 |
||||
 | ||||
2. Drag and Drop operation-simple example
Let's look at some examples, starting with simple drag and drop operation. Create a Visual Basic.net windows application and design a form with control & Drag Drop event procedure as follows
To enable drag & drop for text
1. Place two textboxes and set Allowdrop property of a second textbox to true.
2. Add the following code
Private MouseIsDown As Boolean = False ‘variable declaration
Private Sub TextBox1_MouseDown(ByVal sender As Object, ByVal e As _
System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseDown
'Set a flag to show that the mouse is down.
MouseIsDown = True
End Sub
Private Sub TextBox1_MouseMove(ByVal sender As Object, ByVal e As _
System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseMove
If MouseIsDown Then
' Initiate dragging.
TextBox1.DoDragDrop(TextBox1.Text,DragDropEffects.Copy)
End If
MouseIsDown = False
End Sub
Private Sub TextBox2_DragEnter(ByVal sender As Object, ByVal e As _
System.Windows.Forms.DragEventArgs) Handles TextBox2.DragEnter
'Check the format of the data being dropped.
If (e.Data.GetDataPresent(DataFormats.Text)) Then
' Display the copy cursor.
e.Effect = DragDropEffects.Copy
Else
' Display the no-drop cursor.
e.Effect = DragDropEffects.None
End If
End Sub
Private Sub TextBox2_DragDrop(ByVal sender As Object, ByVal e As _
System.Windows.Forms.DragEventArgs) Handles TextBox2.DragDrop
'Paste the text.
TextBox2.Text = e.Data.GetData(DataFormats.Text)
End Sub
In the above example, the MouseDown event is used to set a flag showing that the mouse is down, and then the DoDragDrop method is called in the MouseMove event. Although you could initiate the drag in the MouseDown event, doing so would create undesirable behavior: Every time a user clicks the control, the no-drag cursor would be displayed.
The DoDragDrop method takes two parameters:
Data parameter, which in this case takes the Text property of the TextBox
allowedEffects parameter, which in this case only allows copying
Also in the MouseMove event the MouseIsDown flag is set to False. Although unnecessary in this example, if you had multiple controls that support dragging you could get a run-time exception.
In the DragEnter event, the GetDataPresent method checks the format of the data being dragged. In this case it is text, so the Effect property is set to Copy, which in turn displays the copy cursor.
In the DragDrop event, the GetData method is used to retrieve the text from the DataObject and assign it to the target TextBox.
The next section provides an example of dragging a different type of data and providing support for both cutting and copying.
To enable drag and drop for a picture
- Add two picturebox control to a form
- Add the following code.
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
System.EventArgs) Handles MyBase.Load
' Enable dropping.
PictureBox2.AllowDrop = True
End Sub
Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As _
System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown
If Not PictureBox1.Image Is Nothing Then
' Set a flag to show that the mouse is down.
m_MouseIsDown = True
End If
End Sub
Private Sub PictureBox1_MouseMove(ByVal sender As Object, ByVal e As _
System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseMove
If m_MouseIsDown Then
' Initiate dragging and allow either copy or move.
PictureBox1.DoDragDrop(PictureBox1.Image, DragDropEffects.Copy Or _
DragDropEffects.Move)
End If
m_MouseIsDown = False
End Sub
Private Sub PictureBox2_DragEnter(ByVal sender As Object, ByVal e As _
System.Windows.Forms.DragEventArgs) Handles PictureBox2.DragEnter
If e.Data.GetDataPresent(DataFormats.Bitmap) Then
'Check for the CTRL key.
If e.KeyState = 9 Then
e.Effect = DragDropEffects.Copy
Else
e.Effect = DragDropEffects.Move
End If
Else
e.Effect = DragDropEffects.None
End if
End sub
Private Sub PictureBox2_DragDrop(ByVal sender As Object, ByVal e As _
System.Windows.Forms.DragEventArgs) Handles PictureBox2.DragDrop
' Assign the image to the PictureBox.
PictureBox2.Image = e.Data.GetData(DataFormats.Bitmap)
' If the CTRL key is not pressed, delete the source picture.
If Not e.KeyState = 8 Then
PictureBox1.Image = Nothing
End If
End Sub
In the above example, note that the AllowDrop property for the second PictureBox control is set in the Form1_Load event. This is necessary because the AllowDrop property is not available at design time.
In the MouseDown event, the code first checks to make sure that there is an image assigned to the PictureBox; otherwise, after you moved the picture, subsequent clicks would raise an exception.
Also note that in both the DragEnter and DragDrop events the code checks to see if the CTRL key is pressed to determine whether to copy or move the picture. Why are the values different? In the DragEnter event, the left mouse button is down, resulting in a value of 8 for the CTRL key plus 1 for the left mouse button. For a list of KeyState enumerations, see DragEventArgs.KeyState Property.
Both examples so far have dealt with dragging between two controls on the same form; they would also work for dragging items between controls on different forms within an application. The next example demonstrates accepting items dropped from another application — in this case, files that are dragged from Windows Explorer.
Screen shots for the above example is as shown below
Fig1: Screenshot for example1 control before being dragged to a target
Fig2: Screenshot for example1 control after being dragged to a target
- Downloadable zip file
|
|