Delphi 5开发人员指南书的疑点

[zjybestzjybest]

//第十三章394页程序使用了两次子类化窗口技术，但是退出程序时只恢复了其中的一个
//窗口函数，但是程序还是能够正常运行真是见鬼了


unit Scwndprc;

interface

uses Forms, Messages;

const
DDGM_FOOMSG = WM_USER;

implementation

uses Windows, SysUtils, Dialogs;

var
WProc: Pointer;

function NewWndProc(Handle: hWnd; Msg, wParam, lParam: Longint): Longint;
stdcall;
{ This is a Win32 API-level window procedure. It handles the messages }
{ received by the Application window. }
begin
if Msg = DDGM_FOOMSG then
{ If it's our user-defined message, then alert the user. }
ShowMessage(Format('Message seen by WndProc! Value is: $%x', [Msg]));
{ Pass message on to old window procedure }
Result := CallWindowProc(WProc, Handle, Msg, wParam, lParam);
end;

initialization
{ Set window procedure of Application window. }
WProc := Pointer(SetWindowLong(Application.Handle, gwl_WndProc,
Integer(@NewWndProc)));
end.





//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
unit Main;

interface

uses
SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
Forms, Dialogs, StdCtrls;

type
TMainForm = class(TForm)
SendBtn: TButton;
PostBtn: TButton;
procedure SendBtnClick(Sender: TObject);
procedure PostBtnClick(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure FormDestroy(Sender: TObject);
private
OldWndProc: Pointer;
WndProcPtr: Pointer;
procedure WndMethod(var Msg: TMessage);
procedure HandleAppMessage(var Msg: TMsg; var Handled: Boolean);
end;

var
MainForm: TMainForm;

implementation

{$R *.DFM}

uses ScWndPrc;


procedure TMainForm.HandleAppMessage(var Msg: TMsg; var Handled: Boolean);
{ OnMessage handler for Application object. }
begin
if Msg.Message = DDGM_FOOMSG then
{ if it's the user-defined message, then alert the user. }
ShowMessage(Format('Message seen by OnMessage! Value is: $%x',
[Msg.Message]));
end;

procedure TMainForm.WndMethod(var Msg: TMessage);
begin
if Msg.Msg = DDGM_FOOMSG then
{ if it's the user-defined message, then alert the user. }
ShowMessage(Format('Message seen by WndMethod! Value is: $%x', [Msg.Msg]));
with Msg do
{ Pass message on to old window procedure. }
Result := CallWindowProc(OldWndProc, Application.Handle, Msg, wParam,
lParam);
end;

procedure TMainForm.SendBtnClick(Sender: TObject);
begin
SendMessage(Application.Handle, DDGM_FOOMSG, 0, 0);
end;

procedure TMainForm.PostBtnClick(Sender: TObject);
begin
PostMessage(Application.Handle, DDGM_FOOMSG, 0, 0);
end;

procedure TMainForm.FormCreate(Sender: TObject);
begin
Application.OnMessage := HandleAppMessage; // set OnMessage handler
WndProcPtr := MakeObjectInstance(WndMethod); // make window proc
{ Set window procedure of application window. }
OldWndProc := Pointer(SetWindowLong(Application.Handle, GWL_WNDPROC,
Integer(WndProcPtr)));
end;

procedure TMainForm.FormDestroy(Sender: TObject);
begin
{ Restore old window procedure for Application window }
SetWindowLong(Application.Handle, GWL_WNDPROC, Longint(OldWndProc));
{ Free our user-created window procedure }
FreeObjectInstance(WndProcPtr);
end;

end.

[FrameSniper]

严重关注！

[alphax]

咋不能啊，你试过了没有？

[zjybestzjybest]

原来的程序是可以正常运行，
所以不明白了为什么能正常运行

[alphax]

Application的WndProc那一个不恢复对程序的退出没有影响

[zjybestzjybest]

为什么没有影响，这就是我的问题

[alphax]

当Application的Window句柄被释放的时候，NewWndProc就再也不会被执行，所以对退出没有影响

[zjybestzjybest]

好象不能这样理解把，那所有的WNDPROC都可以替换，退出时候就可以不复原吗？？？

[wan_ming]

我还没看到这，关注！！！！！！！！！！！！！！！！！！

[alphax]

首先，你要搞清楚这个和SubClass不是一回事，
其次，MainForm的Destroy恢复Application的WndProc，只是为了Application原来的WndProc（这里指NewWndProc)能够运行，如果不恢复，那么在MainForm被Free了以后，一旦Application要处理消息，那么执行：
procedure TMainForm.WndMethod(var Msg: TMessage);
begin
...
with Msg do
Result := CallWindowProc(OldWndProc, Application.Handle, Msg, wParam,
lParam); //在这里就会发生异常，因为MainForm实例释放了，所以存取MainForm的字段OldWndProc会引发AV异常
end;

[myling]

等等，吃饭去，回来如果没喝多再LOOK

[zjybestzjybest]

把wproc做成全局变量在INTERFACE中
我已经做过实验，在程序最后加一句，换回最初的窗口函数，和不替换是一样的。不明白


SetWindowLong(Application.Handle, gwl_WndProc,
Integer(WProc);