WM_PAINT在微软官方定义中，wParam和lParam都没有使用，所以就被Delphi给重定义了这个消息，还增加了DC（Delphi可任意改写消息的结构，只需要保持前4个字节是消息即可，另外要携带微软定义的所有必要信息就行了）

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

procedure TForm1.Button3Click(Sender: TObject);
begin
  ShowMessage(IntToStr(sizeof(TWMPaint)));
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 // 消息里已经自带DC，用这个绘制就可以了
  begin
    if not (csCustomPaint in ControlState) and (ControlCount = 0) then // 针对Windows自带控件，其包含的子控件数量为0，且没有自绘标记。
      inherited // 走向1，主要是针对Windows系统原生控件，微软定义的基本控件，总不会有错，应该内含BeginPaint了吧？
    else
      PaintHandler(Message); // 走向2，主要是针对Delphi的自绘控件，内含BeginPaint
  end
  else
  begin
    DC := GetDC(0); // 走向3，消息里没有自带DC，那么就现取，内含BeginPaint
    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; // 其实这有这里进行赋值
      WMPaint(Message); // 中间有了DC，赶紧使用
      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;

procedure TWinControl.PaintHandler(var Message: TWMPaint);
var
  I, Clip, SaveIndex: Integer;
  DC: HDC;
  PS: TPaintStruct;
begin
  DC := Message.DC; // 使用消息带来的句柄（没准在别处赋值的），比如子控件挨个向父控件发送重绘请求，这样就不用每次都重新给DC赋值了。但是这里不是引用，出了这个函数，Message.DC的值并没有被改变。fixme 还是没全懂
  if DC = 0 then DC := BeginPaint(Handle, PS); // 如果DC是空的，就要取得它（如果是单独的控件，肯定要执行）
  try
    if FControls = nil then PaintWindow(DC) else
    begin
      SaveIndex := SaveDC(DC);
      Clip := SimpleRegion;
      for I := 0 to FControls.Count - 1 do
        with TControl(FControls[I]) do
          if (Visible or (csDesigning in ComponentState) and
            not (csNoDesignVisible in ControlStyle)) and
            (csOpaque in ControlStyle) then
          begin
            Clip := ExcludeClipRect(DC, Left, Top, Left + Width, Top + Height);
            if Clip = NullRegion then Break;
          end;
      if Clip <> NullRegion then PaintWindow(DC);
      RestoreDC(DC, SaveIndex);
    end;
    PaintControls(DC, nil); // 还要把自己的DC带给子控件
  finally
    if Message.DC = 0 then EndPaint(Handle, PS);
  end;
end;

procedure TGraphicControl.WMPaint(var Message: TWMPaint);
begin
  if Message.DC <> 0 then // 注意，如果DC为零，根本就不自绘（没法自绘）
  begin
    Canvas.Lock;
    try
      Canvas.Handle := Message.DC; // 可见的本质上，DC就是一个控件的Canvas句柄，而且是父控件的DC句柄
      try
        Paint;
      finally
        Canvas.Handle := 0;
      end;
    finally
      Canvas.Unlock;
    end;
  end;
end;

procedure TCustomControl.WMPaint(var Message: TWMPaint);
begin
  Include(FControlState, csCustomPaint);
  inherited;
  Exclude(FControlState, csCustomPaint);
end;

procedure TWinControl.WMPrintClient(var Message: TWMPrintClient);
begin
  with Message do
    if Result <> 1 then
      if ((Flags and PRF_CHECKVISIBLE) = 0) or Visible then
        PaintHandler(TWMPaint(Message))
      else
        inherited
    else
      inherited;
end;