TWinControl.DefaultHandler里的CallWindowProc(FDefWndProc)还挺有深意的，TButton对WM_PAINT消息的处理就是靠它来处理的（以前不明白为什么总是要调用inherited，其实就是没有明白TWinControl.DefaultHandler的真正用处，而且还很有用）

我忽然发现：TButton既没有处理WM_PAINT，又没有Paint()或者PaintWindow()，那么它是什么时候被绘制的？

Form1上放2个TButton，然后设置代码：

procedure TForm1.Button1Click(Sender: TObject);
begin
button2.Repaint;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
ShowMessage('good');
end;

在Form1第一次显示时，应该会让这两个Button显示。这两个Button应该会处理WM_PAINT并显示。可是完全找不到相关代码啊。

后来用脑子想了想，TButton是TWinControl，而且是没有图形子控件的，所以它收到WM_PAINT时应该会依次执行：

procedure TButtonControl.WndProc(var Message: TMessage);
begin
case Message.Msg of
WM_LBUTTONDOWN, WM_LBUTTONDBLCLK:
if not (csDesigning in ComponentState) and not Focused then
begin
FClicksDisabled := True;
Windows.SetFocus(Handle);
FClicksDisabled := False;
if not Focused then Exit;
end;
CN_COMMAND:
if FClicksDisabled then Exit;
end;
inherited WndProc(Message);
end;

procedure TWinControl.WndProc(var Message: TMessage);
var
Form: TCustomForm;
begin
case Message.Msg of
WM_SETFOCUS:
begin
Form := GetParentForm(Self);
if (Form <> nil) and not Form.SetFocusedControl(Self) then Exit;
end;
WM_KILLFOCUS:
if csFocusing in ControlState then Exit;
WM_NCHITTEST:
begin
inherited WndProc(Message);
if (Message.Result = HTTRANSPARENT) and (ControlAtPos(ScreenToClient(
SmallPointToPoint(TWMNCHitTest(Message).Pos)), False) <> nil) then
Message.Result := HTCLIENT;
Exit;
end;
WM_MOUSEFIRST..WM_MOUSELAST:
if IsControlMouseMsg(TWMMouse(Message)) then
begin
{ Check HandleAllocated because IsControlMouseMsg might have freed the
window if user code executed something like Parent := nil. }
if (Message.Result = 0) and HandleAllocated then
DefWindowProc(Handle, Message.Msg, Message.wParam, Message.lParam);
Exit;
end;
WM_KEYFIRST..WM_KEYLAST:
if Dragging then Exit;
WM_CANCELMODE:
if (GetCapture = Handle) and (CaptureControl <> nil) and
(CaptureControl.Parent = Self) then
CaptureControl.Perform(WM_CANCELMODE, 0, 0);
end;
inherited WndProc(Message);
end;

procedure TControl.WndProc(var Message: TMessage);
var
Form: TCustomForm;
KeyState: TKeyboardState;
WheelMsg: TCMMouseWheel;
begin
if (csDesigning in ComponentState) then
begin
Form := GetParentForm(Self);
if (Form <> nil) and (Form.Designer <> nil) and
Form.Designer.IsDesignMsg(Self, Message) then Exit
end;
if (Message.Msg >= WM_KEYFIRST) and (Message.Msg <= WM_KEYLAST) then
begin
Form := GetParentForm(Self);
if (Form <> nil) and Form.WantChildKey(Self, Message) then Exit;
end
else if (Message.Msg >= WM_MOUSEFIRST) and (Message.Msg <= WM_MOUSELAST) then
begin
if not (csDoubleClicks in ControlStyle) then
case Message.Msg of
WM_LBUTTONDBLCLK, WM_RBUTTONDBLCLK, WM_MBUTTONDBLCLK:
Dec(Message.Msg, WM_LBUTTONDBLCLK - WM_LBUTTONDOWN);
end;
case Message.Msg of
WM_MOUSEMOVE: Application.HintMouseMessage(Self, Message);
WM_LBUTTONDOWN, WM_LBUTTONDBLCLK:
begin
if FDragMode = dmAutomatic then
begin
BeginAutoDrag;
Exit;
end;
Include(FControlState, csLButtonDown);
end;
WM_LBUTTONUP:
Exclude(FControlState, csLButtonDown);
else
with Mouse do
if WheelPresent and (RegWheelMessage <> 0) and
(Message.Msg = RegWheelMessage) then
begin
GetKeyboardState(KeyState);
with WheelMsg do
begin
Msg := Message.Msg;
ShiftState := KeyboardStateToShiftState(KeyState);
WheelDelta := Message.WParam;
Pos := TSmallPoint(Message.LParam);
end;
MouseWheelHandler(TMessage(WheelMsg));
Exit;
end;
end;
end
else if Message.Msg = CM_VISIBLECHANGED then
with Message do
SendDockNotification(Msg, WParam, LParam);
Dispatch(Message);
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 // 执行这里，会执行TWinControl.DefaultHandler，因为TButton和TButtonControl都没有覆盖DefaultHandler函数
else
PaintHandler(Message);
end
else
begin
DC := GetDC(0);
MemBitmap := CreateCompatibleBitmap(DC, ClientRect.Right, ClientRect.Bottom);
ReleaseDC(0, DC);
MemDC := CreateCompatibleDC(0);
OldBitmap := SelectObject(MemDC, MemBitmap);
try
DC := BeginPaint(Handle, PS);
Perform(WM_ERASEBKGND, MemDC, MemDC);
Message.DC := MemDC;
WMPaint(Message);
Message.DC := 0;
BitBlt(DC, 0, 0, ClientRect.Right, ClientRect.Bottom, MemDC, 0, 0, SRCCOPY);
EndPaint(Handle, PS);
finally
SelectObject(MemDC, OldBitmap);
DeleteDC(MemDC);
DeleteObject(MemBitmap);
end;
end;
end;

所以就真的没办法，只能执行：

procedure TWinControl.DefaultHandler(var Message);
begin
if FHandle <> 0 then
begin
with TMessage(Message) do
begin
if (Msg = WM_CONTEXTMENU) and (Parent <> nil) then
begin
Result := Parent.Perform(Msg, WParam, LParam);
if Result <> 0 then Exit;
end;
case Msg of
WM_CTLCOLORMSGBOX..WM_CTLCOLORSTATIC:
Result := SendMessage(LParam, CN_BASE + Msg, WParam, LParam);
CN_CTLCOLORMSGBOX..CN_CTLCOLORSTATIC:
begin
SetTextColor(WParam, ColorToRGB(FFont.Color));
SetBkColor(WParam, ColorToRGB(FBrush.Color));
Result := FBrush.Handle;
end;
else
if Msg = RM_GetObjectInstance then
Result := Integer(Self)
else
begin
if Msg <> WM_PAINT then // 稍微改造一下，否则程序执行会出错
Result := CallWindowProc(FDefWndProc, FHandle, Msg, WParam, LParam); // 会执行到这里！
end;
end;
if Msg = WM_SETTEXT then
SendDockNotification(Msg, WParam, LParam);
end;
end
else
inherited DefaultHandler(Message);
end;

执行完TWinControl.DefaultHandler后，程序对TButton的WM_PAINT消息就算处理完毕了，会一路返回。

百思不得其解的情况下，忽然灵机一动，TButton是封装了微软的Button，虽然其代码不可见，但也应该是正确处理了WM_PAINT消息的，那么只要把WM_PAINT消息发给这个Button的句柄，不就可以正确显示了？这一切，还多亏了CallWindowProc(FDefWndProc, FHandle, Msg, WParam, LParam);的调用。我想完全屏蔽这句试试效果，结果程序运行出错，因为这样一来许多消息都无法得到处理。那么就加个条件吧：if Msg <> WM_PAINT then 结果发现，两个Button果然灰蒙蒙一片，无法正确显示！但是点击执行事件还是没问题的，而且点击之后，就可以正确显示了，经过研究，又有一番滋味：

procedure TButtonControl.WndProc(var Message: TMessage);
begin
case Message.Msg of
WM_LBUTTONDOWN, WM_LBUTTONDBLCLK:
if not (csDesigning in ComponentState) and not Focused then
begin
FClicksDisabled := True;
Windows.SetFocus(Handle);
FClicksDisabled := False;
if not Focused then Exit;
end;
CN_COMMAND:
if FClicksDisabled then Exit;
end;
inherited WndProc(Message);
end;

procedure TControl.WMLButtonDown(var Message: TWMLButtonDown);
begin
SendCancelMode(Self);
inherited; // 这里，也会调用TWinControl.DefaultHandler，而且传递的消息是WM_LBUTTONDOWN
if csCaptureMouse in ControlStyle then MouseCapture := True;
if csClickEvents in ControlStyle then Include(FControlState, csClicked);
DoMouseDown(Message, mbLeft, []);
end;

procedure TWinControl.DefaultHandler(var Message);
begin
if FHandle <> 0 then
begin
with TMessage(Message) do
begin
if (Msg = WM_CONTEXTMENU) and (Parent <> nil) then
begin
Result := Parent.Perform(Msg, WParam, LParam);
if Result <> 0 then Exit;
end;
case Msg of
WM_CTLCOLORMSGBOX..WM_CTLCOLORSTATIC:
Result := SendMessage(LParam, CN_BASE + Msg, WParam, LParam);
CN_CTLCOLORMSGBOX..CN_CTLCOLORSTATIC:
begin
SetTextColor(WParam, ColorToRGB(FFont.Color));
SetBkColor(WParam, ColorToRGB(FBrush.Color));
Result := FBrush.Handle;
end;
else
if Msg = RM_GetObjectInstance then
Result := Integer(Self)
else
begin
if Msg <> WM_PAINT then // 因为是WM_LBUTTONDOWN，所以照样会传进去
Result := CallWindowProc(FDefWndProc, FHandle, Msg, WParam, LParam);
end;
end;
if Msg = WM_SETTEXT then
SendDockNotification(Msg, WParam, LParam);
end;
end
else
inherited DefaultHandler(Message);
end;

之所以点击以后，会重新出现完整的Button，是因为点击以后，Button的显示情况要变，趁此机会，微软的Button就把它重绘了一遍。真是好复杂呀，好多都是靠猜的。

经测试，TEdit也是同样的效果（没WM_PAINT函数，没Paint，屏蔽WM_PAINT消息后就无法正确显示）。

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理论回答：

应该是调用的Win32标准控件库，绘制代码应该在comctl32.dll中
在TWinControl.CreateWnd中调用CreateParams方法指定基本的外观风格
所以你看TButton有重写CreateParams方法
绘制是交给系统来做的，如果想更改，就要自绘

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新问题：情况1：空窗体放Panel1，不影响

情况2：空窗体放Panel1和Button1，这时候Panel1就受影响了。什么叫做影响？就是Panel1无法得到正确的绘制。

目前不知道为什么。

有那么一点点可能，是拦截了Form1的WM_PAINT，它作为Parent就没有发送消息给子控件。不过TWinControl都是独立的，应该不需要父控件来管理发送呀。

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这个，你看一下Panel和Button的CreateParams下的代码
它们的Style有区别，不知道是不是这里面的原因

另外，你试试将Windows的主题设置“经典”样式