WM_ERASEBKGND官方解释，以及Delphi里所有的使用情况（就是绘制窗口控件背景色，并阻止进一步传递消息）

#define WM_ERASEBKGND                   0x0014

 

Parameters

wParam

A handle to the device context. // 设备上下文的句柄

lParam

This parameter is not used.

Return value

Type: LRESULT

An application should return nonzero if it erases the background; otherwise, it should return zero.

如果它擦除背景，函数返回非零值 ；否则，函数返回零。

Remarks

The DefWindowProc function erases the background by using the class background brush specified by the hbrBackgroundmember of the WNDCLASS structure. If hbrBackground is NULL, the application should process the WM_ERASEBKGNDmessage and erase the background.

DefWindowProc函数通过使用WNDCLASS结构中指定的hbrBackground背景画笔擦除背景。如果hbrBackground是空的，应用程序应该处理WM_ERASEBKGND消息擦除背景。

An application should return nonzero in response to WM_ERASEBKGND if it processes the message and erases the background; this indicates that no further erasing is required. If the application returns zero, the window will remain marked for erasing. (Typically, this indicates that the fErase member of the PAINTSTRUCT structure will be TRUE.)

如果应用程序响应处理WM_ERASEBKGND消息并擦除背景，应返回非零值 ；告诉Windows没有必要再擦除。如果应用程序返回零，Windows仍将标记为删除。（通常情况下，这表明PAINTSTRUCT结构中的fErase为真.)

翻译来自：http://baike.baidu.com/view/9801207.htm

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这个消息在Delphi被重定义：

  TWMEraseBkgnd = packed record
    Msg: Cardinal;
    DC: HDC;
    Unused: Longint;
    Result: Longint;
  end;

它在Controls.pas单元里的相关代码（1处定义，2处使用）：

procedure TWinControl.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  with ThemeServices do
  if ThemesEnabled and Assigned(Parent) and (csParentBackground in FControlStyle) then
    begin
      { Get the parent to draw its background into the control's background. }
      DrawParentBackground(Handle, Message.DC, nil, False);
    end
    else
    begin
      { Only erase background if we're not doublebuffering or painting to memory. }
      if not FDoubleBuffered or
         (TMessage(Message).wParam = TMessage(Message).lParam) then
        FillRect(Message.DC, ClientRect, FBrush.Handle);
    end;

  Message.Result := 1; // 相当于说，WM_ERASEBKGND的传递到此为止，无论如何都不要继续传递了 end;
procedure TWinControl.WMPaint(var Message: TWMPaint);
var
  DC, MemDC: HDC;
  MemBitmap, OldBitmap: HBITMAP;
  PS: TPaintStruct;
begin
  if not FDoubleBuffered or (Message.DC <> 0) then
  begin
    if not (csCustomPaint in ControlState) and (ControlCount = 0) then
      inherited
    else
      PaintHandler(Message);
  end
  else
  begin
    DC := GetDC(0); // 整个屏幕的DC，借用一下而已
    MemBitmap := CreateCompatibleBitmap(DC, ClientRect.Right, ClientRect.Bottom); // 图像不宜过大，所以设置成最大就是屏幕大小
    ReleaseDC(0, DC); // 还掉了
    MemDC := CreateCompatibleDC(0);
    OldBitmap := SelectObject(MemDC, MemBitmap);
    try
      DC := BeginPaint(Handle, PS); // 重新取得当前控件的DC
      Perform(WM_ERASEBKGND, MemDC, MemDC); // 结合上面的TWinControl.WMEraseBkgnd，可以观察到，要得就是这种效果，即双缓冲第一次绘制时候，需要删除背景
      Message.DC := MemDC; // 构建一个消息，把MemDC传入，当前控件和子控件都在MemDC上画，不在原先的DC上作画了
      WMPaint(Message); // 递归调用，会引发DC<>0的情况，从而正常绘制所有子控件（绘制子控件不需要双缓冲了），否则根本据不会进入绘制子控件的流程
      Message.DC := 0;  // 子控件绘制完毕，不管后续消息流动到何处，不要在消息包含的DC上绘制东西了
      BitBlt(DC, 0, 0, ClientRect.Right, ClientRect.Bottom, MemDC, 0, 0, SRCCOPY); // 把MemDC上的内容拷贝到DC上
      EndPaint(Handle, PS);
    finally
      SelectObject(MemDC, OldBitmap); // 还掉
      DeleteDC(MemDC); // 还掉
      DeleteObject(MemBitmap); // 还掉 end;
  end;
end;

