第8章 计时器

8.1 计时器的基本知识

（1）SetTimer时间参数：1毫秒到4294 967 295毫秒（近50天）

（2）Windows本身处理BIOS中断，应用程序不需介入，Windows会每个计时器保持一个计数值，硬件时钟滴答一次，这个值减1.当计数为0时，发送WM_TIMER到消息队列，同时计数值重新恢复到原始值。

（3）WIN98的计时器周期约为55毫秒，Windows NT大约为10毫秒。即SetTimer参数小于这个数值，则根据操作系统取其中的一个值。SetTimer会把指定的时间间隔舍入到时钟滴答的整数倍，如1000ms/54.925=18.2个时钟滴答，舍入后为18个时钟滴答，即实际的间隔为989ms。

（4）计时器消息不是异步的。在指定的时间内可能因程序忙而收到不到WM_TIMER消息。WM_TIMER消息与WM_PAINT消息都是低优先级的，只有当队列没其他消息时，程序才会接收它们。

8.2 使用计时器的三种方法

（1）处理WM_TIMER消息

①SetTimer（hwnd,ID_TIMER,uiMsecInterval,NULL）;//ID_TIMER>0,不能为0.

②KillTimer(hwnd,ID_TIMER);

③消息参数：wParam=ID_TIMER; lParam =0;

例： 定义2个计时器

#define TIMER_SEC 1

#define TIMER_MIN 2

//设置定时器

SetTimer(hwnd,TIMER_SEC,1000,NULL);  //1秒

SetTimer(hwnd,TIMER_MIN,60000,NULL); //1分钟

//WM_TIMER处理逻辑

case WM_TIMER:

   switch(wParam)

   {

case TIMER_SEC:

    [每秒钟一次的处理]

     Break;

case TIMER_MIN:

    [每分钟一次的处理]

     Break;

}

return 0;

【Beeper1程序】

/*------------------------------------------------------------
BEEPER1.C -- Timer Demo Program No.1
(c) Charles Petzold, 1998
------------------------------------------------------------*/
#include <windows.h>
#define ID_TIMER 1
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    PSTR szCmdLine, int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("Beeper1");
    HWND         hwnd;
    MSG          msg;
    WNDCLASS     wndclass;
    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0;
    wndclass.cbWndExtra = 0;
    wndclass.hInstance = hInstance;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName = NULL;
    wndclass.lpszClassName = szAppName;
    if (!RegisterClass(&wndclass))
    {
        MessageBox(NULL, TEXT("This program requires Windows NT!"),
            szAppName, MB_ICONERROR);
        return 0;
    }

    hwnd = CreateWindow(szAppName,                  // window class name
        TEXT("Beeper1 Timer Demo"), // window caption
        WS_OVERLAPPEDWINDOW,        // window style
        CW_USEDEFAULT,              // initial x position
        CW_USEDEFAULT,              // initial y position
        CW_USEDEFAULT,              // initial x size
        CW_USEDEFAULT,              // initial y size
        NULL,                       // parent window handle
        NULL,                       // window menu handle
        hInstance,                  // program instance handle
        NULL);                     // creation parameters

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static BOOL fFlipFlop = FALSE;
    HBRUSH hBrush;
    HDC         hdc;
    PAINTSTRUCT ps;
    RECT        rect;

    switch (message)
    {
    case WM_CREATE:
        SetTimer(hwnd, ID_TIMER, 1000, NULL);
        return 0;
    case WM_TIMER:
        MessageBeep(-1);
        fFlipFlop = !fFlipFlop;
        InvalidateRect(hwnd, NULL, FALSE);
        return 0;
    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);

        GetClientRect(hwnd, &rect);
        hBrush = CreateSolidBrush(fFlipFlop ? RGB(255, 0, 0) : RGB(0, 0, 255));
        FillRect(hdc, &rect, hBrush);
        DeleteObject(hBrush);
        EndPaint(hwnd, &ps);
        return 0;

    case WM_DESTROY:
        KillTimer(hwnd, ID_TIMER);
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, message, wParam, lParam);
}

