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The TextObserver form, with controls to display the subject s data
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Note that the Observer base class above was created by adding a form to a WinForms project, and adding the empty Notify() method No controls were added to the form, although any controls added would have been inherited by classes that inherited this form and this may have been useful in some circumstances Ideally, ObserverForm would be abstract (de ned with the MustInherit keyword), and Notify() would be de ned as MustOverride However, changing the de nitions to make this so upsets the Form Designer and so we must simply remember to override the Notify() method for each observer (with a proper abstract class, the Form Designer reminds you and will not compile code in which abstract methods have not been property overridden) With a form de nition to inherit from, the next step is to create some concrete observer forms You can either build the assembly as it is (without any concrete observers) to enable visual inheritance, or simply change the inherits statement at the top of the new forms to reference ObserverForm I nd this was much faster and easier To demonstrate the principle, we need two observer forms, since the point of the Observer Pattern is that the subject only needs to know there is an observer, not how it does its job The rst will display the subject information as text in three text boxes (txtWidth, txtHeight and txtFile) Figure 1010 and Listing 1021
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Public Class TextObserverForm Inherits ObserverForm + Windows Form Designer Generated Code Public Overrides Sub Notify(ByVal S As Subject) txtWidthText = SWidthToString() txtHeightText = SHeightToString() txtFileText = SGraphicFile End Sub Private Sub btnOK_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) Handles btnOKClick MeHide() End Sub End Class Listing 1021: The code behind the TextObserverForm
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Figure 1011 The GraphicObserverForm, with a PictureBox to display the subject
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The second observer will display the picture in the speci ed picture le at the speci ed Width and Height, so it will only require a PictureBox control to do this Figure 1011 and Listing 1022
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Public Class GraphicObserverForm Inherits ObserverForm + Windows Form Designer Generated Code Public Overrides Sub Notify(ByVal S As Subject) If Not picObserverImage Is Nothing Then picObserverImageDispose() End If picObserverSizeMode = _ PictureBoxSizeModeStretchImage picObserverWidth = SWidth picObserverHeight = SHeight picObserverImage = New Bitmap(SGraphicFile) End Sub Private Sub btnOK_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) Handles btnOKClick MeHide() End Sub End Class Listing 1022: Code behind GraphicObserverForm
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The Notify() code in GraphicObserverForm makes sure that the PictureBox control will resize the picture to t its speci ed dimensions (PictureBoxSizeModeStretchImage) and load the speci ed image le into it Note that in each observer, we simply pass a reference to the subject directly to the observer s Notify() method However, this may require us to break encapsulation
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The main form to control the observer demonstration
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in some cases, and then more speci c parameters would need to be de ned for the Notify() method As a nal step, we need some way of creating a subject and adding observers to it We can return to the default form of the WinForms application (renamed as MainForm) and add a few buttons to it, as shown in Figure 1012 Code on this form (Listing 1023) does as it says on the button captions
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Public Class MainForm Inherits SystemWindowsFormsForm + Windows Form Designer Generated Code Private S As Subject Private Sub btnCreate_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles btnCreateClick S = New Subject(0, 0, "") UpdateSubject() End Sub Private Sub UpdateSubject() If Not S Is Nothing Then SWidth = InputBox("Enter width of subject", _ "Picture Width", 100) SHeight = InputBox("Enter height of subject", _ "Picture Height", 100) Dim dlg As OpenFileDialog = New OpenFileDialog() With dlg Filter = "JPeg Files|*jpg|All Files|**" If ShowDialog = DialogResultOK Then SGraphicFile = FileName End If End With dlgDispose() dlg = Nothing End If End Sub
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