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<h1>Transparent Persistence Example</h1>
<p>In order to support <span class="MCTextPopup"><a href="javascript:void(0);" class="MCTextPopupSpot" onclick="FMCTextPopup( event, this ); return false;">TP<img style="border: none;margin-left: 5px;" src="../../../SkinSupport/ExpandingClosed.gif" MadCap:altsrc="../../../SkinSupport/ExpandingOpen.gif" class="MCExpandingIcon" onload="if ( typeof( FMCPreloadImage ) == 'function' ) { FMCPreloadImage( '../../../SkinSupport/ExpandingOpen.gif' ); }" /></a><span class="MCTextPopupBody" style="display: none; ">Transparent Persistence</span></span>, the persistent objects need to implement the <span class="PrimaryActivatable">IActivatable</span>-interface.</p>
<p>An object which implements the <span class="PrimaryActivatable">IActivatable</span>-interface is responsible for activating itself. For this purpose the class needs a field to keep its activator. This field is only used by the <a href="../../activation_concept/transparent_activation_framework.htm">transparent activation framework</a>. Therefore it's marked as transient, to avoid that it's stored in the database. </p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">public class Person : IActivatable
{
[NonSerialized] private IActivator activator;</pre>
<div class="codesnippet-meta">Person.cs: Implement the required activatable interface and add activator
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Class Person
Implements IActivatable
<Transient()> _
Private m_activator As IActivator</pre>
<div class="codesnippet-meta">Person.vb: Implement the required activatable interface and add activator
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<p>Then implement the two methods of the <span class="PrimaryActivatable">IActivatable</span>-interface. The bind-method binds an activator to the object. It's called by the transparent activation framework. The activate-method needs to be called before any read or write operation on the object. Since these two methods are always the same, you can move the implementation to a common super class or to a static utility class.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">public void Bind(IActivator activator)
{
if (this.activator == activator)
{
return;
}
if (activator != null && null != this.activator)
{
throw new InvalidOperationException("Object can only be bound to one activator");
}
this.activator = activator;
}
public void Activate(ActivationPurpose activationPurpose)
{
if (null != activator)
{
activator.Activate(activationPurpose);
}
}
</pre>
<div class="codesnippet-meta">Person.cs: Implement the activatable interface methods
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Sub Bind(ByVal activator As IActivator) _
Implements IActivatable.Bind
If m_activator Is activator Then
Exit Sub
End If
If activator IsNot Nothing AndAlso m_activator IsNot Nothing Then
Throw New InvalidOperationException("Object can only be bound to one activator")
End If
m_activator = activator
End Sub
Public Sub Activate(ByVal activationPurpose As ActivationPurpose) _
Implements IActivatable.Activate
If m_activator IsNot Nothing Then
m_activator.Activate(activationPurpose)
End If
End Sub
</pre>
<div class="codesnippet-meta">Person.vb: Implement the activatable interface methods
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<p>Now the important part. Every time a field of the class is accessed you need to call the activate-method with the purpose. For example in every <span class="Primarygetter/setter">property</span> or other method. Probably the best way is to use only <![CDATA[ ]]><span class="Primarygetter/setter">property</span> <![CDATA[ ]]>even within the class to access fields. And the <span class="Primarygetter/setter">property</span> ensures that the activate-method is called.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">public string Name
{
get
{
Activate(ActivationPurpose.Read);
return name;
}
set
{
Activate(ActivationPurpose.Write);
name = value;
}
}
public override string ToString()
{
// use the getter/setter withing the class,
// to ensure the activate-method is called
return Name;
}
</pre>
<div class="codesnippet-meta">Person.cs: Call the activate method on every field access
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Property Name() As String
Get
Activate(ActivationPurpose.Read)
Return m_name
End Get
Set(ByVal value As String)
Activate(ActivationPurpose.Write)
m_name = value
End Set
End Property
Public Overloads Overrides Function ToString() As String
' use the getter/setter withing the class,
' to ensure the activate-method is called
Return Name
End Function
</pre>
<div class="codesnippet-meta">Person.vb: Call the activate method on every field access
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<p> Implementing the <span class="PrimaryActivatable">IActivatable</span>-interface manually for every class is repetitive and error prone. That's why this process can be automated. <a href="tp_enhanced_example.htm" target="" title="" alt="" class="MCXref" xrefformat="See "{paratext}"">See "TP Enhancement"</a></p>
<p>The last step is to enable transparent persistence via configuration.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IEmbeddedConfiguration configuration = Db4oEmbedded.NewConfiguration();
configuration.Common.Add(new TransparentPersistenceSupport());
IObjectContainer container = Db4oEmbedded.OpenFile(configuration, DatabaseFileName);</pre>
<div class="codesnippet-meta">TransparentActivationExamples.cs: Activate transparent persistence
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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 configuration As IEmbeddedConfiguration = Db4oEmbedded.NewConfiguration()
configuration.Common.Add(New TransparentPersistenceSupport())
Dim container As IObjectContainer = Db4oEmbedded.OpenFile(configuration, DatabaseFileName)</pre>
<div class="codesnippet-meta">TransparentActivationExamples.vb: Activate transparent persistence
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<p>Now transparent persistence is enabled. You have to store the object only once initially. After that, changes are automatically stored on the commit call.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">using (IObjectContainer container = OpenDatabaseTP())
{
Person person = Person.PersonWithHistory();
container.Store(person);
}
using (IObjectContainer container = OpenDatabaseTP())
{
Person person = QueryByName(container, "Joanna the 10");
Person beginOfDynasty = person.Mother;
// With transparent persistence enabled, you can navigate deeply
// nested object graphs. db4o will ensure that the objects
// are loaded from the database.
while (null != beginOfDynasty.Mother)
{
beginOfDynasty = beginOfDynasty.Mother;
}
Console.WriteLine(beginOfDynasty.Name);
// Updating a object doesn't requires no store call.
// Just change the objects and the call commit.
beginOfDynasty.Name = "New Name";
container.Commit();
}
using (IObjectContainer container = OpenDatabaseTP())
{
Person person = QueryByName(container, "New Name");
// The changes are stored, due to transparent persistence
Console.WriteLine(person.Name);
}</pre>
<div class="codesnippet-meta">TransparentActivationExamples.cs: Transparent persistence in action
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</div>
<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Using container As IObjectContainer = OpenDatabaseTP()
Dim joanna As Person = Person.PersonWithHistory()
container.Store(joanna)
End Using
Using container As IObjectContainer = OpenDatabaseTP()
Dim joanna As Person = QueryByName(container, "Joanna the 10")
Dim beginOfDynasty As Person = joanna.Mother
' With transparent persistence enabled, you can navigate deeply
' nested object graphs. db4o will ensure that the objects
' are loaded from the database.
While beginOfDynasty.Mother IsNot Nothing
beginOfDynasty = beginOfDynasty.Mother
End While
Console.WriteLine(beginOfDynasty.Name)
' Updating a object doesn't requires no store call.
' Just change the objects and the call commit.
beginOfDynasty.Name = "New Name"
container.Commit()
End Using
Using container As IObjectContainer = OpenDatabaseTP()
Dim joanna As Person = QueryByName(container, "New Name")
' The changes are stored, due to transparent persistence
Console.WriteLine(joanna.Name)
End Using</pre>
<div class="codesnippet-meta">TransparentActivationExamples.vb: Transparent persistence in action
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