（2）利用回调函数处理——将忽略窗口过程中的WM_TIMER消息处理代码

  ①VOID CALLBACK MyTimerProc(HWNDhwnd,UINT message,UINT iTimerID,DWORD dwTime)

  ②SetTimer(hwnd,iTimerID,iMsecInterval,MyTimerProc);

    ★注意：从回调函数中得到的dwTime等于GetTickCount返回的值，指的是自Windows启动到现在的毫秒数。

【Beeper2程序】

/*------------------------------------------------------------
BEEPER2.C -- Timer Demo Program No.2
(c) Charles Petzold, 1998
------------------------------------------------------------*/
#include <windows.h>
#define ID_TIMER 1
VOID CALLBACK TimerProc(HWND, UINT, UINT, DWORD);
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    PSTR szCmdLine, int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("Beeper2");
    HWND         hwnd;
    MSG          msg;
    WNDCLASS     wndclass;
    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0;
    wndclass.cbWndExtra = 0;
    wndclass.hInstance = hInstance;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName = NULL;
    wndclass.lpszClassName = szAppName;
    if (!RegisterClass(&wndclass))
    {
        MessageBox(NULL, TEXT("This program requires Windows NT!"),
            szAppName, MB_ICONERROR);
        return 0;
    }

    hwnd = CreateWindow(szAppName,                  // window class name
        TEXT("Beeper2 Timer Demo"), // window caption
        WS_OVERLAPPEDWINDOW,        // window style
        CW_USEDEFAULT,              // initial x position
        CW_USEDEFAULT,              // initial y position
        CW_USEDEFAULT,              // initial x size
        CW_USEDEFAULT,              // initial y size
        NULL,                       // parent window handle
        NULL,                       // window menu handle
        hInstance,                  // program instance handle
        NULL);                     // creation parameters

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}
VOID CALLBACK TimerProc(HWND hwnd, UINT message, UINT iTimerID, DWORD dwTime)
{
    HBRUSH hBrush;
    HDC         hdc;
    RECT        rect;
    static BOOL fFlipFlop = FALSE;
    MessageBeep(-1);
    fFlipFlop = !fFlipFlop;
    hdc = GetDC(hwnd);
    GetClientRect(hwnd, &rect);
    hBrush = CreateSolidBrush(fFlipFlop ? RGB(255, 0, 0) : RGB(0, 0, 255));
    FillRect(hdc, &rect, hBrush);
    DeleteObject(hBrush);
    ReleaseDC(hwnd, hdc);
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{

    switch (message)
    {
    case WM_CREATE:
        SetTimer(hwnd, ID_TIMER, 1000, TimerProc);
        return 0;
        // case WM_TIMER:
        //MessageBeep(-1);
        //fFlipFlop = !fFlipFlop;
        //InvalidateRect(hwnd, NULL, FALSE);
        //return 0;

    case WM_DESTROY:
        KillTimer(hwnd, ID_TIMER);
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, message, wParam, lParam);
}

（3）方法三——与第2种相似，但SetTimer的设置如下。（一般很少这样用）

参数	设置	备注
hwnd	NULL	SetTimer、KillTimer、回调函数的hwnd都要设为NULL;
第2个参数	0	被忽略，填0
第4个参数	TimerProc	回调函数
返回值	计时器的ID	如返回0，说明没有可用的计时器

8.3 使用计时器作为时钟

8.3.1 数字时钟

（1）获取系统时间：GetLocalTime（当地时间）和GetSystemTime（协调世界时UTC）

（2）显示数字和冒号——自定义函数DisplayDigit和DisplayColon

 

①BOOL fSevenSegment[10][7]——用来控制0-9这10个数字的LED相应段开与关

②static POINT ptSegment[7][6]——表示LED中7个六边形的顶点坐标。

③数字宽度为42个单位，高度为72个单位，冒号是12个单位宽。6个数字加冒号共276个单位宽度。

（3）国际化——控制面板【区域和语言选项】中设置格式化日期和时间

    ①显示12或24小时格式。

        //获得是否是24小时格式

        GetLocaleInfo(LOCALE_USER_DEFAULT,LOCALE_ITIME, szBuffer, 2);

        f24Hour = (szBuffer[0] =='1');

    ②小时数字是0不显示的处理，见DisplayTwoDigits函数。

    //获取是否显示小时的第1个0.

