背景图片函数

WM_PAINT与WM_ERASEBKGND

当WM_PAINT不是由InvalidateRect产生时,即由最大化，最小化等产生时，或者移动产生（移动有时只会产生WM_ERASEBKGND消息)系统先发送WM_ERASEBKGND消息,再发送WM_PAINT消息.

如果处理WM_ERASEBKGND消息时返回FALSE，BeginPaint标记pt.fErase 为TRUE,如果处理WM_ERASEBKGND时返回TRUE,BeginPaint标记pt.fErase为FALSE.

当WM_PAINT由InvalidateRect产生时,先发送WM_PAINT消息（异步),如果InvalidateRect的bErase为TRUE,BeginPaint检查到更新区域需要删除背景,向窗口发送一个WM_ERASEBKGND消息,如果处理WM_ERASEBKGND消息时返回FALSE，BeginPaint标记pt.fErase 为TRUE,如果处理WM_ERASEBKGND时返回TRUE,BeginPaint标记pt.fErase为FALSE.

如果pt.fErase标记为TRUE,指示应用程序应该处理背景,但是应用程序不一定需要处理,pt.fErase只是作为一个标记.

补充：DefWindowProc(hWnd, message, wParam, lParam)处理WM_ERASEBKGND消息时默认用下面的画刷清除背景

wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);

WM_ERASEBKGND返回TRUE和返回FALSE是一个规范，一般情况下没有什么区别，但是如果什么时候用到了，会根据函数返回值判断后续处理。因此最好按照要求返回数据.

最后再提供一些测试代码，让大家更好的理解上面的内容。

// WM_ERASEBKGNDTest.cpp : Defines the entry point for the application. //

#include \"stdafx.h\"

#include \"WM_ERASEBKGNDTest.h\" #define MAX_LOADSTRING 100 // Global Variables:

HINSTANCE hInst; // current instance

TCHAR szTitle[MAX_LOADSTRING]; // The title bar text

TCHAR szWindowClass[MAX_LOADSTRING]; // the main window class name // Forward declarations of functions included in this code module: ATOM MyRegisterClass(HINSTANCE hInstance); BOOL InitInstance(HINSTANCE, int);

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM); INT_PTR CALLBACK About(HWND, UINT, WPARAM, LPARAM); int APIENTRY _tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPTSTR lpCmdLine, int nCmdShow) {

UNREFERENCED_PARAMETER(hPrevInstance); UNREFERENCED_PARAMETER(lpCmdLine); // TODO: Place code here. MSG msg;

HACCEL hAccelTable; // Initialize global strings

LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING); LoadString(hInstance, IDC_WM_ERASEBKGNDTEST, szWindowClass, MAX_LOADSTRING); MyRegisterClass(hInstance); // Perform application initialization: if (!InitInstance (hInstance, nCmdShow)) {

return FALSE; }

hAccelTable = LoadAccelerators(hInstance,

MAKEINTRESOURCE(IDC_WM_ERASEBKGNDTEST));

// Main message loop:

while (GetMessage(&msg, NULL, 0, 0)) {

if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg)) {

TranslateMessage(&msg); DispatchMessage(&msg); } }

return (int) msg.wParam; } //

// FUNCTION: MyRegisterClass() //

// PURPOSE: Registers the window class. //

// COMMENTS: //

// This function and its usage are only necessary if you want this code // to be compatible with Win32 systems prior to the 'RegisterClassEx' // function that was added to Windows 95. It is important to call this function // so that the application will get 'well formed' small icons associated // with it. //

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_WM_ERASEBKGNDTEST)); wcex.hCursor = LoadCursor(NULL, IDC_ARROW); wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1); // wcex.hbrBackground = NULL;

wcex.lpszMenuName = MAKEINTRESOURCE(IDC_WM_ERASEBKGNDTEST); wcex.lpszClassName = szWindowClass;

wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL)); return RegisterClassEx(&wcex); } //

// FUNCTION: InitInstance(HINSTANCE, int) //

// PURPOSE: Saves instance handle and creates main window //

// COMMENTS: //

// In this function, we save the instance handle in a global variable and // create and display the main program window. //

BOOL InitInstance(HINSTANCE hInstance, int nCmdShow) {

HWND hWnd;

hInst = hInstance; // Store instance handle in our global variable

hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL); if (!hWnd) {

return FALSE; }

ShowWindow(hWnd, nCmdShow); UpdateWindow(hWnd); return TRUE; } //

// FUNCTION: WndProc(HWND, UINT, WPARAM, LPARAM) //

// PURPOSE: Processes messages for the main window. //

// WM_COMMAND - process the application menu // WM_PAINT - Paint the main window

// WM_DESTROY - post a quit message and return // //

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) {

int wmId, wmEvent; PAINTSTRUCT ps; HDC hdc; switch (message) {

case WM_COMMAND: wmId = LOWORD(wParam); wmEvent = HIWORD(wParam); // Parse the menu selections: switch (wmId) {

case IDM_ABOUT:

DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About); break;

case IDM_EXIT: DestroyWindow(hWnd);

break; default:

return DefWindowProc(hWnd, message, wParam, lParam); } break;

case WM_LBUTTONDOWN: RECT rect;

GetClientRect(hWnd, &rect); InvalidateRect(hWnd, &rect, TRUE); MessageBeep(2); break;

//case WM_ERASEBKGND:

// OutputDebugString(_T(\"Enter WM_ERASEBKGND/n\")); // return 1; // break;

case WM_PAINT:

OutputDebugString(_T(\"Enter WM_PAINT/n\")); hdc = BeginPaint(hWnd, &ps); if (ps.fErase == TRUE)

OutputDebugString(_T(\"ps.fErase == TRUE/n/n\")); // TODO: Add any drawing code here... EndPaint(hWnd, &ps); break;

case WM_DESTROY: PostQuitMessage(0); break; default:

if (WM_ERASEBKGND == message)

OutputDebugString(_T(\"Enter WM_ERASEBKGND/n\")); return DefWindowProc(hWnd, message, wParam, lParam); }

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

// Message handler for about box.

INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam) {

UNREFERENCED_PARAMETER(lParam); switch (message) {

case WM_INITDIALOG: return (INT_PTR)TRUE; case WM_COMMAND:

if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL) {

EndDialog(hDlg, LOWORD(wParam)); return (INT_PTR)TRUE; } break; }

return (INT_PTR)FALSE; }