s WINDOWS FORMS UI in Visual Basic .NET

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CHAPTER 9 s WINDOWS FORMS UI
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Figure 9-24. The ReadValuesOnChange property on the ProjectBindingSource control
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Because ProjectEdit inherits from WinPart, it must override the GetIdValue() method: Protected Overrides Function GetIdValue() As Object Return mProject End Function Remember that this method is supposed to return a unique identifying value for the user control; and the best thing to use is the business object being edited. By using the business object itself as the unique identifier, the code in MainForm can ensure that any attempt by the user to open a second user control to edit the same Project object will simply result in the original user control being brought to the forefront. There s one more thing to do, though. Recall that MainForm calls ToString() on the identifier value to populate the display in the Documents menu. Yet calling ToString() on a Project object will return the object s Id value: a Guid. This is obviously not desirable, since it would be preferable to show the object s Name property value. To solve this, the ToString() method is overridden to return that value: Public Overrides Function ToString() As String Return mProject.Name End Function This way, the Documents menu will show the project s Name rather than its Id.
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Loading the User Control
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A parameterized constructor is used to create the form, allowing the calling code to pass in a reference to the Project object that is to be viewed or edited. This constructor sets the DataSource properties of the BindingSource controls to initiate data binding:
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CHAPTER 9 s WINDOWS FORMS UI
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Private WithEvents mProject As Project Public Sub New(ByVal project As Project) InitializeComponent() mProject = project mProject.BeginEdit() Me.RoleListBindingSource.DataSource = RoleList.GetList Me.ProjectBindingSource.DataSource = mProject ApplyAuthorizationRules() End Sub First though, it stores the object reference in an instance field for later use. It also calls the business object s BeginEdit() method, thus invoking the n-level undo capability discussed in 3. This method call tells the object to take a snapshot of its current state, so the object can be restored to this exact state later if the user clicks the Cancel or Close button without saving the data first. With that done, data binding is initialized: Me.RoleListBindingSource.DataSource = RoleList.GetList Me.ProjectBindingSource.DataSource = mProject The DataSource property of RoleListBindingSource is set to a RoleList object. If this is the first time GetList() has been called, this will incur a call to the database; otherwise, the collection is returned from the cache implemented in 8. The DataSource property of projectBindingList is set to the Project object passed into the constructor as a parameter. By accepting this object as a parameter, ProjectEdit can be used to add new objects, or edit or view existing objects. It isn t concerned with where the object came from, just that it is a Project object that is to be displayed or edited. This technique increases the reusability of the ProjectEdit user control. Notice that the DataSource of ResourcesBindingSource is not explicitly set. This is because the Windows Forms designer understands the relationship between the Project object and its Resources property, and sets up the binding automatically. The last line of code in the constructor calls ApplyAuthorizationRules(), which is responsible for applying all authorization rules to the controls on the form.
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Authorization Rules
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Authorization rules are applied in two cases: as the form loads, and any time the user logs into or out of the application. This second scenario causes a CurrentPrincipalChanged event to be raised, and that event is handled in the code: Private Sub ProjectEdit_CurrentPrincipalChanged( _ ByVal sender As Object, ByVal e As System.EventArgs) _ Handles Me.CurrentPrincipalChanged ApplyAuthorizationRules() End Sub Like in the constructor, the ApplyAuthorizationRules() method is called to actually apply the rules and update the display. Of course, not much code is required in ApplyAuthorizationRules() because the ReadWriteAuthorization control will do most of the hard work. The only real code in the method exists to deal with the various buttons on the form (which aren t data bound) and the DataGridView control:
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