VCL中消息处理初探

TObject是基类，所以我们先看一下TObject的DISPATCH方法。Dispatch根据传入的message来寻找相应的消息处理方法，如果找不到的话，就继续向上到父类的消息处理方法表中寻找响应的处理方法，一直到找到为止，如果找到顶还没有，则调用DefaultHandle来处理该消息。message可以是任何的类型，Dispatch假设message的头两位是消息的ID，它就是根据ID来寻找消息处理方法的。虽然任何类型的message都可以被接受，但是TObject的子类还是希望传入的message参数是TMessage的记录类型或其他证明的记录类型。
以下声明和注释摘自与system.pas:
{ 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;

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