Detailed explanation of messages when creating and destroying windows



Usually we understand that the message WM_CREATE is issued when the window is created, but there is a message WM_NCCREATE that is issued before this message.
Namely:
WM_NCCREATE Corresponding to
WM_CREATE
, the message WM_DESTROY, WM_NCDESTROY is issued when the window is closed.
Sequence:
WM_DESTROY
WM_NCDESTROY
It should be noted here that when a window is closed, if there is a child window in this window, the WM_DESTROY and WM_NCDESTROY messages of the child window will be sent first, and then the WM_DESTROY and WM_NCDESTROY messages of the parent window will be sent.
Sequence:
WM_DESTROY //CHILD, child window message
WM_NCDESTROY //CHILD, child window message
WM_DESTROY //Parent, parent window message     
WM_NCDESTROY //Parent, parent window message
The following is an example to verify the execution sequence:
1. Create a new MFC project , the project name is TestCreate, then there is already a default form TestCreate, and the settings are as follows:

2. Next, add the subform TestChild without setting it.
3. Add the open subform event response:

1. #include "TestChild.h"
2. void  CTestCreateDlg::OnButton2() //Open the subform
3. {
4.     if (!frm.GetSafeHwnd())
5.     {
6.         frm.Create(IDD_TESTCHILD_DIALOG,this);
7.     }
8.     frm.ShowWindow(SW_SHOW);
9. }

4. Add a message handler to the parent form

1. LRESULT CTestCreateDlg::DefWindowProc(UINT message, WPARAM wParam, LPARAM lParam) 
2. {
3.     // TODO: Add your specialized code here and/or call the base class
4.     switch(message)
5.     {
6.     case WM_CREATE:
7.         TRACE0("----------------------WM_CREATE/n");
8.         break;
9.     case WM_NCCREATE:
10.         TRACE0("----------------------WM_NCCREATE/n");
11.         break;
12.     case WM_DESTROY:
13.         TRACE0("----------------------WM_DESTROY/n");
14.         break;
15.     case WM_NCDESTROY:
16.         TRACE0("----------------------WM_NCDESTROY/n");
17.         break;
18.     default:
19.         break;
20.     }
21.     return CDialog::DefWindowProc(message, wParam, lParam);
22. }

5. Add a message handler to the subform

1. LRESULT CTestChild::DefWindowProc(UINT message, WPARAM wParam, LPARAM lParam) 
2. {
3.     // TODO: Add your specialized code here and/or call the base class
4.     switch(message)
5.     {
6.     case WM_CREATE:
7.         TRACE0("----------------------WM_CREATE_CHILD/n");
8.         break;
9.     case WM_NCCREATE:
10.         TRACE0("----------------------WM_NCCREATE_CHILD/n");
11.         break;
12.     case WM_DESTROY:
13.         TRACE0("----------------------WM_DESTROY_CHILD/n");
14.         break;
15.     case WM_NCDESTROY:
16.         TRACE0("----------------------WM_NCDESTROY_CHILD/n");
17.         break;
18.     default:
19.         break;
20.     }
21.     return CDialog::DefWindowProc(message, wParam, lParam);
22. }

到此，一个测试例程就做好了，Alt+2打开Output窗口，运行。
点击按钮打开子窗体，然后切换到父窗体，并点击关闭按钮，可以看到输出结果：
----------------------WM_NCCREATE
Loaded 'C:/WINDOWS/system32/version.dll', no matching symbolic information found.
Loaded 'C:/WINDOWS/system32/MSCTFIME.IME', no matching symbolic information found.
----------------------WM_CREATE
----------------------WM_NCCREATE_CHILD
----------------------WM_CREATE_CHILD
----------------------WM_DESTROY_CHILD
----------------------WM_NCDESTROY_CHILD
----------------------WM_DESTROY
----------------------WM_NCDESTROY