     GetLocaleInfo(LOCALE_USER_DEFAULT,LOCALE_ITLZERO, szBuffer, 2);

     fSuppress= (szBuffer[0] == '0');

【DigClock程序】
【效果图】

/*------------------------------------------------------------
DIGCLOCK.C -- Digital Clock
(c) Charles Petzold, 1998
------------------------------------------------------------*/
#include <windows.h>
#define ID_TIMER 1
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    PSTR szCmdLine, int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("DigClock");
    HWND         hwnd;
    MSG          msg;
    WNDCLASS     wndclass;
    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0;
    wndclass.cbWndExtra = 0;
    wndclass.hInstance = hInstance;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName = NULL;
    wndclass.lpszClassName = szAppName;
    if (!RegisterClass(&wndclass))
    {
        MessageBox(NULL, TEXT("This program requires Windows NT!"),
            szAppName, MB_ICONERROR);
        return 0;
    }

    hwnd = CreateWindow(szAppName,                  // window class name
        TEXT("Digital Clock"), // window caption
        WS_OVERLAPPEDWINDOW,        // window style
        CW_USEDEFAULT,              // initial x position
        CW_USEDEFAULT,              // initial y position
        CW_USEDEFAULT,              // initial x size
        CW_USEDEFAULT,              // initial y size
        NULL,                       // parent window handle
        NULL,                       // window menu handle
        hInstance,                  // program instance handle
        NULL);                     // creation parameters

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}
void DisplayDigit(HDC hdc, int iNumber)
{
    static BOOL fSevenSegment[10][7] = {
        1, 1, 1, 0, 1, 1, 1, //0
        0, 0, 1, 0, 0, 1, 0, //1
        1, 0, 1, 1, 1, 0, 1, //2
        1, 0, 1, 1, 0, 1, 1, //3
        0, 1, 1, 1, 0, 1, 0, //4
        1, 1, 0, 1, 0, 1, 1, //5
        1, 1, 0, 1, 1, 1, 1, //6
        1, 0, 1, 0, 0, 1, 0, //7
        1, 1, 1, 1, 1, 1, 1, //8
        1, 1, 1, 1, 0, 1, 1 };//9
    //定义数字8的每个笔划（共7笔，每笔形状为6边形）
    static POINT ptSegment[7][6] = {
        7, 6, 11, 2, 31, 2, 35, 6, 31, 10, 11, 10,       //第0笔
        6, 7, 10, 11, 10, 31, 6, 35, 2, 31, 2, 11,      //第1笔
        36, 7, 40, 11, 40, 31, 36, 35, 32, 31, 32, 11,  //第2笔
        7, 36, 11, 32, 31, 32, 35, 36, 31, 40, 11, 40,  //第3笔
        6, 37, 10, 41, 10, 61, 6, 65, 2, 61, 2, 41,     //第4笔
        36, 37, 40, 41, 40, 61, 36, 65, 32, 61, 32, 41, //第5笔
        7, 66, 11, 62, 31, 62, 35, 66, 31, 70, 11, 70 }; //第6笔

