{ TObject.Dispatch accepts any data type as its Message parameter. The
first 2 bytes of the data are taken as the message id to search for
in the object's message methods. TDispatchMessage is an example of
such a structure with a word field for the message id.
}
TDispatchMessage = record
MsgID: Word;
end;
类的继承关系如下：

TObject->TPersistent->TComponent->TControl
TControl是所以可视化组件的父类，TControl提供了一个新的方法，WndProc:
procedure TControl.WndProc(var Message: TMessage);
var
Form: TCustomForm;
KeyState: TKeyboardState;
WheelMsg: TCMMouseWheel;
begin
//如果处在设计期
if (csDesigning in ComponentState) then
begin
Form := GetParentForm(Self);//得到拥有该组件的窗体
if (Form <> nil) and (Form.Designer <> nil) and
Form.Designer.IsDesignMsg(Self, Message) then Exit //消息由窗体来处理
end;
//窗体可以为其拥有的组件来处理键盘消息。
if (Message.Msg >= WM_KEYFIRST) and (Message.Msg <= WM_KEYLAST) then
begin
Form := GetParentForm(Self);
if (Form <> nil) and Form.WantChildKey(Self, Message) then Exit;
end
//关于鼠标的消息
else if (Message.Msg >= WM_MOUSEFIRST) and (Message.Msg <= WM_MOUSELAST) then
begin
//如果组件不可以接受和处理双击消息，就将双击消息映射为单击消息。
if not (csDoubleClicks in ControlStyle) then
case Message.Msg of
WM_LBUTTONDBLCLK, WM_RBUTTONDBLCLK, WM_MBUTTONDBLCLK:
Dec(Message.Msg, WM_LBUTTONDBLCLK - WM_LBUTTONDOWN);
end;
case Message.Msg of
WM_MOUSEMOVE: Application.HintMouseMessage(Self, Message);//如果是鼠标移动的消息，则出现hint窗口
WM_LBUTTONDOWN, WM_LBUTTONDBLCLK://如果是左键被按下，或者双击，如果是自动拖动模式，则开始拖动，并将左键按下的状态加入组件的状态。
begin
if FDragMode = dmAutomatic then
begin
BeginAutoDrag;
Exit;
end;
Include(FControlState, csLButtonDown);
end;
WM_LBUTTONUP:
Exclude(FControlState, csLButtonDown); //如果是左键放开，则将左键按下的状态剔除。
else
with Mouse do
if WheelPresent and (RegWheelMessage <> 0) and //如果鼠标有滚轮，并且滚轮滑动时发出了消息
(Message.Msg = RegWheelMessage) then
begin
GetKeyboardState(KeyState); //将256虚拟键的状态拷贝到缓存中去
with WheelMsg do //填充记录
begin
Msg := Message.Msg;
ShiftState := KeyboardStateToShiftState(KeyState);
WheelDelta := Message.WParam;
Pos := TSmallPoint(Message.LParam);
end;
MouseWheelHandler(TMessage(WheelMsg)); //派发鼠标滚轮的消息
Exit;
end;
end;
end
else if Message.Msg = CM_VISIBLECHANGED then
with Message do
SendDockNotification(Msg, WParam, LParam); //处理自定义消息
Dispatch(Message); //派发未处理的消息
end;
但是只有TWinControl可以获得焦点：
procedure TWinControl.WndProc(var Message: TMessage);
var
Form: TCustomForm;
begin
case Message.Msg of
WM_SETFOCUS: //设置控件的焦点
begin
Form := GetParentForm(Self);
if (Form <> nil) and not Form.SetFocusedControl(Self) then Exit;
end;
WM_KILLFOCUS:
if csFocusing in ControlState then Exit;
//当鼠标有活动的时候发出该消息，如果鼠标没有被捕捉到，则消息发往鼠标下面的那个窗口，否则消息将发往捕捉到鼠标的那个窗口。
WM_NCHITTEST:
begin
inherited WndProc(Message); //调用父类的处理方法
//如果窗体被挡住并且在指定的点没有控件，则返回结果为在client区。
if (Message.Result = HTTRANSPARENT) and (ControlAtPos(ScreenToClient(
SmallPointToPoint(TWMNCHitTest(Message).Pos)), False) <> nil) then
Message.Result := HTCLIENT;
Exit;
end;
WM_MOUSEFIRST..WM_MOUSELAST:
if IsControlMouseMsg(TWMMouse(Message)) then //鼠标消息是否直接发往组件的窗体子组件
begin
{ Check HandleAllocated because IsControlMouseMsg might have freed the
window if user code executed something like Parent := nil. }
if (Message.Result = 0) and HandleAllocated then
DefWindowProc(Handle, Message.Msg, Message.wParam, Message.lParam);//调用默认的的消息处理方法对该消息进行默认处理。
Exit;
end;
WM_KEYFIRST..WM_KEYLAST:
if Dragging then Exit;
WM_CANCELMODE:
if (GetCapture = Handle) and (CaptureControl <> nil) and
(CaptureControl.Parent = Self) then
CaptureControl.Perform(WM_CANCELMODE, 0, 0);
end;
inherited WndProc(Message);
end;

TApplication在程序中发挥着重要的作用：
Application.Run;

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;
//一个消息循环直到Terminated为True时才退出。
repeat
try
HandleMessage;
except
HandleException(Self);
end;
until Terminated;
end;
finally
FRunning := False;
end;
end;

procedure TApplication.HandleMessage;
var
Msg: TMsg;
begin
if not ProcessMessage(Msg) then Idle(Msg);
end;

function TApplication.ProcessMessage(var Msg: TMsg): Boolean;
var
Handled: Boolean;
begin
Result := False;
if PeekMessage(Msg, 0, 0, 0, PM_REMOVE) then //从现成的消息循环中取出消息并放入指定的消息结构中。
begin
Result := True;
if Msg.Message <> WM_QUIT then //如果不是退出消息则进行相应的处理
begin
Handled := False;
if Assigned(FOnMessage) then FOnMessage(Msg, Handled);
if not IsHintMsg(Msg) and not Handled and not IsMDIMsg(Msg) and
not IsKeyMsg(Msg) and not IsDlgMsg(Msg) then
begin
TranslateMessage(Msg);
DispatchMessage(Msg);
end;
end
else
FTerminate := True;
end;
end;