación de base de datos y seguir ejecutando otros procesos sin esperar a que se complete la operación de base de datos. Sin embargo, muchos escenarios obligan a realizar alguna acción una vez que la operación de base de datos ha finalizado. En una aplicación de Windows, por ejemplo, es posible que quiera delegar la operación de larga duración a un subproceso en segundo plano, al mismo tiempo que permite que el subproceso de la interfaz de usuario siga respondiendo. Sin embargo, cuando se completa la operación de base de datos, desea utilizar los resultados para rellenar el formulario. Este tipo de escenario se implementa mejor con una devolución de llamada.
Una devolución de llamada se define especificando un delegado AsyncCallback en el método BeginExecuteNonQuery, BeginExecuteReader o BeginExecuteXmlReader. Se llama al delegado cuando se completa la operación. Puede pasar a delegado una referencia a SqlCommand, lo que facilita el acceso al objeto SqlCommand y llama al método End adecuado sin tener que usar una variable global.
Ejemplo
La siguiente aplicación Windows muestra el uso del método BeginExecuteNonQuery, que ejecuta una instrucción Transact-SQL que incluye un retraso de unos pocos segundos (emulando un comando de ejecución prolongada).
En este ejemplo se muestran varias técnicas importantes, como llamar a un método que interactúa con el formulario desde un subproceso independiente. Además, en este ejemplo se muestra cómo debe impedir que los usuarios ejecuten simultáneamente un comando varias veces y cómo debe asegurarse de que el formulario no se cierra antes de que se llame al procedimiento de devolución de llamada.
Para configurar este ejemplo, cree una aplicación Windows. Coloque un control Button y dos controles Label en el formulario (aceptando el nombre predeterminado de cada control). Agregue el código siguiente a la clase del formulario y modifique la cadena de conexión según sea necesario para su entorno.
' Add these to the top of the class:
Imports System
Imports System.Data
Imports System.Data.SqlClient
' Add this code to the form's class:
' You'll need this delegate in order to display text from a
' thread other than the form's thread. See the HandleCallback
' procedure for more information.
' This same delegate matches both the DisplayStatus
' and DisplayResults methods.
Private Delegate Sub DisplayInfoDelegate(ByVal Text As String)
' This flag ensures that the user doesn't attempt
' to restart the command or close the form while the
' asynchronous command is executing.
Private isExecuting As Boolean
' This example maintains the connection object
' externally, so that it's available for closing.
Private connection As SqlConnection
Private Function GetConnectionString() As String
' To avoid storing the connection string in your code,
' you can retrieve it from a configuration file.
' If you have not included "Asynchronous Processing=true"
' in the connection string, the command will not be able
' to execute asynchronously.
Return "..." & _
"Asynchronous Processing=true"
End Function
Private Sub DisplayStatus(ByVal Text As String)
Me.Label1.Text = Text
End Sub
Private Sub DisplayResults(ByVal Text As String)
Me.Label1.Text = Text
DisplayStatus("Ready")
End Sub
Private Sub Form1_FormClosing(ByVal sender As Object, _
ByVal e As System.Windows.Forms.FormClosingEventArgs) _
Handles Me.FormClosing
If isExecuting Then
MessageBox.Show(Me, "Can't close the form until " & _
"the pending asynchronous command has completed. " & _
"Please wait...")
e.Cancel = True
End If
End Sub
Private Sub Button1_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Button1.Click
If isExecuting Then
MessageBox.Show(Me, _
"Already executing. " & _
"Please wait until the current query " & _
"has completed.")
Else
Dim command As SqlCommand
Try
DisplayResults("")
DisplayStatus("Connecting...")
connection = New SqlConnection(GetConnectionString())
' To emulate a long-running query, wait for
' a few seconds before working with the data.
' This command doesn't do much, but that's the point--
' it doesn't change your data, in the long run.