    for (int iSeg = 0; iSeg < 7; iSeg++)
    {
        if (fSevenSegment[iNumber][iSeg])
        {
            Polygon(hdc, ptSegment[iSeg], 6);
        }
    }
}
void DisplayTwoDigits(HDC hdc, int iNumber, BOOL fSuppress)
{
    //将0显示为“ 0”或“00”
    if (!fSuppress || (iNumber / 10 != 0))
        DisplayDigit(hdc, iNumber / 10);  //执行到这里，0显示为“00”
    OffsetWindowOrgEx(hdc, -42, 0, NULL);
    DisplayDigit(hdc, iNumber % 10);
    OffsetWindowOrgEx(hdc, -42, 0, NULL);//每个数字的宽度为42个像素
}
void DisplayColon(HDC hdc)
{
    POINT ptColon[2][4] = {
        2, 21, 6, 17, 10, 21, 6, 25,
        2, 51, 6, 47, 10, 51, 6, 55 };
    Polygon(hdc, ptColon[0], 4);
    Polygon(hdc, ptColon[1], 4);
    OffsetWindowOrgEx(hdc, -12, 0, NULL);  //冒号的宽度为12像素
}
void DisplayTime(HDC hdc, BOOL f24Hour, BOOL fSuppress)
{
    SYSTEMTIME st;
    GetLocalTime(&st);
    //显示小时，24小时格式变或12小时格式
    if (f24Hour)
    {
        DisplayTwoDigits(hdc, st.wHour, fSuppress);
    }
    else
    {
        //12小时格式，将01224，统一显示为12
        DisplayTwoDigits(hdc, (st.wHour %= 12) ? st.wHour : 12, fSuppress);
    }

    DisplayColon(hdc);
    //显示分钟
    DisplayTwoDigits(hdc, st.wMinute, FALSE);
    DisplayColon(hdc);
    //显示秒数
    DisplayTwoDigits(hdc, st.wSecond, FALSE);
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static BOOL f24Hour, fSuppress;
    static HBRUSH hBrushRed;
    static int cxClient, cyClient;
    TCHAR  szBuffer[2];
    HDC         hdc;
    PAINTSTRUCT ps;

    switch (message)
    {
    case WM_CREATE:
        SetTimer(hwnd, ID_TIMER, 1000, NULL);
        hBrushRed = CreateSolidBrush(RGB(255, 0, 0));
        //继续执行
    case WM_SETTINGCHANGE:
        //获得是否是24小时格式
        GetLocaleInfo(LOCALE_USER_DEFAULT, LOCALE_ITIME, szBuffer, 2);
        f24Hour = (szBuffer[0] == '1');
        //获取是否显示小时的第1个0.
        GetLocaleInfo(LOCALE_USER_DEFAULT, LOCALE_ITLZERO, szBuffer, 2);
        fSuppress = (szBuffer[0] == '0');
        InvalidateRect(hwnd, NULL, TRUE);
        return 0;
    case WM_SIZE:
        cxClient = LOWORD(lParam);
        cyClient = HIWORD(lParam);
        return 0;
    case WM_TIMER:
        InvalidateRect(hwnd, NULL, TRUE);
        return 0;
    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);
        SetMapMode(hdc, MM_ISOTROPIC); //各向同性
        SetWindowExtEx(hdc, 276, 72, NULL);//共6个数字，每个数字宽42，2对显号，每对宽12
        SetViewportExtEx(hdc, cxClient, cyClient, NULL);
        //输出中心移动到客户区中央
        SetWindowOrgEx(hdc, 138, 36, NULL);
        SetViewportOrgEx(hdc, cxClient / 2, cyClient / 2, NULL);
        SelectObject(hdc, GetStockObject(NULL_PEN));
        SelectObject(hdc, (HGDIOBJ)hBrushRed);
        DisplayTime(hdc, f24Hour, fSuppress);
        EndPaint(hwnd, &ps);
        return 0;

    case WM_DESTROY:
        KillTimer(hwnd, ID_TIMER);
        DeleteObject(hBrushRed);
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, message, wParam, lParam);
}

 8.3.2 模拟时钟

（1）坐标系统的设置——各向同性、y轴向上为正，坐标轴原点在客户区中央。

    ①逻辑窗口范围（1000,1000）

    ②视口范围（cxClient/2,cyClient/2）;

