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。
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