还可以打印控件到其它DC设备上：

// Draws the windowed control to a device context.
procedure TWinControl.PaintTo(DC: HDC; X, Y: Integer);
var
  I, EdgeFlags, BorderFlags, SaveIndex: Integer;
  R: TRect;
begin
  Include(FControlState, csPaintCopy);
  SaveIndex := SaveDC(DC);
  MoveWindowOrg(DC, X, Y);
  IntersectClipRect(DC, 0, 0, Width, Height);
  BorderFlags := 0;
  EdgeFlags := 0;
  if GetWindowLong(Handle, GWL_EXSTYLE) and WS_EX_CLIENTEDGE <> 0 then
  begin
    EdgeFlags := EDGE_SUNKEN;
    BorderFlags := BF_RECT or BF_ADJUST
  end else
  if GetWindowLong(Handle, GWL_STYLE) and WS_BORDER <> 0 then
  begin
    EdgeFlags := BDR_OUTER;
    BorderFlags := BF_RECT or BF_ADJUST or BF_MONO;
  end;
  if BorderFlags <> 0 then
  begin
    SetRect(R, 0, 0, Width, Height);
    DrawEdge(DC, R, EdgeFlags, BorderFlags);
    MoveWindowOrg(DC, R.Left, R.Top);
    IntersectClipRect(DC, 0, 0, R.Right - R.Left, R.Bottom - R.Top);
  end;
  Perform(WM_ERASEBKGND, DC, 0);
  Perform(WM_PAINT, DC, 0);
  if FWinControls <> nil then
    for I := 0 to FWinControls.Count - 1 do
      with TWinControl(FWinControls[I]) do
        if Visible then PaintTo(DC, Left, Top);
  RestoreDC(DC, SaveIndex);
  Exclude(FControlState, csPaintCopy);
end;

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Controls.pas里还有一处定义，但是它只有在TSpeedButton里被使用：

procedure PerformEraseBackground(Control: TControl; DC: HDC);
var
  LastOrigin: TPoint;
begin
  GetWindowOrgEx(DC, LastOrigin);
  SetWindowOrgEx(DC, LastOrigin.X + Control.Left, LastOrigin.Y + Control.Top, nil);
  Control.Parent.Perform(WM_ERASEBKGND, Integer(DC), Integer(DC)); // TSpeedButton自己不是Windows窗口，只能出此下策
  SetWindowOrgEx(DC, LastOrigin.X, LastOrigin.Y, nil);
end;

procedure TSpeedButton.Paint;
const
  DownStyles: array[Boolean] of Integer = (BDR_RAISEDINNER, BDR_SUNKENOUTER);
  FillStyles: array[Boolean] of Integer = (BF_MIDDLE, 0);
var
  PaintRect: TRect;
  DrawFlags: Integer;
  Offset: TPoint;
  Button: TThemedButton;
  ToolButton: TThemedToolBar;
  Details: TThemedElementDetails;
begin
  if not Enabled then
  begin
    FState := bsDisabled;
    FDragging := False;
  end
  else if FState = bsDisabled then
    if FDown and (GroupIndex <> 0) then
      FState := bsExclusive
    else
      FState := bsUp;
  Canvas.Font := Self.Font;

  if ThemeServices.ThemesEnabled then
  begin
    PerformEraseBackground(Self, Canvas.Handle);
    // 其它代码
  end;
end;

