Introduction:
The so-called child window is a window in a parent window, which is also called the main window. Sub-window is also a window, it has its own callback function, its own window class, its own creation. It is located in an area of the main window. We can set the size and location of the sub-window and its related properties. Subwindows are related to the main window, and they access each other through handles and messages.

Here we first introduce the common sub-windows.

I. Creation of Subwindows

1. Register window class

Subwindow class parameters are basically the same as the main window. Generally, only four parameters need to be modified.

// After registering the main window, modify four parameters
wc.cbClsExtra = sizeof(long);    // Reserved space
wc.hIcon = NULL;                 // Icon set to empty
wc.lpfnWndProc = ChildWndProc;   // callback
wc.lpszClassName = TEXT("ChildWindow");   // Subwindow class name

// Register subwindow classes
RegisterClassEx(&wc);

2. Create subwindows

Comparison of CreateWindow parameters:

Whether the sub-window display can be modified by calling Show Window function, the size and location of the window can be modified by Move Window function.

// Create 25 sub-windows
case WM_CREATE:
{
	for (i = 0; i < 5; i++)
	{
		for (j = 0; j < 5; j++)
		{
		 childhwnd[i][j]=CreateWindow(TEXT("ChildWindow"), NULL, 
				           WS_CHILD|WS_VISIBLE,
							0, 0, 0 ,0, 
							hwnd, (HMENU)(x++),
						 (HINSTANCE)GetWindowLong (hwnd, GWL_HINSTANCE), NULL);
		}
	}
break;
}

3. Define subwindow callback function

LRESULT CALLBACK WndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam);  // Main window callback function
LRESULT CALLBACK ChildWndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam);  // Subwindow callback function

LRESULT CALLBACK ChildWndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
	switch(Message)
	{
	case WM_LBUTTONDOWN:
	{
		ShowWindow(hwnd, SW_HIDE);
		break;
	}
	default:
		return DefWindowProc(hwnd, Message, wParam, lParam);   //Let the system process messages, this statement must be added
	}
	return 0;
}

II. THE CONNECTION BETWEEN SUBWINDOWS AND MAIN WINDOWS

1. Main window accesses sub-windows (sending messages)

• Get window information: GetWindowLongPrt
• Gets the ID number of the specified control: GetDlgCtrlID
• Gets a handle to the specified ID: GetDlgItem
• Change the position and size of the specified window: MoveWindow
• Sends the specified message to one or more windows: SendMessage

GetWindowLongPrt Function: Get window information

LONG_PTR GetWindowLongPtr(HWND hWnd,   // Window handle
                          int nIndex);   // Get the specified value of the information

GetDlgCtrlID Function: Gets the specified control's ID Number
ID For subforms,Put this in the form ID Number passed to hMenu Parameters.

int GetDlgCtrlID(HWND hwndCtl);  // Subwindow handle

GetDlgItem Function: Gets the specified ID Handle to

HWND childhwnd = GetDlgItem(Parenthwnd,ID);

BOOL MoveWindow( HWND hWnd, int X, int Y, int nWidth, int nHeight, 
                 BOOL bRepaint; // TRUE redrawing, FALSE not redrawing

SendMessage Function: Sends a specified message to one or more windows
SendMessage The function calls the window program for the specified window until the window program has processed the message before returning.
PostMessage The function returns a message immediately after it is sent to a thread's message queue.

LRESULT SendMessage(HWND hWnd，UINT Msg，WPARAM wParam，LPARAM IParam)

2. Subwindows access the main window (send messages)

• Gets a parent window handle for the specified child window: GetParent
• Sends the specified message to one or more windows: SendMessage

HWND GetParent(HWND hWnd)；

3. The Application of Sub-windows: Turn the Boards

Introduction:
Here, 25 sub-windows are created, 25 sub-windows are visible at the beginning. The main window and the sub-window are respectively pressed by the left mouse button. The main window receives the message to display the sub-window, and the sub-window receives the message to hide the sub-window. ShowWindow can realize this. When the sub-window is displayed, the picture can not be seen, and if the sub-window is not displayed, the picture can be seen. When the sub-window hides, the sub-window disappears and the mouse press message is unacceptable. It should be noted here that when we click on the sub-window, the main window can not receive the key messages, and the sub-window processes the messages through its own callback function.

