#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 = ) then
inherited
else
PaintHandler(Message);
end
else
begin
DC := GetDC(); // 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(, DC); // 还掉了
MemDC := CreateCompatibleDC();
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 := ; // 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, , , ClientRect.Right, ClientRect.Bottom, MemDC, , , 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, , , Width, Height);
BorderFlags := ;
EdgeFlags := ;
if GetWindowLong(Handle, GWL_EXSTYLE) and WS_EX_CLIENTEDGE <> then
begin
EdgeFlags := EDGE_SUNKEN;
BorderFlags := BF_RECT or BF_ADJUST
end else
if GetWindowLong(Handle, GWL_STYLE) and WS_BORDER <> then
begin
EdgeFlags := BDR_OUTER;
BorderFlags := BF_RECT or BF_ADJUST or BF_MONO;
end;
if BorderFlags <> then
begin
SetRect(R, , , Width, Height);
DrawEdge(DC, R, EdgeFlags, BorderFlags);
MoveWindowOrg(DC, R.Left, R.Top);
IntersectClipRect(DC, , , R.Right - R.Left, R.Bottom - R.Top);
end;
Perform(WM_ERASEBKGND, DC, );
Perform(WM_PAINT, DC, );
if FWinControls <> nil then
for I := to FWinControls.Count - do
with TWinControl(FWinControls[I]) do
if Visible then PaintTo(DC, Left, Top);
RestoreDC(DC, SaveIndex);
Exclude(FControlState, csPaintCopy);
end;
------------------------------------------------------------------------------------------------------
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, );
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 <> ) 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;
------------------------------------------------------------------------------------------------------
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 := to MDIChildCount - 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 <> ) then
begin
Windows.GetClientRect(FClientHandle, R);
FillRect(TWMEraseBkGnd(Message).DC, R, Brush.Handle);
end;
Result := ;
end;
$3F://!
begin
Default;
if FFormStyle = fsMDIForm then
ShowMDIClientEdge(FClientHandle, (MDIChildCount = ) or
not MaximizedChildren);
end;
WM_PAINT:
begin
DC := TWMPaint(Message).DC;
if DC = then
TWMPaint(Message).DC := BeginPaint(ClientHandle, PS);
try
if DC = 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 = 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 <> 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 := ;
for I := to FWindowHooks.Count - 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;
------------------------------------------------------------------------------------------------------
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 := ;
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 :=
else
inherited;
end; procedure TButton.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
if ThemeServices.ThemesEnabled then
Message.Result :=
else
DefaultHandler(Message);
end; procedure TScrollBar.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
DefaultHandler(Message);
end;
------------------------------------------------------------------------------------------------------
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 <> 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 := to (R.Right - R.Left) div Picture.Width do
for J := 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 <> then
Canvas.Handle := ;
Message.Result := ;
end;
end
else
begin
Canvas.Brush.Color := Color;
Canvas.Brush.Style := bsSolid;
Canvas.FillRect(ClientRect);
inherited;
end;
end;
------------------------------------------------------------------------------------------------------
还有dbcgrids.pas和DbCtrls.pas单元里的：
procedure TDBCtrlPanel.WMEraseBkgnd(var Message: TMessage);
begin
Message.Result := ;
end; procedure TDBCtrlGrid.WMEraseBkgnd(var Message: TMessage);
begin
Message.Result := ;
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, , Integer(PChar(S)));
SendMessage(FPaintControl.Handle, WM_ERASEBKGND, Message.DC, );
SendMessage(FPaintControl.Handle, WM_PAINT, Message.DC, );
end;
end;
------------------------------------------------------------------------------------------------------
还有ShadowWnd.pas单元里的：
procedure TShadowWindow.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
Message.Result := ;
end;
------------------------------------------------------------------------------------------------------
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, )
else
FillRect(Rect(, , MemBitmap.Width, MemBitmap.Height)); end;
end;
------------------------------------------------------------------------------------------------------
还有OleCtrls.pas单元里的：
procedure TOleControl.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
if FMiscStatus and OLEMISC_INVISIBLEATRUNTIME = then
DefaultHandler(Message) else
inherited;
end;
------------------------------------------------------------------------------------------------------
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 := ;
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 := ;
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, , ) and UISF_HIDEFOCUS) = ) then
InflateRect(R, -, -);
ThemeServices.DrawParentBackground(Handle, Message.DC, nil, False, @R);
Message.Result := ;
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 = ))) 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, );
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。