WM_PAINT与WM_ERASEBKGND（用户操作和API这两种情况产生消息的顺序有所不同）

1）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。

2）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;
}

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