win32 api 双缓冲绘图的1点疑问

win32 api 双缓冲绘图的一点疑问

#include "stdafx.h"
#include "Win32Project1.h"

#define MAX_LOADSTRING 100

// 全局变量:
HINSTANCE hInst;// 当前实例
TCHAR szTitle[MAX_LOADSTRING];// 标题栏文本
TCHAR szWindowClass[MAX_LOADSTRING];// 主窗口类名

// 此代码模块中包含的函数的前向声明:
ATOMMyRegisterClass(HINSTANCE hInstance);
BOOLInitInstance(HINSTANCE, int);
LRESULT CALLBACKWndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACKAbout(HWND, UINT, WPARAM, LPARAM);

int APIENTRY _tWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPTSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
UNREFERENCED_PARAMETER(hPrevInstance);
UNREFERENCED_PARAMETER(lpCmdLine);

 // TODO: 在此放置代码。
MSG msg;
HACCEL hAccelTable;

// 初始化全局字符串
LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
LoadString(hInstance, IDC_WIN32PROJECT1, szWindowClass, MAX_LOADSTRING);
MyRegisterClass(hInstance);

// 执行应用程序初始化:
if (!InitInstance (hInstance, nCmdShow))
{
return FALSE;
}

hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WIN32PROJECT1));

// 主消息循环:
while (GetMessage(&msg, NULL, 0, 0))
{
if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}

return (int) msg.wParam;
}



//
//  函数: MyRegisterClass()
//
//  目的: 注册窗口类。
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{
WNDCLASSEX wcex;

wcex.cbSize = sizeof(WNDCLASSEX);

wcex.style= CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc= WndProc;
wcex.cbClsExtra= 0;
wcex.cbWndExtra= 0;
wcex.hInstance= hInstance;
wcex.hIcon= LoadIcon(hInstance, MAKEINTRESOURCE(IDI_WIN32PROJECT1));
wcex.hCursor= LoadCursor(NULL, IDC_ARROW);
wcex.hbrBackground= (HBRUSH)(COLOR_WINDOW+1);
wcex.lpszMenuName= MAKEINTRESOURCE(IDC_WIN32PROJECT1);
wcex.lpszClassName= szWindowClass;
wcex.hIconSm= LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

return RegisterClassEx(&wcex);
}

//
//   函数: InitInstance(HINSTANCE, int)
//
//   目的: 保存实例句柄并创建主窗口
//
//   注释:
//
//        在此函数中，我们在全局变量中保存实例句柄并
//        创建和显示主程序窗口。
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   HWND hWnd;

   hInst = hInstance; // 将实例句柄存储在全局变量中

   hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPED | WS_MINIMIZEBOX | WS_MAXIMIZE | WS_SYSMENU ,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL); 
 

   if (!hWnd)
   {
      return FALSE;
   }

   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

//
//  函数: WndProc(HWND, UINT, WPARAM, LPARAM)
//
//  目的: 处理主窗口的消息。
//
//  WM_COMMAND- 处理应用程序菜单
//  WM_PAINT- 绘制主窗口
//  WM_DESTROY- 发送退出消息并返回
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
int wmId, wmEvent;
PAINTSTRUCT ps;
HDC hdc;
static HDC hMemdc;
static HBITMAP hMembmp;
static RECT rect;
switch (message)
{
case WM_CREATE:

GetClientRect(hWnd,&rect);
hMemdc=CreateCompatibleDC(NULL);
hMembmp=CreateCompatibleBitmap(hMemdc,rect.right,rect.bottom);
SelectObject(hMemdc,hMembmp);
FillRect(hMemdc,&rect,(HBRUSH)GetStockObject(WHITE_BRUSH));
break;
case WM_LBUTTONDOWN:
SelectObject(hMemdc,(HBRUSH)GetStockObject(BLACK_PEN));
Rectangle(hMemdc,0,0,100,100);
break;
case WM_RBUTTONDOWN:
case WM_PAINT:
hdc = BeginPaint(hWnd, &ps);


BitBlt(hdc,0,0,rect.right,rect.bottom, hMemdc,0,0,SRCCOPY);
EndPaint(hWnd,&ps);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;

default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
return 0;
}

单击左键在内存DC的左上角画矩形，右键发送WM_PAINT消息。但是，遇到一个问题，只有在窗口覆盖了屏幕左上角区域时，才能在屏幕上画出矩形。如果屏幕左上角区域不在窗口内是时候，无法画出矩形 API win32 GDI
[解决办法]
CreateCompatibleDC
The CreateCompatibleDC function creates a memory device context (DC) compatible with the specified device.

HDC CreateCompatibleDC(
HDC hdc // handle to DC
);
Parameters
hdc
[in] Handle to an existing DC. If this handle is NULL, the function creates a memory DC compatible with the application's current screen.

win32 api 双缓冲绘图的1点疑问

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