TForm的显示过程

新建一个空窗体项目，然后运行，此时首先运行：

procedure TApplication.Run;
begin
  FRunning := True;
  try
    AddExitProc(DoneApplication);
    if FMainForm <> nil then
    begin
      case CmdShow of
        SW_SHOWMINNOACTIVE: FMainForm.FWindowState := wsMinimized;
        SW_SHOWMAXIMIZED: MainForm.WindowState := wsMaximized;
      end;
      if FShowMainForm then
        if FMainForm.FWindowState = wsMinimized then
          Minimize else
          FMainForm.Visible := True;
      repeat
        try
          HandleMessage;
        except
          HandleException(Self);
        end;
      until Terminated;
    end;
  finally
    FRunning := False;
  end;
end;

调用 MainForm.WindowState := wsMaximized;
其中 类属性WindowState调用SetWindowState
调用 FMainForm.Visible := True;
其中 类属性Visible调用SetVisible虚函数，间接调用TControl.SetVisible（相当于UpdateWindow API）

第一个步骤：

procedure TCustomForm.SetWindowState(Value: TWindowState);
const
  ShowCommands: array[TWindowState] of Integer = (SW_SHOWNORMAL, SW_MINIMIZE, SW_SHOWMAXIMIZED);
begin
  if FWindowState <> Value then
  begin
    FWindowState := Value;
    if not (csDesigning in ComponentState) and Showing then
      ShowWindow(Handle, ShowCommands[Value]);
  end;
end;

第二个步骤：：

procedure TCustomForm.SetVisible(Value: Boolean);
begin
  if fsCreating in FFormState then
    if Value then
      Include(FFormState, fsVisible) else
      Exclude(FFormState, fsVisible)
  else
  begin
    if Value and (Visible <> Value) then SetWindowToMonitor;
    inherited Visible := Value;
  end;
end;

procedure TControl.SetVisible(Value: Boolean);
begin
  if FVisible <> Value then
  begin
    VisibleChanging;
    FVisible := Value;
    Perform(CM_VISIBLECHANGED, Ord(Value), 0);
    RequestAlign;
  end;
end;

然后故事就长了：

procedure TWinControl.CMVisibleChanged(var Message: TMessage);
begin
  if not FVisible and (Parent <> nil) then RemoveFocus(False);
  if not (csDesigning in ComponentState) or
    (csNoDesignVisible in ControlStyle) then UpdateControlState;
end;

procedure TWinControl.UpdateControlState;
var
  Control: TWinControl;
begin
  Control := Self;
  while Control.Parent <> nil do
  begin
    Control := Control.Parent;
    if not Control.Showing then Exit;
  end;
  if (Control is TCustomForm) or (Control.FParentWindow <> 0) then UpdateShowing;
end;

procedure TWinControl.UpdateShowing;
var
  ShowControl: Boolean;
  I: Integer;
begin
  ShowControl := (FVisible or (csDesigning in ComponentState) and
    not (csNoDesignVisible in ControlStyle)) and
    not (csReadingState in ControlState);
  if ShowControl then
  begin
    if FHandle = 0 then CreateHandle;
    if FWinControls <> nil then
      for I := 0 to FWinControls.Count - 1 do
        TWinControl(FWinControls[I]).UpdateShowing;
  end;
  if FHandle <> 0 then
    if FShowing <> ShowControl then
    begin
      FShowing := ShowControl;
      try
        Perform(CM_SHOWINGCHANGED, 0, 0);
      except
        FShowing := not ShowControl;
        raise;
      end;
    end;
end;

procedure TWinControl.CMShowingChanged(var Message: TMessage);
const
  ShowFlags: array[Boolean] of Word = (
    SWP_NOSIZE + SWP_NOMOVE + SWP_NOZORDER + SWP_NOACTIVATE + SWP_HIDEWINDOW,
    SWP_NOSIZE + SWP_NOMOVE + SWP_NOZORDER + SWP_NOACTIVATE + SWP_SHOWWINDOW);
begin
  SetWindowPos(FHandle, 0, 0, 0, 0, 0, ShowFlags[FShowing]);
end;

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第三步，调用了ShowWindow API和SetWindowPos API以后（不知道这两个API那个更重要），当系统空闲时（因为没发现调用UpdateWindow API），Windows先擦除Form1的背景，后向Form1发WM_PAINT消息，由TCustomForm接收：

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;
end;

擦除背景绝对重要，只有擦除了背景，才能在上面作画，否则作画全部无效：

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

在TWinControl.WMPaint函数里下调试点：

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);
  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;

很明显执行的是 not FDoubleBuffered逻辑，说明TForm的双缓冲默认是关闭的。然后执行PaintHandler

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);
      RestoreDC(DC, SaveIndex);
    end;
    PaintControls(DC, nil);
  finally
    if Message.DC = 0 then EndPaint(Handle, PS);
  end;
end;

因为是空窗体，所以执行PaintWindow（如有子控件执行PaintControls），即：

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

最后执行Paint

procedure TCustomForm.Paint;
begin
  if Assigned(FOnPaint) then FOnPaint(Self);
end;

然后就通过FOnPaint自动调用了程序员定义的事件。

总结：TForm和TCustomControl有点差不多，都各自包含了一个Canvas，属于自绘控件，至于它们的显示函数的区别如下：
WMPaint函数一样加上了csCustomPaint风格，只是TForm在窗口最小化的情况下是绘制图标 （入口函数，毕竟Windows直接把消息发给这个函数）
PaintWindow函数完全一样（除了TForm在设计期间要显示格子）。
Paint函数略有点不一样，在TCustomControl里完全空函数，反正它就是抽象类，等着被继承和覆盖；在TForm里有内容，即直接调用程序员函数

我的另一篇文章，注释比较详细，可一并参考：一行代码设置TForm颜色的前世今生
http://www.cnblogs.com/findumars/p/4117783.html

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其中有意思的一个小技巧是调用父类属性：
TCustomForm.SetVisible(Value: Boolean);函数里，调用父类的属性居然可以这样调用：
inherited Visible := Value;
如果不写inherited，就变成调用本类的属性了，就会调用不同的Set函数，这样效果就完全不一样了。