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Forms.pas单元里的定义与使用：

procedure TCustomForm.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  if not IsIconic(Handle) then inherited else
  begin
    Message.Msg := WM_ICONERASEBKGND; // 当窗口处于最小化状态的时候，改变消息值
    DefaultHandler(Message); // 当窗口是MDI时，还应该改变当前窗口的图标，否则相当于不执行 end;
end;

procedure TCustomForm.ClientWndProc(var Message: TMessage);

  procedure Default;
  begin
    with Message do
      Result := CallWindowProc(FDefClientProc, ClientHandle, Msg, wParam, lParam);
  end;

  function MaximizedChildren: Boolean;
  var
    I: Integer;
  begin
    for I := 0 to MDIChildCount - 1 do
      if MDIChildren[I].WindowState = wsMaximized then
      begin
        Result := True;
        Exit;
      end;
    Result := False;
  end;

var
  DC: HDC;
  PS: TPaintStruct;
  R: TRect;
begin
  with Message do
    case Msg of
      WM_NCHITTEST:
        begin
          Default;
          if Result = HTCLIENT then Result := HTTRANSPARENT;
        end;
      WM_ERASEBKGND: begin
          FillRect(TWMEraseBkGnd(Message).DC, ClientRect, Brush.Handle);
          { Erase the background at the location of an MDI client window }
          if (FormStyle = fsMDIForm) and (FClientHandle <> 0) then
          begin
            Windows.GetClientRect(FClientHandle, R);
            FillRect(TWMEraseBkGnd(Message).DC, R, Brush.Handle);
          end;
          Result := 1;
        end;
      $3F://!
        begin
          Default;
          if FFormStyle = fsMDIForm then
            ShowMDIClientEdge(FClientHandle, (MDIChildCount = 0) or
              not MaximizedChildren);
        end;
      WM_PAINT:
        begin
          DC := TWMPaint(Message).DC;
          if DC = 0 then
            TWMPaint(Message).DC := BeginPaint(ClientHandle, PS);
          try
            if DC = 0 then
            begin
              GetWindowRect(FClientHandle, R);
              R.TopLeft := ScreenToClient(R.TopLeft);
              MoveWindowOrg(TWMPaint(Message).DC, -R.Left, -R.Top);
            end;
            PaintHandler(TWMPaint(Message));
          finally
            if DC = 0 then
              EndPaint(ClientHandle, PS);
          end;
        end;
    else
      Default;
    end;
end;

进一步解释TCustomForm.WMEraseBkgnd转发来的消息：

procedure TCustomForm.DefaultHandler(var Message);
begin
  if ClientHandle <> 0 then
    with TMessage(Message) do
      if Msg = WM_SIZE then
        Result := DefWindowProc(Handle, Msg, wParam, lParam) else
        Result := DefFrameProc(Handle, ClientHandle, Msg, wParam, lParam)
  else
    inherited DefaultHandler(Message)
end;

// 处理图标消息 message WM_ICONERASEBKGND
procedure TCustomForm.WMIconEraseBkgnd(var Message: TWMIconEraseBkgnd); 
begin
  if FormStyle = fsMDIChild then
  if (FormStyle = fsMDIChild) and not (csDesigning in ComponentState) then
    FillRect(Message.DC, ClientRect, Application.MainForm.Brush.Handle)
  else inherited;
end;