Dim commandText As String = _
"WAITFOR DELAY '0:0:05';" & _
"UPDATE Production.Product " & _
"SET ReorderPoint = ReorderPoint + 1 " & _
"WHERE ReorderPoint Is Not Null;" & _
"UPDATE Production.Product " & _
"SET ReorderPoint = ReorderPoint - 1 " & _
"WHERE ReorderPoint Is Not Null"
command = New SqlCommand(commandText, connection)
connection.Open()
DisplayStatus("Executing...")
isExecuting = True
' Although it's not required that you pass the
' SqlCommand object as the second parameter in the
' BeginExecuteNonQuery call, doing so makes it easier
' to call EndExecuteNonQuery in the callback procedure.
Dim callback As New _
AsyncCallback(AddressOf HandleCallback)
' Once the BeginExecuteNonQuery method is called,
' the code continues--and the user can interact with
' the form--while the server executes the query.
command.BeginExecuteNonQuery(callback, command)
Catch ex As Exception
isExecuting = False
DisplayStatus($"Ready (last error: {ex.Message})")
If connection IsNot Nothing Then
connection.Close()
End If
End Try
End If
End Sub
Private Sub HandleCallback(ByVal result As IAsyncResult)
Try
' Retrieve the original command object, passed
' to this procedure in the AsyncState property
' of the IAsyncResult parameter.
Dim command As SqlCommand = _
CType(result.AsyncState, SqlCommand)
Dim rowCount As Integer = _
command.EndExecuteNonQuery(result)
Dim rowText As String = " rows affected."
If rowCount = 1 Then
rowText = " row affected."
End If
rowText = rowCount & rowText
' You may not interact with the form and its contents
' from a different thread, and this callback procedure
' is all but guaranteed to be running from a different
' thread than the form. Therefore you cannot simply call
' code that displays the results, like this:
' DisplayResults(rowText)
' Instead, you must call the procedure from the form's
' thread. One simple way to accomplish this is to call
' the Invoke method of the form, which calls the delegate
' you supply from the form's thread.
Dim del As New _
DisplayInfoDelegate(AddressOf DisplayResults)
Me.Invoke(del, rowText)
Catch ex As Exception
' Because you're now running code in a separate thread,
' if you don't handle the exception here, none of your
' other code will catch the exception. Because none of
' your code is on the call stack in this thread, there's
' nothing higher up the stack to catch the exception if
' you don't handle it here. You can either log the
' exception or invoke a delegate (as in the non-error
' case in this example) to display the error on the form.
' In no case can you simply display the error without
' executing a delegate as in the Try block here.
' You can create the delegate instance as you
' invoke it, like this:
Me.Invoke(New _
DisplayInfoDelegate(AddressOf DisplayStatus), _
$"Ready (last error: {ex.Message}")
Finally
isExecuting = False
If connection IsNot Nothing Then
connection.Close()
End If
End Try
End Sub
// Add these to the top of the class, if they're not already there:
using System;
using System.Data;
using System.Data.SqlClient;
// Hook up the form's Load event handler (you can double-click on
// the form's design surface in Visual Studio), and then add
// this code to the form's class:
// You'll need this delegate in order to display text from a thread
// other than the form's thread. See the HandleCallback
// procedure for more information.
// This same delegate matches both the DisplayStatus
// and DisplayResults methods.
private delegate void DisplayInfoDelegate(string Text);
// This flag ensures that the user doesn't attempt
// to restart the command or close the form while the
// asynchronous command is executing.
private bool isExecuting;
// This example maintains the connection object
// externally, so that it's available for closing.
private SqlConnection connection;
private static string GetConnectionString()
{
// To avoid storing the connection string in your code,
// you can retrieve it from a configuration file.
