How to: Replace a Standard PDF Viewer Control Command with a Custom Command

• Oct 12, 2020
• 3 minutes to read

This example shows how to modify the existing PDF Viewer command functionality.

1. Create a custom command provider class inherited from the PdfCommandProvider class and override its members to create a new DelegateCommand.

   In this example the CustomPdfCommandProvider class overrides ZoomInCommandInternal and ZoomOutCommandInternal properties to zoom in/out to a custom zoom factor.

   The delegate command constructor accepts ZoomIn and CanZoomIn (ZoomOut and CanZoomOut) delegates.

   View Example

   C#

   public class CustomPdfCommandProvider : PdfCommandProvider {
   
           ICommand zoomInCommandInternal;
           protected override ICommand ZoomInCommandInternal {
               get { return zoomInCommandInternal ?? (zoomInCommandInternal = new DelegateCommand(ZoomIn, CanZoomIn)); }
           }
   
           ICommand zoomOutCommandInternal;
           protected override ICommand ZoomOutCommandInternal {
               get { return zoomOutCommandInternal ?? (zoomOutCommandInternal = new DelegateCommand(ZoomOut, CanZoomOut)); }
           }
   
           void ZoomIn() {
               //...           
           }
   
           bool CanZoomIn() {
               //...      
           }
   
           void ZoomOut() {
               //...           
           }
   
           bool CanZoomOut() {
               //...      
           }
   }
   

   VB.NET

   Public Class CustomPdfCommandProvider
           Inherits PdfCommandProvider
   
           Private _ZoomInCommandInternal As DelegateCommand
           Protected Overrides ReadOnly Property ZoomInCommandInternal() As ICommand
               Get
                   If _ZoomInCommandInternal IsNot Nothing Then
                       Return _ZoomInCommandInternal
                   Else
                       _ZoomInCommandInternal = New DelegateCommand(AddressOf ZoomIn, AddressOf CanZoomIn)
                       Return _ZoomInCommandInternal
                   End If
               End Get
           End Property
   
           Private _ZoomOutCommandInternal As DelegateCommand
           Protected Overrides ReadOnly Property ZoomOutCommandInternal() As ICommand
               Get
                   If _ZoomOutCommandInternal IsNot Nothing Then
                       Return _ZoomOutCommandInternal
                   Else
                       _ZoomOutCommandInternal = New DelegateCommand(AddressOf ZoomOut, AddressOf CanZoomOut)
                       Return _ZoomOutCommandInternal
                   End If
               End Get
           End Property
   
           Private Sub ZoomIn()
           End Sub
   
           Private Function CanZoomIn() As Boolean
           End Function
   
           Private Sub ZoomOut()
           End Sub
   
           Private Function CanZoomOut() As Boolean
           End Function
   
   End Class
   

2. Implement ZoomIn and CanZoomIn (ZoomOut and CanZoomOut) methods that the corresponding delegate command will call when the command is executed.

   View Example

   C#

   //...
           readonly List<double> factors = new List<double> { 0.15, 0.3, 0.45, 1, 1.25, 1.5, 2, 5 };
           public PdfViewerControl Control { get { return DocumentViewer as PdfViewerControl; } }
   
           void ZoomIn() {
               if (Control == null) return;
   
               foreach (var zoomFactor in factors) {
                   if (Control.ZoomFactor < zoomFactor) {
                       Control.ZoomFactor = zoomFactor;
                       break;
                   }
               }
           }
   
           bool CanZoomIn() {
               return Control != null && Control.Document != null && Control.ZoomFactor < factors[factors.Count - 1];
           }
   
           void ZoomOut() {
               if (Control == null) return;
   
               for (int i = factors.Count - 1; i >= 0; i--) {
                   if (factors[i] < Control.ZoomFactor) {
                       Control.ZoomFactor = factors[i];
                       break;
                   }
               }
           }
   
           bool CanZoomOut() {
               return Control != null && Control.Document != null && Control.ZoomFactor > factors[0];
           }
   //...
   

   VB.NET

   Class SurroundingClass
       ReadOnly factors As List(Of Double) = New List(Of Double) From {
           0.15,  0.3,  0.45, 1, 1.25, 1.5, 2,  5
       }
   
       Public ReadOnly Property Control As PdfViewerControl
           Get
               Return TryCast(DocumentViewer, PdfViewerControl)
           End Get
       End Property
   
       Private Sub ZoomIn()
           If Control Is Nothing Then Return
   
           For Each zoomFactor In factors
   
               If Control.ZoomFactor < zoomFactor Then
                   Control.ZoomFactor = zoomFactor
                   Exit For
               End If
           Next
       End Sub
   
       Private Function CanZoomIn() As Boolean
           Return Control IsNot Nothing AndAlso Control.Document IsNot Nothing AndAlso Control.ZoomFactor < factors(factors.Count - 1)
       End Function
   
       Private Sub ZoomOut()
           If Control Is Nothing Then Return
   
           For i As Integer = factors.Count - 1 To 0
   
               If factors(i) < Control.ZoomFactor Then
                   Control.ZoomFactor = factors(i)
                   Exit For
               End If
           Next
       End Sub
   
       Private Function CanZoomOut() As Boolean
           Return Control IsNot Nothing AndAlso Control.Document IsNot Nothing AndAlso Control.ZoomFactor > factors(0)
       End Function
   End Class
   

3. Use the created CustomPdfCommandProvider to substitute the default command provider.

   View Example

   XAML

   xmlns:local="clr-namespace:DXSample"
   xmlns:pdfViewer="http://schemas.devexpress.com/winfx/2008/xaml/pdf"
   
       <pdfViewer:PdfViewerControl.CommandProvider>
            <local:CustomPdfCommandProvider/>            
       </pdfViewer:PdfViewerControl.CommandProvider>