还有TApplication对这个消息的使用：

procedure TApplication.WndProc(var Message: TMessage);
var
  procedure Default;
  begin
    with Message do
      Result := DefWindowProc(FHandle, Msg, WParam, LParam);
  end;

begin
  try
    Message.Result := 0;
    for I := 0 to FWindowHooks.Count - 1 do
      if TWindowHook(FWindowHooks[I]^)(Message) then Exit;
    CheckIniChange(Message);
    with Message do
      case Msg of
        WM_SYSCOMMAND:
          case WParam and $FFF0 of
            SC_MINIMIZE: Minimize;
            SC_RESTORE: Restore;
          else
            Default;
          end;
        WM_CLOSE:
          if MainForm <> nil then MainForm.Close;
        WM_PAINT:
          if IsIconic(FHandle) then DrawAppIcon else Default;
        WM_ERASEBKGND: // 改变消息，然后转发 begin
            Message.Msg := WM_ICONERASEBKGND;
            Default;
          end;
        WM_NULL:
          CheckSynchronize;
      else
        Default;
      end;
  except
    HandleException(Self);
  end;
end;

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再看StdCtrls.pas单元里的定义与使用：

procedure TCustomComboBox.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  if Style = csSimple then
  begin
    FillRect(Message.DC, ClientRect, Parent.Brush.Handle);
    Message.Result := 1;
  end
  else
    DefaultHandler(Message);
end;

procedure TButtonControl.WMEraseBkGnd(var Message: TWMEraseBkGnd);
begin
  { Under theme services the background is drawn in CN_CTLCOLORSTATIC. }
  if ThemeServices.ThemesEnabled then
    Message.Result := 1
  else
    inherited;
end;

procedure TButton.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  if ThemeServices.ThemesEnabled then
    Message.Result := 1
  else
    DefaultHandler(Message);
end;

procedure TScrollBar.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  DefaultHandler(Message);
end;

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还有ExtCtrls.pas单元里的：

procedure TCustomControlBar.WMEraseBkgnd(var Message: TWmEraseBkgnd);
var
  R: TRect;
  I, J: Integer;
  Save: Boolean;
begin
  if Message.DC <> 0 then
    Canvas.Handle := Message.DC;
  if Picture.Graphic <> nil then
  begin

    try
      R := ClientRect;
      Save := FDrawing;
      FDrawing := True;
      try
        { Tile image across client area }
        for I := 0 to (R.Right - R.Left) div Picture.Width do
          for J := 0 to (R.Bottom - R.Top) div Picture.Height do
            Canvas.Draw(I * Picture.Width, J * Picture.Height, Picture.Graphic);
      finally
        FDrawing := Save;
      end
    finally
      if Message.DC <> 0 then
        Canvas.Handle := 0;
      Message.Result := 1;
    end;
  end
  else
  begin
    Canvas.Brush.Color := Color;
    Canvas.Brush.Style := bsSolid;
    Canvas.FillRect(ClientRect);
    inherited;
  end;
end;

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还有dbcgrids.pas和DbCtrls.pas单元里的：

procedure TDBCtrlPanel.WMEraseBkgnd(var Message: TMessage);
begin
  Message.Result := 1;
end;

procedure TDBCtrlGrid.WMEraseBkgnd(var Message: TMessage);
begin
  Message.Result := 1;
end;procedure TDBMemo.WMPaint(var Message: TWMPaint);
var
  S: string;
begin
  if not (csPaintCopy in ControlState) then inherited else
  begin
    if FDataLink.Field <> nil then
      if FDataLink.Field.IsBlob then
      begin
        if FAutoDisplay then
          S := AdjustLineBreaks(FDataLink.Field.AsString) else
          S := Format('(%s)', [FDataLink.Field.DisplayLabel]);
      end else
        S := FDataLink.Field.DisplayText;
    SendMessage(FPaintControl.Handle, WM_SETTEXT, 0, Integer(PChar(S)));
    SendMessage(FPaintControl.Handle, WM_ERASEBKGND, Message.DC, 0);
    SendMessage(FPaintControl.Handle, WM_PAINT, Message.DC, 0);
  end;
end;

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还有ShadowWnd.pas单元里的：

procedure TShadowWindow.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  Message.Result := 0;
end;

