Introduction
This article discusses the construction of an image viewer user control that may be used to display images from a directory containing a collection of image files. The control displays the previous and next images in the file along with the current image. The current image may be opened from the user control into the default image application on the user’s machine based upon that machine’s file associations. Image thumbnails are used to supply the previous, current, and next images from the directory. As shown in the demonstration, the user control could be added to an application where you might need to allow users to preview or visually scan images prior to opening them up for viewing or edit.
Figure 1: Image Viewer User Control Loaded into a Test Application.
Figure 2: Opening an Image into the Default Image Application
(Microsoft Office Picture Manager shown).
Getting Started
In order to get started, unzip the included project and open the solution in the Visual Studio 2008 environment. There are two projects contained in the solution. ImageViewerVB is the first project and it is a control library project containing a single user control. The second project (TestImageViewerVB) is a test application which displays a single form used to contain the user control.
In the solution explorer, you should note these files:
Figure 3: Solution Explorer.
The Image Viewer User Control (ImageViewerVB.vb)
The Image Viewer user control contains everything necessary to browse to the image folder and to view the current, next, and previous images. The control itself contains a text box and browse button used to open a folder browser dialog box; when the user navigates to the desired folder and selects it, the path the folder will be shown in the textbox control and the browse button click event handler will trigger the collection of all of the image files contained in the selected folder and will start up the display of the images. The control contains three picture boxes used to display the previous, current, and next images. The previous and next images are accompanied by two buttons used to move backwards and forwards through the image collection. The paths to each of the image files are contained in an array list and an integer value with class scope is used to maintain the current position within that array list.
If you'd care to open the code view up in the IDE, you will see that the code file begins with the following using statements; most included are per the default configuration but System.IO was added to support the directory operations used within the code:
Imports System.IO
Imports System.Collections
Following the imports, the class is defined and a constructor added as are a collection of local member variables used to maintain the folder path, the image file paths (in an array list), the image list position, and the images themselves (previous, current, and next). I'm not doing anything in the constructor here but if you want to; for example, you might want to point the folder browser dialog to a default folder location.
Public Class ImageViewer
Private mFolder As String
Private mImageList As ArrayList
Private mImagePosition As Integer
Private mPreviousImage As Image
Private mCurrentImage As Image
Private mNextImage As Image
Public Sub New()
InitializeComponent()
End Sub
Next up is the user control’s load event handler; this demonstration does not do anything in the load event but you may wish to use this handler to do something here.
Private Sub ImageViewer_Load(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles MyBase.Load
End Sub
The next section of the code is the browse button click event handler. In this section of the code, the handler opens the folder browser dialog box which the user may then use to set the path to the image folder. Once the folder has been set, the handler loops through that folder and locates all BMP, JPEG, and GIF files; the file paths of any image files found are added to an array list (BMPs, GIFs, and JPEGs). Once the array list is populated with the file paths, the click event handler wraps up but setting the list index position to zero and then it calls the SetImages method which sets up the current, previous, and next images into the appropriate picture box. The code is annotated and should be easy enough to follow:
Private Sub btnBrowse_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnBrowse.Click
Me.folderBrowserDialog1.Description = "Select the image directory."
Me.folderBrowserDialog1.ShowNewFolderButton = False
Dim result As DialogResult = folderBrowserDialog1.ShowDialog()
If result = DialogResult.OK Then
mFolder = folderBrowserDialog1.SelectedPath
If (Not String.IsNullOrEmpty(mFolder)) Then
txtImageDirectory.Text = mFolder
Else
Return
End If
mImageList = New ArrayList()
Dim Dir As New DirectoryInfo(mFolder)
Dim f As FileInfo
For Each f In Dir.GetFiles("*.*")
Select Case (f.Extension.ToUpper())
Case ".JPG"
mImageList.Add(f.FullName)
Case ".BMP"
mImageList.Add(f.FullName)
Case ".GIF"
mImageList.Add(f.FullName)
Case Else
End Select
Next
mImagePosition = 0
SetImages()
End If
End Sub
The next bit of code is the SetImages function; this function is used to set the images for each of the picture boxes (previous, current, and next image). This code creates thumbnails of the images stored as file paths in the images array list and then sets the picture boxes to display the thumbnails rather than the full images. The code is written to respond differently when hitting the limits of the array list used to store the file paths to the actual images; at the limits, the previous or next images will be blanked.
