Question

我只想画一个椭圆：

case WM_PAINT:
        hdc = BeginPaint(parentWindow, &ps);
        Ellipse(hdc, x, y, width, height);
        EndPaint(parentWindow, &ps);

，然后使用计时器每秒绘制一个带有一些新参数的新椭圆来擦除它：

case WM_CREATE:
        SetTimer(hWnd, 1, 1000, NULL);
        break;
case WM_TIMER:
        x += 5;
        InvalidateRect(hWnd, NULL, TRUE);
        break;

但椭圆不会被删除和分层：

但是，我试图跟踪WM_ERASEBKGND，它确实是每个InvalidateRect发送的。

完整代码：

#include <Windows.h>
#include <windowsx.h>
#include <tchar.h>
#include <iostream>


TCHAR szWindowClass[] = TEXT("CreateThreadWindow");
TCHAR szAppName[] = TEXT("CreateThreadExample");

BOOL InitWindow(HINSTANCE, int);
ATOM MyRegisterClass(HINSTANCE);
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

HWND parentWindow;
MSG msg;

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    MyRegisterClass(hInstance);
    if (!InitWindow(hInstance, nCmdShow))
        return FALSE;
    BOOL bRet;
    while ((bRet = GetMessage(&msg, (HWND)NULL, 0, 0)) != 0)
    {
        if (bRet == -1)
            return FALSE;
        else
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }
    return (int)msg.wParam;
}

ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASS wndClass;
    memset(&wndClass, 0, sizeof(wndClass));
    wndClass.lpfnWndProc = WndProc;
    wndClass.hInstance = hInstance;
    wndClass.lpszMenuName = NULL;
    wndClass.lpszClassName = szWindowClass;
    return RegisterClass(&wndClass);
}

BOOL InitWindow(HINSTANCE hInstance, int nCmdShow)
{
    parentWindow = CreateWindow(szWindowClass, szAppName, WS_OVERLAPPEDWINDOW,
        300, 0, 600, 600, NULL, NULL, hInstance, NULL);
    ShowWindow(parentWindow, nCmdShow);
    return TRUE;
}



LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wparam, LPARAM lparam)
{
    PAINTSTRUCT ps;
    HDC hdc;
    static int x = 0, y = 0, width = 200, height = 100;
    switch (message) {
    case WM_ERASEBKGND:
        _RPT1(0, "%s\n", "erase");
        break;
    case WM_PAINT:
        hdc = BeginPaint(hWnd, &ps);
        Ellipse(hdc, x, y, width, height);
        EndPaint(hWnd, &ps);
        break;
    case WM_CREATE:
        SetTimer(hWnd, 1, 1000, NULL);
        break;
    case WM_TIMER:
        x += 5;
        InvalidateRect(hWnd, NULL, TRUE);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    default:
        return DefWindowProc(hWnd, message, wparam, lparam);
    }
}

Answer 1

您的代码没有删除任何内容。它只是在指定的坐标处绘制一个椭圆。之前绘制的椭圆仍在那里。

您提到了WM_ERASEBKGND消息，但有两个原因导致您无效：

在窗口过程（WndProc）中，您显式处理WM_ERASEBKGND消息，这意味着它不会传递给默认窗口过程（DefWindowProc）。编写窗口过程的更好方法如下：

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wparam, LPARAM lparam)
{
    static int x = 0, y = 0, width = 200, height = 100;
    switch (message) {
    case WM_ERASEBKGND:
    {
        _RPT1(0, "%s\n", "erase");
        break;
    }
    case WM_PAINT:
    {
        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(hWnd, &ps);
        Ellipse(hdc, x, y, width, height);
        EndPaint(hWnd, &ps);
        return 0;
    }
    case WM_CREATE:
    {
        SetTimer(hWnd, 1, 1000, NULL);
        break;
    }
    case WM_TIMER:
    {
        x += 5;
        InvalidateRect(hWnd, NULL, TRUE);
        return 0;
    }
    case WM_DESTROY:
    {
        PostQuitMessage(0);
        return 0;
    }
    default:
        break;
    }

    return DefWindowProc(hWnd, message, wparam, lparam);
}

现在，除非您从return标签内明确case，否则每次都会调用默认窗口过程。

当您注册窗口类（MyRegisterClass内）时，将WNDCLASS structure的所有字段归零，然后显式初始化其中的几个字段。您不显式初始化hbrBackground字段，因此将其设置为0.当hbrBackground为0时，

当此成员为NULL时，应用程序必须在请求在其客户区域中绘制时绘制自己的背景。要确定是否必须绘制背景，应用程序可以处理WM_ERASEBKGND消息或测试fErase函数填充的PAINTSTRUCT结构的BeginPaint成员。

这意味着默认窗口过程没有响应WM_ERASEBKGND消息，因为您没有给窗口提供背景画笔。

您需要将hbrBackground设置为COLOR_WINDOW + 1之类的内容，否则您需要向WM_ERASEBKGND消息处理程序添加代码以自行删除窗口的背景。

WM_ERASEBKGND

hbrBackground

WM_ERASEBKGND

WM_PAINT

case WM_PAINT:
{
    PAINTSTRUCT ps;
    HDC hdc = BeginPaint(hWnd, &ps);

    // Erase background of entire client area.
    RECT rcClient;
    GetClientRect(hWnd, &rcClient);
    FillRect(hdc, &rcClient, reinterpret_cast<HBRUSH>(COLOR_WINDOW+1));

    // Do normal drawing.
    Ellipse(hdc, x, y, width, height);

    EndPaint(hWnd, &ps);
    return 0;
}

WINAPI。不能用椭圆擦除窗口的背景

1 个答案: