WINAPI.无法用椭圆擦除窗口的背景

Question

我只是想绘制一个椭圆:

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

你的代码没有删除任何东西.它只是在指定的坐标处绘制一个椭圆.之前绘制的椭圆仍然存在.

你提到了这个WM_ERASEBKGND消息,但有两个原因导致你不适合你:

1. 在窗口过程(WndProc)中,您WM_ERASEBKGND显式处理消息,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,除非您明确地return从case标签内部调用,否则每次都会调用默认窗口过程.

2. 当您注册窗口类(内部MyRegisterClass)时,您将WNDCLASS结构的所有字段归零,然后显式初始化其中的几个.您没有显式初始化该hbrBackground字段,因此它被设置为0.当hbrBackground为0时,

   当该成员是NULL,应用程序必须在请求在其客户区域中绘制时绘制其自己的背景.要确定是否必须绘制背景,应用程序可以处理WM_ERASEBKGND消息或测试由函数填充fErase的PAINTSTRUCT结构的成员BeginPaint.

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

   您将需要设置hbrBackground类似的东西COLOR_WINDOW + 1,或者您需要向WM_ERASEBKGND消息处理程序添加代码以自行擦除窗口的背景.

或者,或许更好的选择是完全忘记WM_ERASEBKGND消息,就像许多Windows程序员那样,因为这种两步擦除和绘制方法往往会导致闪烁.将hbrBackground字段设置为NULL,不响应WM_ERASEBKGND消息执行任何操作,并在WM_PAINT处理程序的顶部执行删除操作:

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;
}