The code is as follows:

#include <windows.h>
#include"strsafe.h"//secure string operation
// Define callback function
LRESULT CALLBACK WndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam);
LRESULT CALLBACK ChildWndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam);

HINSTANCE hInst;   // Used to save the instance handle, used to create a sub-window CreateWindow below
#define NUM 5//macro definition

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) {
	WNDCLASSEX wc;
	HWND hwnd;
	MSG msg;

//  Register the main window class
	memset(&wc, 0, sizeof(wc));
	wc.style = CS_HREDRAW | CS_VREDRAW;
	wc.cbSize = sizeof(WNDCLASSEX);
	wc.lpfnWndProc = WndProc;
	wc.hInstance = hInstance;
	wc.hCursor = LoadCursor(NULL, IDC_ARROW);
	wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
	wc.lpszClassName = TEXT("WindowClass");
	wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
	wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);

	if (!RegisterClassEx(&wc)) {
		MessageBox(NULL, TEXT("Window Registration Failed!"), TEXT("Error!"), MB_ICONEXCLAMATION | MB_OK);
		return 0;
	}
 
 // Register subwindow classes
	wc.cbClsExtra = sizeof(long);
	wc.hIcon = NULL;
	wc.lpfnWndProc = ChildWndProc;
	wc.lpszClassName = TEXT("ChildWindow");
	RegisterClassEx(&wc);

// Create the main window
	hwnd = CreateWindowEx(WS_EX_CLIENTEDGE, TEXT("WindowClass"), TEXT("Caption"), WS_CAPTION|WS_SYSMENU,
		CW_USEDEFAULT, /* x */
		CW_USEDEFAULT, /* y */
		520, /* width */
		542, /* height */
		NULL, NULL, hInstance, NULL);

	hInst = hInstance;   // Save instance handle

	ShowWindow(hwnd, nCmdShow);
	UpdateWindow(hwnd);

	if (hwnd == NULL) {
		MessageBox(NULL, TEXT("Window Creation Failed!"), TEXT("Error!"), MB_ICONEXCLAMATION | MB_OK);
		return 0;
	}
	while (GetMessage(&msg, NULL, 0, 0) > 0) {
		TranslateMessage(&msg);
		DispatchMessage(&msg);
	}
	return msg.wParam;
}

// Principal function callback function
LRESULT CALLBACK WndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
	PAINTSTRUCT ps;
	HDC hdc,mdc;
	static int cxClient, cyClient,x=0,cx,cy;
	static HWND childhwnd[NUM][NUM];
	HBITMAP bmp;
	int i, j;
	WCHAR str[100];

	switch (Message)
	{

// Create 25 sub-windows
	case WM_CREATE:
	{
		for (i = 0; i < NUM; i++)
		{
			for (j = 0; j < NUM; j++)
			{
				childhwnd[i][j]=CreateWindowEx(0,TEXT("ChildWindow"), NULL, WS_CHILD|WS_VISIBLE,
							100*i, /* x */
							100*j, /* y */
							98, /* width */
							98, /* height */
							hwnd, (HMENU)(x++), hInst, NULL);
			}
		}
		break;
	}
	
	// Put Zhao Liying-mei's picture on it, as mentioned in the previous blog.
	case WM_PAINT:
	{
		hdc = BeginPaint(hwnd, &ps);
		mdc = CreateCompatibleDC(hdc);
		bmp = (HBITMAP)LoadImage(hInst, MAKEINTRESOURCEW(101), IMAGE_BITMAP, 500, 500, LR_CREATEDIBSECTION);
		SelectObject(mdc, bmp);
		BitBlt(hdc, 0, 0, 500, 500, mdc, 0, 0, SRCCOPY);
		EndPaint(hwnd, &ps);
		break;
	}

   // The main window responds to the left key and displays the sub-windows.
	case WM_LBUTTONDOWN:
	{
		cx = LOWORD(lParam);
		cy = HIWORD(lParam);
		i = cx/100;
		j =  cy/100;
		ShowWindow(childhwnd[i][j], SW_SHOW);  // Display subwindows
	
		break;
	}
	case WM_DESTROY:
	{
		PostQuitMessage(0);
		break;
	}

	default:
		return DefWindowProc(hwnd, Message, wParam, lParam);
	}

	return 0;
}

// Subwindow callback function
LRESULT CALLBACK ChildWndProc(HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam)
{
	switch(Message)
	{
	// Subwindows respond to the left key and hide subwindows
	case WM_LBUTTONDOWN:
	{
		ShowWindow(hwnd, SW_HIDE);
		break;
	}
	
	// Other message processing, can't forget ah
	default:
		return DefWindowProc(hwnd, Message, wParam, lParam);
	}
	return 0;
}

The author says:
This world is really good and deceived! The more you don't resist, the less fearful others are, the more you like to cause trouble and bully. She always thinks she is so great. Damn, your family is looking at my mom bullying, right? If something happens, they will be angry with my mom. My mom just went up for a few days and deliberately came to my mom's place to find something. The curse is still so ugly. Really, if I'm not here, otherwise I really want to rush up. My mom is not in good health and can't be in a hurry. My mother's heart aches. This person really is, his personality is questionable also said that my mother, if you are good, how can you and his mother-in-law meet and quarrel, have stayed at his mother's house for more than 20 years, you have the ability not to go back, work in the factory, three days two days is not with this quarrel or that quarrel, and my mother or mother's cousin, there is such a quarrel. Sister ah, I have never seen under the sun, since your family also said, relatives can go, can not go, I also think it is the best, I do not want to touch your light in the future, the road to the sky one side. Others scolded my mother for being upset. I hope you can understand it here.