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<h1><a name="kanchor81"></a>Event Registry API</h1>
<p>You can register to events of the db4o-database. You can used these events to implement all kinds of additional functionality. Take a look a few example use-cases. <a href="possible_usecases.htm" target="" title="" alt="" class="MCXref" xrefformat="See "{paratext}"">See "Possible Usecases"</a></p>
<p>There's an event for each database operation. Most of the time there are two events for an operation. One is fired before the operation starts, the other when the operation ends.</p>
<h2>Register to an Event</h2>
<p>You can gain access to the events via a event registry. These three steps show how to register to events.</p>
<p>First obtain a <span class="PrimaryEventRegistry">IEventRegistry</span>-instance from the object container.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IEventRegistry events = EventRegistryFactory.ForObjectContainer(container);</pre>
<div class="codesnippet-meta">EventRegistryExamples.cs: Obtain the event-registry
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim events As IEventRegistry = EventRegistryFactory.ForObjectContainer(container)</pre>
<div class="codesnippet-meta">EventRegistryExamples.vb: Obtain the event-registry
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<p>Now you can register your event-handlers on the event registry.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">events.Committing += HandleCommitting;</pre>
<div class="codesnippet-meta">EventRegistryExamples.cs: register for a event
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">AddHandler events.Committing, AddressOf HandleCommitting</pre>
<div class="codesnippet-meta">EventRegistryExamples.vb: register for a event
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<p>Then implement your event handling.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">private static void HandleCommitting(object sender,
CommitEventArgs commitEventArgs)
{
// handle the event
}
</pre>
<div class="codesnippet-meta">EventRegistryExamples.cs: implement your event handling
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Private Shared Sub HandleCommitting(ByVal sender As Object, _
ByVal commitEventArgs As CommitEventArgs)
' handle the event
End Sub</pre>
<div class="codesnippet-meta">EventRegistryExamples.vb: implement your event handling
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<h2>Cancelable Events</h2>
<p>Some events can cancel the operation. All events which have a CancellableObjectEventArgs-parameter can cancel the operation. When you cancel in a event, the operation won't be executed. For example:</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IEventRegistry events = EventRegistryFactory.ForObjectContainer(container);
events.Creating +=
delegate(object sender, CancellableObjectEventArgs args)
{
if (args.Object is Person)
{
Person p = (Person) args.Object;
if (p.Name.Equals("Joe Junior"))
{
args.Cancel();
}
}
};
</pre>
<div class="codesnippet-meta">EventRegistryExamples.cs: Cancel store operation
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim events As IEventRegistry = EventRegistryFactory.ForObjectContainer(container)
AddHandler events.Creating, AddressOf HandleCreatingEvent</pre>
<div class="codesnippet-meta">EventRegistryExamples.vb: Cancel store operation
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Private Shared Sub HandleCreatingEvent(ByVal sender As Object, _
ByVal args As CancellableObjectEventArgs)
If TypeOf args.[Object] Is Person Then
Dim p As Person = DirectCast(args.[Object], Person)
If p.Name.Equals("Joe Junior") Then
args.Cancel()
End If
End If
End Sub</pre>
<div class="codesnippet-meta">EventRegistryExamples.vb: Cancel store operation Handler
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<h2>Register Events On The Server</h2>
<p>When you want to register for the events on the server, you should register it on the server-container. </p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IObjectServer server =
Db4oClientServer.OpenServer(DatabaseFileName, PortNumber);
IEventRegistry eventsOnServer =
EventRegistryFactory.ForObjectContainer(server.Ext().ObjectContainer());</pre>
<div class="codesnippet-meta">EventRegistryExamples.cs: register for events on the server
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim server As IObjectServer = _
Db4oClientServer.OpenServer(DatabaseFileName, PortNumber)
Dim eventsOnServer As IEventRegistry = _
EventRegistryFactory.ForObjectContainer(server.Ext().ObjectContainer())</pre>
<div class="codesnippet-meta">EventRegistryExamples.vb: register for events on the server
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<h2>Commit-Events</h2>
<p>Commit-events bring a collection of the added, updated and deleted object with it. You can iterate over these objects. The updated- and added-collections contain LazyObjectReferences, the deleted-event a FrozenObjectInfos. Note that you may cannot get deleted object-instance anymore, but only the meta-info. Furthermore the object doesn't need to be activated. So when you need to read information out if it, ensure that you've activated it first.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IEventRegistry events = EventRegistryFactory.ForObjectContainer(container);
events.Committed +=
delegate(object sender, CommitEventArgs args)
{
foreach (LazyObjectReference reference in args.Added)
{
Console.WriteLine("Added " + reference.GetObject());
}
foreach (LazyObjectReference reference in args.Updated)
{
Console.WriteLine("Updated " + reference.GetObject());
}
foreach (FrozenObjectInfo reference in args.Deleted)
{
//the deleted info might doesn't contain the object anymore and
//return the null.
Console.WriteLine("Deleted " + reference.GetObject());
}
};</pre>
<div class="codesnippet-meta">EventRegistryExamples.cs: Commit-info
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim events As IEventRegistry = EventRegistryFactory.ForObjectContainer(container)
AddHandler events.Committed, AddressOf HandlingCommitEvent</pre>
<div class="codesnippet-meta">EventRegistryExamples.vb: Commit-info
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Private Shared Sub HandlingCommitEvent(ByVal sender As Object, ByVal args As CommitEventArgs)
For Each reference As LazyObjectReference In args.Added
Console.WriteLine("Added " & reference.GetObject())
Next
For Each reference As LazyObjectReference In args.Updated
Console.WriteLine("Updated " & reference.GetObject())
Next
For Each reference As FrozenObjectInfo In args.Deleted
'the deleted info might doesn't contain the object anymore and
'return the null.
Console.WriteLine("Deleted " & reference.GetObject())
Next
End Sub</pre>
<div class="codesnippet-meta">EventRegistryExamples.vb: Commit-info Handler
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<h2>Pitfalls and Limitations</h2>
<ul>
<li value="1">All embedded clients-/session share the same event registry. So you need to register the events only on one.</li>
<li value="2">You cannot call recursively the event-producing operation within the event-handler. For example in the storing-event you cannot call store. In the committing-event you cannot call commit.</li>
<li value="3">In client-server mode, each client has it's own event-registry, and therefore only sees its own events. Except the committed-event. <a href="../../client-server/events_in_cs_mode.htm" target="" title="" alt="" class="MCXref" xrefformat="See "{paratext}"">See "Events In Client Server-Mode"</a></li>
</ul>
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