Quickly clear the TreeView control

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Quickly clear the TreeView control with reverse deletion

While VB's TreeView control offers the Clear method to remove the nodes
from a TreeView, you've probably noticed that this command takes forever
to execute. As a result, you may have searched for an alternative
method. Believe it or not, one way to ramp up the TreeView's speed when
clearing nodes is to delete them one by one in reverse order. To do so,
you create a loop that steps backwards through the nodes, then issue the
Remove method. The following code shows you how to implement this
technique:

Private Sub TrvwClear()
Dim x As Integer
With TreeView1.Nodes
For x = .Count To 1 Step -1
.Remove x
Next x
End With
End Sub

With this code in place, the TreeView clears much faster than it does
with the Clear method.

Of course, if you want to use the API, you can improve this performance
even further. We'll cover this improvement in the second part of this
tip.

________________________________________________________________________

In the last tip, we showed you how to quickly clear the TreeView control of nodes by looping through the nodes in reverse
order and then removing them. However, by using a simple API function, you can improve the performance of this
technique.

In the previous tip, we used the following code:

Private Sub TrvwClear()
Dim x As Integer
With TreeView1.Nodes
For x = .Count To 1 Step -1
.Remove x
Next x
End With
End Sub

Unfortunately, as VB loops through the TreeView's nodes and removes them, it also repaints the control to reflect the
remaining nodes. This behavior slows down the process considerably.

What's needed is a way to prevent the TreeView from repainting itself as it removes the nodes. Fortunately, the Windows
API provides a great way to do just that. The SendMessage() function supports the WM_SETREDRAW message. With
this message, your code can dictate whether a window should redraw (repaint) itself. Pass in True (or 1 in API parlance) to
allow the window to repaint, or False (0) to prevent it. With the addition of the API to our code, the clear procedure now
looks like this:

Private Const WM_SETREDRAW = &HB
Private Declare Function SendMessage Lib "user32" Alias _
"SendMessageA" (ByVal hwnd As Long, ByVal wMsg As _
Long, ByVal wParam As Long, lParam As Any) As Long

Private Sub TrvwClear()
Dim x As Integer
With TreeView1
SendMessage .hwnd, WM_SETREDRAW, 0, 0
For x = .Nodes.Count To 1 Step -1
.Nodes.Remove x
Next x
SendMessage .hwnd, WM_SETREDRAW, 1, 0
End With
End Sub

This combination of the API and reverse deletion provides the quickest way to clear the TreeView control.

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