研究一下TForm.WMPaint过程（也得研究WM_ERASEBKGND）——TForm虽然继承自TWinControl，但是自行模仿了TCustomControl的全部行为，一共三种自绘的覆盖方法，比TCustomControl还多一种

先擦除背景：

procedure TCustomForm.WMEraseBkgnd(var Message: TWMEraseBkgnd);
begin
if not IsIconic(Handle) then inherited
else
begin
Message.Msg := WM_ICONERASEBKGND;
DefaultHandler(Message);
end;
end;

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); // Brush的颜色事先读取好了
end;

Message.Result := 1;
end;

然后进行绘制（背景色已经事先存在，无论后面绘制了什么都不影响背景色，如果不绘制，就全部都是背景色）：

procedure TCustomForm.WMPaint(var Message: TWMPaint);
var
DC: HDC;
PS: TPaintStruct;
begin
if not IsIconic(Handle) then
begin
ControlState := ControlState + [csCustomPaint]; // 模仿1
inherited; // 模仿2
ControlState := ControlState - [csCustomPaint];
end
else
begin
DC := BeginPaint(Handle, PS);
DrawIcon(DC, 0, 0, GetIconHandle);
EndPaint(Handle, PS);
end;
end;

inherited会调用：

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); // 走这里
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;

继续：

procedure TWinControl.PaintHandler(var Message: TWMPaint);
var
I, Clip, SaveIndex: Integer;
DC: HDC;
PS: TPaintStruct;
begin
DC := Message.DC;
if DC = 0 then DC := BeginPaint(Handle, PS);
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); // 走这里
RestoreDC(DC, SaveIndex);
end;
PaintControls(DC, nil);
finally
if Message.DC = 0 then EndPaint(Handle, PS);
end;
end;

TCustomForm有相应的覆盖函数：

procedure TCustomForm.PaintWindow(DC: HDC); // 模仿3
begin
FCanvas.Lock;
try
FCanvas.Handle := DC;
try
if FDesigner <> nil then FDesigner.PaintGrid else Paint; // 模仿4
finally
FCanvas.Handle := 0;
end;
finally
FCanvas.Unlock;
end;
end;

Paint会调用：

procedure TCustomForm.Paint; // Paint是dynamic函数，也是虚函数
begin
if Assigned(FOnPaint) then FOnPaint(Self); // 巨变：这里直接调用程序员事件，而不是等着程序员覆盖Paint函数（那样做也可以，另外还可直接覆盖PaintWindow虚函数，所以一共有3种方法，即：覆盖OnPaint事件，覆盖PaintWindow虚函数，覆盖Paint虚函数）
end;

这个FOnPaint来自：

property OnPaint: TNotifyEvent read FOnPaint write FOnPaint stored IsForm;

它会调用我写的事件内容：

procedure TForm1.FormPaint(Sender: TObject);
begin
//
end;

即使为空，也丝毫不影响整个Form1的显示。也许像上面那样写会被编译器删除，那么我这样写：

procedure TForm1.FormPaint(Sender: TObject);
begin
tag := 100;
end;

还是丝毫不影响整个Form1的显示。为什么会不影响呢？因为背景色提前就绘制在上面了，后面的OnPaint无论是否绘制，都不影响它的存在，顶多覆盖一小部分区域。比如：

procedure TForm1.FormPaint(Sender: TObject);
begin
Canvas.Brush.Color := clRed;
Canvas.Rectangle(0, 0, 100, 100);
end;

也就是覆盖了一个角，剩下的还是背景色。

---------------------------------------------------------------------

这里测试了覆盖Paint函数，OnPaint的代码保留，但是效果只有左上角一个小绿块，而没有红色方块。如果加上inherited（IDE会自动帮你加上，也就是推荐使用），那么红的绿的方块都有，比较有意思：

procedure TForm1.Paint;
begin
// inherited;
Canvas.Brush.Color := clGreen;
Canvas.Rectangle(0, 0, 50, 50);
end;

---------------------------------------------------------------------

唯一有个问题是，InitInheritedComponent读取dfm的颜色以后，是什么时候把它赋值给FBrush.Color的？它与{$R *.dfm}是什么关系？