Further, the code clears out the images once the picture boxes are set, and garbage collection is called to clean it up each time the function is called. I found that, without garbage collection, the control would throw an out of memory exception when scanning through large numbers of images in one setting. Again, this code is annotated and should be easy enough to follow:
Private Sub SetImages()
mPreviousImage = Nothing
mNextImage = Nothing
mCurrentImage = Nothing
If mImagePosition > 0 Then
Try
Dim prevCallback As Image.GetThumbnailImageAbort = _
New Image.GetThumbnailImageAbort(AddressOf ThumbnailCallback)
Dim prevBmp As _
New Bitmap(mImageList(mImagePosition - 1).ToString())
mPreviousImage = _
prevBmp.GetThumbnailImage(160, 100, _
prevCallback, IntPtr.Zero)
picPreviousImage.Image = mPreviousImage
prevBmp = Nothing
prevCallback = Nothing
mPreviousImage = Nothing
Catch
End Try
Else
picPreviousImage.Image = Nothing
End If
If mImagePosition < mImageList.Count Then
Try
Dim currentCallback = _
New Image.GetThumbnailImageAbort(AddressOf
ThumbnailCallback)
Dim currentBmp As _
New Bitmap(mImageList(mImagePosition).ToString())
mCurrentImage = _
currentBmp.GetThumbnailImage(320, 200, currentCallback,
IntPtr.Zero)
picCurrentImage.Image = mCurrentImage
currentBmp = Nothing
mCurrentImage = Nothing
currentCallback = Nothing
Catch
End Try
End If
If mImagePosition < mImageList.Count - 1 Then
Try
Dim nextCallback As _
New Image.GetThumbnailImageAbort(AddressOf
ThumbnailCallback)
Dim nextBmp As _
New Bitmap(mImageList(mImagePosition + 1).ToString())
mNextImage = _
nextBmp.GetThumbnailImage(160, 100, nextCallback,
IntPtr.Zero)
picNextImage.Image = mNextImage
nextBmp = Nothing
nextCallback = Nothing
mNextImage = Nothing
Catch
End Try
Else
picNextImage.Image = Nothing
End If
GC.Collect()
End Sub
The next bit of code is the code called if the thumbnail image attempt aborts; this is required by the method used to generate the thumbnail.
Public Function ThumbnailCallback() As Boolean
Return False
End Function
The previous image button click event handler is next up; it is used to update the image position within the array list and to recall the SetImages method when the index position is updated:
Private Sub btnPreviousImage_Click(ByVal sender As System.Object, ByVal e
As System.EventArgs) Handles btnPreviousImage.Click
If (mImagePosition >= 0 And Not picPreviousImage.Image Is Nothing)
Then
mImagePosition -= 1
SetImages()
End If
End Sub
The next image button click event handler is next up; it is used to update the image position within the array list and to recall the SetImages method when the index position is updated:
Private Sub btnNextImage_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnNextImage.Click
If (mImagePosition <= (mImageList.Count - 2)) Then
mImagePosition += 1
SetImages()
End If
End Sub
The last method is used to open the current image into the default image viewing application as based upon the machine’s file associations. This method merely calls the process start function and passes it the file path to the current image file.
Private Sub btnOpenImage_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnOpenImage.Click
System.Diagnostics.Process.Start(
_mImageList(mImagePosition).ToString())
End Sub
The Text Form (Form1.vb)
The test form contained with the test application contains a single instance of the Image Viewer user control; the form itself has no specific code leaving nothing to report here. The test application was set as the start up project and, when executed, the test form opens up and displays the user control.
Summary
This solution provides a user control that may be used to provide a user interface that may be helpful if the user of an application will need to browse for image files. The user control provides the controls necessary to browse for a folder containing images along with the means to preview the currently selected, previous, and next images. Through the control, the end user may browse through all of the image files contained in the folder and, if desired, open all the images into a new application.
History
- 3rd June, 2008: Initial version