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还有Tabs.pas单元里的：

procedure TTabSet.Paint;
var
  TabStart, LastTabPos: Integer;
  TabPos: TTabPos;
  Tab: Integer;
  Leading: TEdgeType;
  Trailing: TEdgeType;
  isFirst, isLast, isSelected, isPrevSelected: Boolean;
  R: TRect;
begin
  if not HandleAllocated then Exit;

  MemBitmap.Canvas.Font := Self.Canvas.Font;

  { draw background of tab area }
  with MemBitmap.Canvas do
  begin
    Brush.Bitmap := BrushBitmap;
    if ThemeServices.ThemesEnabled and ParentBackground then
      Perform(WM_ERASEBKGND, MemBitmap.Canvas.Handle, 0)
    else
      FillRect(Rect(0, 0, MemBitmap.Width, MemBitmap.Height));

  end;
end;

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还有OleCtrls.pas单元里的：

procedure TOleControl.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  if FMiscStatus and OLEMISC_INVISIBLEATRUNTIME = 0 then
    DefaultHandler(Message) else
    inherited;
end;

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还有ComCtrls.pas单元里的5处定义和2处使用：

procedure TPageControl.WMEraseBkGnd(var Message: TWMEraseBkGnd);
begin
  if (not ThemeServices.ThemesEnabled) or (not ParentBackground) then
    inherited
  else
    Message.Result := 1;
end;

procedure TCustomStatusBar.WMEraseBkGnd(var Message: TWMEraseBkGnd);
var
  Details: TThemedElementDetails;
begin
  if ThemeServices.ThemesEnabled then
  begin
    Details := ThemeServices.GetElementDetails(tsStatusRoot);
    ThemeServices.DrawElement(Message.DC, Details, ClientRect, nil);
    Message.Result := 1;
  end
  else
    inherited;
end;

procedure TTrackBar.WMEraseBkGnd(var Message: TWMEraseBkGnd);
var
  R: TRect;
begin
  if ThemeServices.ThemesEnabled then
  begin
    R := ClientRect;
    if Focused and ((Perform(WM_QUERYUISTATE, 0, 0) and UISF_HIDEFOCUS) = 0) then
      InflateRect(R, -1, -1);
    ThemeServices.DrawParentBackground(Handle, Message.DC, nil, False, @R);
    Message.Result := 1;
  end
  else
    inherited;
end;

procedure TToolBar.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  if not Transparent then
    inherited else
    DefaultHandler(Message);
end;

procedure TCoolBar.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
  if not ThemeServices.ThemesEnabled and (IsBackgroundDirty or (IsAutoSized and (Bands.Count = 0))) then
    inherited;
  DefaultHandler(Message);
end;

procedure TCustomListView.WndProc(var Message: TMessage);
begin
  if not (csDesigning in ComponentState) and ((Message.Msg = WM_LBUTTONDOWN) or
    (Message.Msg = WM_LBUTTONDBLCLK)) and not Dragging and (DragMode = dmAutomatic) then
  begin
    if not IsControlMouseMsg(TWMMouse(Message)) then
    begin
      ControlState := ControlState + [csLButtonDown];
      Dispatch(Message);
    end;
  end
  else if not (((Message.Msg = WM_PAINT) or (Message.Msg = WM_ERASEBKGND)) and
    Items.FNoRedraw) then
    inherited WndProc(Message);
end;

procedure TCoolBar.PaintWindow(DC: HDC);
begin
  Perform(WM_ERASEBKGND, DC, 0);
  inherited;
end;

总结：其实做的事情不多，子类要么就是发WM_ERASEBKGND消息给父类，要么就是改变它的消息值（使它变成另外一个消息），而父控件做的事情也很少，最主要给控件上背景色，还有就是阻止它进一步传递。

做实验：

1. 窗体上放一个覆盖了WM_ERASEBKGND消息的控件，比如TButton

2. 放一个不覆盖WM_ERASEBKGND消息的空间，比如TPanel，看看到底会出什么事情

备注：其实这种消息很难调试，只能全部记录在日志里。