// If you have not included "Asynchronous Processing=true" in the
// connection string, the command will not be able
// to execute asynchronously.
return "..." + "Asynchronous Processing=true";
}
private void DisplayStatus(string Text)
{
this.label1.Text = Text;
}
private void DisplayResults(string Text)
{
this.label1.Text = Text;
DisplayStatus("Ready");
}
private void Form1_FormClosing(object sender, System.Windows.Forms.FormClosingEventArgs e)
{
if (isExecuting)
{
MessageBox.Show(this, "Can't close the form until " +
"the pending asynchronous command has completed. Please " +
"wait...");
e.Cancel = true;
}
}
private void button1_Click(object sender, System.EventArgs e)
{
if (isExecuting)
{
MessageBox.Show(this, "Already executing. Please wait until " +
"the current query has completed.");
}
else
{
SqlCommand command = null;
try
{
DisplayResults("");
DisplayStatus("Connecting...");
connection = new SqlConnection(GetConnectionString());
// To emulate a long-running query, wait for
// a few seconds before working with the data.
// This command doesn't do much, but that's the point--
// it doesn't change your data, in the long run.
string commandText =
"WAITFOR DELAY '0:0:05';" +
"UPDATE Production.Product " +
"SET ReorderPoint = ReorderPoint + 1 " +
"WHERE ReorderPoint Is Not Null;" +
"UPDATE Production.Product " +
"SET ReorderPoint = ReorderPoint - 1 " +
"WHERE ReorderPoint Is Not Null";
command = new SqlCommand(commandText, connection);
connection.Open();
DisplayStatus("Executing...");
isExecuting = true;
// Although it's not required that you pass the
// SqlCommand object as the second parameter in the
// BeginExecuteNonQuery call, doing so makes it easier
// to call EndExecuteNonQuery in the callback procedure.
AsyncCallback callback = new AsyncCallback(HandleCallback);
// Once the BeginExecuteNonQuery method is called,
// the code continues--and the user can interact with
// the form--while the server executes the query.
command.BeginExecuteNonQuery(callback, command);
}
catch (Exception ex)
{
isExecuting = false;
DisplayStatus($"Ready (last error: {ex.Message})");
if (connection != null)
{
connection.Close();
}
}
}
}
private void HandleCallback(IAsyncResult result)
{
try
{
// Retrieve the original command object, passed
// to this procedure in the AsyncState property
// of the IAsyncResult parameter.
SqlCommand command = (SqlCommand)result.AsyncState;
int rowCount = command.EndExecuteNonQuery(result);
string rowText = " rows affected.";
if (rowCount == 1)
{
rowText = " row affected.";
}
rowText = rowCount + rowText;
// You may not interact with the form and its contents
// from a different thread, and this callback procedure
// is all but guaranteed to be running from a different thread
// than the form. Therefore you cannot simply call code that
// displays the results, like this:
// DisplayResults(rowText)
// Instead, you must call the procedure from the form's thread.
// One simple way to accomplish this is to call the Invoke
// method of the form, which calls the delegate you supply
// from the form's thread.
DisplayInfoDelegate del =
new DisplayInfoDelegate(DisplayResults);
this.Invoke(del, rowText);
}
catch (Exception ex)
{
// Because you're now running code in a separate thread,
// if you don't handle the exception here, none of your other
// code will catch the exception. Because none of your
// code is on the call stack in this thread, there's nothing
// higher up the stack to catch the exception if you don't
// handle it here. You can either log the exception or
// invoke a delegate (as in the non-error case in this
// example) to display the error on the form. In no case
// can you simply display the error without executing a
// delegate as in the try block here.
// You can create the delegate instance as you
// invoke it, like this:
this.Invoke(new DisplayInfoDelegate(DisplayStatus),
$"Ready (last error: {ex.Message}");
}
finally
{
isExecuting = false;
if (connection != null)
{
connection.Close();
}
}
}
private void Form1_Load(object sender, System.EventArgs e)
{
this.button1.Click += new System.EventHandler(this.button1_Click);
this.FormClosing += new System.Windows.Forms.
FormClosingEventHandler(this.Form1_FormClosing);
}