TControl.GetDeviceContext会给图形控件建立新的坐标原点和建立新的剪裁区域

这是取得DC句柄的其中一种方法（会重定义原点和建立新的剪裁区）：

function TControl.GetDeviceContext(var WindowHandle: HWnd): HDC;
begin
if Parent = nil then
raise EInvalidOperation.CreateFmt(SParentRequired, [Name]);
Result := Parent.GetDeviceContext(WindowHandle);
SetViewportOrgEx(Result, Left, Top, nil);       // 注意，根据当前图形控件的坐标建立新的原点
IntersectClipRect(Result, 0, 0, Width, Height); // 注意，只针对当前图形控件的区域建立新的剪裁区
end;

function TWinControl.GetDeviceContext(var WindowHandle: HWnd): HDC;
begin
if csDesigning in ComponentState then
Result := GetDCEx(Handle, 0, DCX_CACHE or DCX_CLIPSIBLINGS)
else
Result := GetDC(Handle);
if Result = 0 then raise EOutOfResources.CreateRes(@SWindowDCError);
WindowHandle := FHandle;
end;

--------------------- 在两个地方会调用GetDeviceContext函数 -----------------
注意，只有TCanvas.RequiredState会调用TControlCanvas.CreateHandle函数

procedure TControlCanvas.CreateHandle;
begin
if FControl = nil then inherited CreateHandle else
begin
if FDeviceContext = 0 then
begin
with CanvasList.LockList do
try
if Count >= CanvasListCacheSize then FreeDeviceContext;
FDeviceContext := FControl.GetDeviceContext(FWindowHandle);
Add(Self);
finally
CanvasList.UnlockList;
end;
end;
Handle := FDeviceContext;
UpdateTextFlags;
end;
end;

function TControl.PaletteChanged(Foreground: Boolean): Boolean;
var
OldPalette, Palette: HPALETTE;
WindowHandle: HWnd;
DC: HDC;
begin
Result := False;
if not Visible then Exit;
Palette := GetPalette;
if Palette <> 0 then
begin
DC := GetDeviceContext(WindowHandle);
OldPalette := SelectPalette(DC, Palette, not Foreground);
if RealizePalette(DC) <> 0 then Invalidate;
SelectPalette(DC, OldPalette, True);
ReleaseDC(WindowHandle, DC);
Result := True;
end;
end;

--------------------- 调用调色板变化的5个函数 -----------------

procedure TControl.SetZOrderPosition(Position: Integer);
var
I, Count: Integer;
ParentForm: TCustomForm;
begin
TControl(ParentForm).PaletteChanged(True);
end;

procedure TWinControl.WMPaletteChanged(var Message: TMessage);
begin
Message.Result := Longint(PaletteChanged(False));
end;

procedure TWinControl.WMSysColorChange(var Message: TWMSysColorChange);
begin
Graphics.PaletteChanged;
Perform(CM_SYSCOLORCHANGE, 0, 0);
end;

function TWinControl.PaletteChanged(Foreground: Boolean): Boolean;
var
I: Integer;
begin
Result := inherited PaletteChanged(Foreground);
if Visible then
for I := ControlCount - 1 downto 0 do
begin
if Foreground and Result then Exit;
Result := Controls[I].PaletteChanged(Foreground) or Result;
end;
end;

procedure TWinControl.WMQueryNewPalette(var Message: TMessage);
begin
Include(FControlState, csPalette);
Message.Result := Longint(PaletteChanged(True));
end;

procedure TWinControl.WMPaletteChanged(var Message: TMessage);
begin
Message.Result := Longint(PaletteChanged(False));
end;