Introduction
In this article, I will describe implementing native WinForms TreeView and ListView descendants that are flicker-free. The solution will be described in several iterations which are connected to actual development in time. All this is created for and used in my freeware file manager Nomad.NET.
The Problem
If you ever use default WinForms TreeView or ListView controls, you already notice massive flickering when control size changes (for example, when you change form size with mouse). In this article, I will try to find workarounds for this behavior and eliminate flickering by using double buffering. When I first noticed this problem, I did some Googling and didn't find any solutions, so I have invented several solutions by myself.
First Attempt with TreeView
Let me first mention why this problem occurs. In general, control painting cycle consists of two phases: erasing and painting. First Windows sends a message to erase the background, then to paint. When the control receives erase message, if fill control surface with solid color, then control draws content on the surface. When user changes form size, Windows invokes many painting cycles, and in every cycle erase and painting occurs, but because painting occurs with delay after erasing, user will see noticeable flickering.
So my first idea was to eat erasing messages and exclude treeview labels from painting cycle by using clipping region.
To do this, I overrode default WndProc method, and catch WM_ERASEBKGND and WM_PAINT messages. This solution worked well for me for about a year, but some parts still flicker (tree lines, buttons and icons), and sometimes (rarely) the selected node was painted with glitches.
protected override void WndProc(ref Message m)
{
switch (m.Msg)
{
case WM_ERASEBKGND:
m.Result = (IntPtr)1;
return;
case WM_PAINT:
Region BkgndRegion = null;
IntPtr RgnHandle = CreateRectRgn(0, 0, 0, 0);
try
{
RegionResult Result = GetUpdateRgn(Handle, RgnHandle, false);
if ((Result == RegionResult.SIMPLEREGION) ||
(Result == RegionResult.COMPLEXREGION))
BkgndRegion = Region.FromHrgn(RgnHandle);
}
finally
{
DeleteObject(RgnHandle);
}
using (BkgndRegion)
{
if ((BkgndRegion != null) && BkgndRegion.IsVisible(ClientRectangle))
{
int I = 0;
TreeNode Node = TopNode;
while ((Node != null) && (I++ <= VisibleCount))
{
BkgndRegion.Exclude(Node.Bounds);
Node = Node.NextVisibleNode;
}
if (BkgndRegion.IsVisible(ClientRectangle))
{
using (Brush BackBrush = new SolidBrush(BackColor))
using (Graphics Canvas = Graphics.FromHwnd(Handle))
Canvas.FillRegion(BackBrush, BkgndRegion);
}
}
}
break;
}
base.WndProc(ref m);
}
Second Attempt
When I started moving my project to Windows Vista, I found that Microsoft implemented native double-buffering, and my task was only to enable such functionality. To do this, I would send TVM_SETEXTENDEDSTYLE message with TVS_EX_DOUBLEBUFFER style after control windows handle creation. This worked very well, but only under Windows Vista.
public DbTreeView()
{
SetStyle(ControlStyles.OptimizedDoubleBuffer |
ControlStyles.AllPaintingInWmPaint, true);
}
private void UpdateExtendedStyles()
{
int Style = 0;
if (DoubleBuffered)
Style |= TVS_EX_DOUBLEBUFFER;
if (Style != 0)
SendMessage(Handle, TVM_SETEXTENDEDSTYLE,
(IntPtr)TVS_EX_DOUBLEBUFFER, (IntPtr)Style);
}
protected override OnHandleCreated(EventArgs e)
{
base.OnHandleCreated(e);
UpdateExtendedStyles();
}
Third Attempt
After implementing native double-buffering under Vista, I started thinking about older systems, especially Windows XP, because it has a large user base. So I have returned to my first attempt and started investigation. First I tried to substitute owner draw hdc by overriding NM_CUSTOMDRAW message, but without luck (it seems than Windows does not detect handle substitution and ignores it).
Then I remembered the WM_PRINT message. This message is used to draw a control on the user supplied surface and usually used for printing. But why not use this message to draw control on bitmap, and then draw this bitmap in the paint cycle?
The first version uses background bitmap for offscreen painting, and it works! After proving the concept, I started the optimization process and bitmap was replaced with .NET BufferedGraphics class. This class is specially created for implementing double-buffering in controls.
protected override void OnSizeChanged(EventArgs e)
{
base.OnSizeChanged(e);
if (bg != null)
{
bg.Dispose();
bg = null;
}
}
protected override void WndProc(ref Message m)
{
switch (m.Msg)
{
case WM_ERASEBKGND:
if (!DoubleBuffered)
{
m.Result = (IntPtr)1;
return;
}
break;
case WM_PAINT:
if (!DoubleBuffered)
{
PAINTSTRUCT PS = new PAINTSTRUCT();
IntPtr hdc = BeginPaint(Handle, ref PS);
try
{
if (bg == null)
bg = BufferedGraphicsManager.Current.Allocate
(hdc, ClientRectangle);
IntPtr bgHdc = bg.Graphics.GetHdc();
SendMessage(Handle, WM_PRINT, bgHdc, (IntPtr)PRF_CLIENT);
bg.Graphics.ReleaseHdc(bgHdc);
bg.Render(hdc);
}
finally
{
EndPaint(Handle, ref PS);
}
return;
}
break;
}
base.WndProc(ref m);
}
Final Solution
Now I have a working solution, but I want to make it more elegant and with less native interoperability. After additional investigation, I have decided to use internal Control logic to do this. Control class has UserPaint style and this style means that all painting occurs in user code, and underlying control painting is not invoked at all. So I have set UserPaint style, as well as OptimizedDoubleBuffer and AllPaintInWmPaint (these styles enable internal control double buffering support). After setting these styles, all I need is to override OnPaint control and simply paint control using WM_PRINT message. Sounds good, but does not work, because control with UserPaint style intercepts not only default painting but also default printing. So instead of sending WM_PRINT message, I have created dummy WM_PRINTCLIENT message and dispatch it directly to default window procedure.
public DbTreeView()
{
SetStyle(ControlStyles.OptimizedDoubleBuffer |
ControlStyles.AllPaintingInWmPaint, true);
if (Environment.OSVersion.Version.Major < 6)
SetStyle(ControlStyles.UserPaint, true);
}
protected override void OnPaint(PaintEventArgs e)
{
if (GetStyle(ControlStyles.UserPaint))
{
Message m = new Message();
m.HWnd = Handle;
m.Msg = WM_PRINTCLIENT;
m.WParam = e.Graphics.GetHdc();
m.LParam = (IntPtr)PRF_CLIENT;
DefWndProc(ref m);
e.Graphics.ReleaseHdc(m.WParam);
}
base.OnPaint(e);
}
ListView
The same approach is valid for ListView, with one exception, native double buffering is available for ListView starting from Windows XP. And even more support for native double buffering exists in .NET 2 ListView control, all I need is to enable double-buffering.
public DbListView()
{
SetStyle(ControlStyles.OptimizedDoubleBuffer |
ControlStyles.AllPaintingInWmPaint, true);
}
For older systems, I have used exactly the same solution as for treeview.
Known Issues
- Due to a bug in CommCtrl
TreeView control on Windows 2000 and earlier, node lines and buttons are drawn on a black surface if default window color is used. To avoid this bug, we simply explicitly set the background color after control creation.
- Double-buffered
ListView on Windows 2000 and earlier still have some flickering in Details mode. This occurs because listview header is another control without double buffering. It is not an issue for me, so I leave the header untouched. This issue can be fixed by subclass default listview header, and implementing the kind of solution described in "Third attempt" section.
History
- 15th June, 2009: Initial post
