Timer di thread (C# e Visual Basic)
La classe System.Threading.Timer risulta utile per l'esecuzione periodica di un'attività su un thread distinto.È ad esempio possibile utilizzare un timer di thread per verificare lo stato e l'integrità di un database o per il backup di file critici.
Esempio di timer di thread
Nell'esempio seguente viene avviata un'attività ogni due secondi e un flag viene utilizzato per inizializzare il metodo Dispose, che consente l'arresto del timer.In questo esempio lo stato viene inviato alla finestra di output.
Private Class StateObjClass
' Used to hold parameters for calls to TimerTask.
Public SomeValue As Integer
Public TimerReference As System.Threading.Timer
Public TimerCanceled As Boolean
End Class
Public Sub RunTimer()
Dim StateObj As New StateObjClass
StateObj.TimerCanceled = False
StateObj.SomeValue = 1
Dim TimerDelegate As New System.Threading.TimerCallback(AddressOf TimerTask)
' Create a timer that calls a procedure every 2 seconds.
' Note: There is no Start method; the timer starts running as soon as
' the instance is created.
Dim TimerItem As New System.Threading.Timer(TimerDelegate, StateObj,
2000, 2000)
' Save a reference for Dispose.
StateObj.TimerReference = TimerItem
' Run for ten loops.
While StateObj.SomeValue < 10
' Wait one second.
System.Threading.Thread.Sleep(1000)
End While
' Request Dispose of the timer object.
StateObj.TimerCanceled = True
End Sub
Private Sub TimerTask(ByVal StateObj As Object)
Dim State As StateObjClass = CType(StateObj, StateObjClass)
' Use the interlocked class to increment the counter variable.
System.Threading.Interlocked.Increment(State.SomeValue)
System.Diagnostics.Debug.WriteLine("Launched new thread " & Now.ToString)
If State.TimerCanceled Then
' Dispose Requested.
State.TimerReference.Dispose()
System.Diagnostics.Debug.WriteLine("Done " & Now)
End If
End Sub
private class StateObjClass
{
// Used to hold parameters for calls to TimerTask.
public int SomeValue;
public System.Threading.Timer TimerReference;
public bool TimerCanceled;
}
public void RunTimer()
{
StateObjClass StateObj = new StateObjClass();
StateObj.TimerCanceled = false;
StateObj.SomeValue = 1;
System.Threading.TimerCallback TimerDelegate =
new System.Threading.TimerCallback(TimerTask);
// Create a timer that calls a procedure every 2 seconds.
// Note: There is no Start method; the timer starts running as soon as
// the instance is created.
System.Threading.Timer TimerItem =
new System.Threading.Timer(TimerDelegate, StateObj, 2000, 2000);
// Save a reference for Dispose.
StateObj.TimerReference = TimerItem;
// Run for ten loops.
while (StateObj.SomeValue < 10)
{
// Wait one second.
System.Threading.Thread.Sleep(1000);
}
// Request Dispose of the timer object.
StateObj.TimerCanceled = true;
}
private void TimerTask(object StateObj)
{
StateObjClass State = (StateObjClass)StateObj;
// Use the interlocked class to increment the counter variable.
System.Threading.Interlocked.Increment(ref State.SomeValue);
System.Diagnostics.Debug.WriteLine("Launched new thread " + DateTime.Now.ToString());
if (State.TimerCanceled)
// Dispose Requested.
{
State.TimerReference.Dispose();
System.Diagnostics.Debug.WriteLine("Done " + DateTime.Now.ToString());
}
}
I timer di thread risultano particolarmente utili quando l'oggetto System.Windows.Forms.Timer non è disponibile, ad esempio quando si sviluppano applicazioni console.