（2）坐标旋轴——用来画刻度和时分针

（3）stPrevious用来判断小时和分钟是否要更新。如果要，用白色画笔在之前的时间位置重绘一遍，相于当抹掉。如果不需要，只有秒针被抹掉。然后，用黑笔画所有指针一次。

【Clock程序】
效果图

/*------------------------------------------------------------
CLOCK.C -- Analog Clock Program
(c) Charles Petzold, 1998
------------------------------------------------------------*/
#include <windows.h>
#include <math.h>
#define  TWOPI (2*3.14159)
#define  ID_TIMER 1
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    PSTR szCmdLine, int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("Clock");
    HWND         hwnd;
    MSG          msg;
    WNDCLASS     wndclass;
    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0;
    wndclass.cbWndExtra = 0;
    wndclass.hInstance = hInstance;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName = NULL;
    wndclass.lpszClassName = szAppName;
    if (!RegisterClass(&wndclass))
    {
        MessageBox(NULL, TEXT("This program requires Windows NT!"),
            szAppName, MB_ICONERROR);
        return 0;
    }

    hwnd = CreateWindow(szAppName,                  // window class name
        TEXT("Analog Clock"),      // window caption
        WS_OVERLAPPEDWINDOW,        // window style
        CW_USEDEFAULT,              // initial x position
        CW_USEDEFAULT,              // initial y position
        CW_USEDEFAULT,              // initial x size
        CW_USEDEFAULT,              // initial y size
        NULL,                       // parent window handle
        NULL,                       // window menu handle
        hInstance,                  // program instance handle
        NULL);                     // creation parameters

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}
void SetIsotropic(HDC hdc, int cxClient, int cyClient)
{
    SetMapMode(hdc, MM_ISOTROPIC);
    SetWindowExtEx(hdc, 1000, 1000, NULL); //逻辑高度和逻辑宽度
    SetViewportExtEx(hdc, cxClient / 2, -cyClient / 2, NULL); //y轴向上为正
    SetViewportOrgEx(hdc, cxClient / 2, cyClient / 2, NULL); //坐标原点位于客户区中央
}
void RotatePoint(POINT pt[], int iNum, int iAngle)
{
    POINT ptTemp;
    for (int i = 0; i < iNum; i++)
    {
        ptTemp.x = (int)(pt[i].x*cos(iAngle*TWOPI / 360) + pt[i].y*sin(iAngle*TWOPI / 360));
        ptTemp.y = (int)(pt[i].y*cos(iAngle*TWOPI / 360) - pt[i].x*sin(iAngle*TWOPI / 360));
        pt[i] = ptTemp;
    }
}
//画针面的刻度
void DrawClock(HDC hdc)
{
    POINT pt[3];
    for (int iAngle = 0; iAngle < 360; iAngle += 6)
    {
        pt[0].x = 0;
        pt[0].y = 900;
        RotatePoint(pt, 1, iAngle);

        pt[2].x = pt[2].y = (iAngle % 5) ? 33 : 100; //圆的大小（直径）

        //Ellipse左上角的点
        pt[0].x -= pt[2].x / 2;
        pt[0].y -= pt[2].y / 2;
        //Ellipse右下角的点
        pt[1].x = pt[0].x + pt[2].x;
        pt[1].y = pt[0].y + pt[2].y;

        SelectObject(hdc, GetStockObject(BLACK_BRUSH));
        Ellipse(hdc, pt[0].x, pt[0].y, pt[1].x, pt[1].y);
    }
}
void DrawHands(HDC hdc, SYSTEMTIME* pst, BOOL fChange)
{
    static POINT pt[3][5] = {
        0, -150, 100, 0, 0, 600, -100, 0, 0, -150, //时针
        0, -200, 50, 0, 0, 800, -50, 0, 0, -200,   //分针
        0, 0, 0, 0, 0, 0, 0, 0, 0, 800 };           //秒针
    int iAngle[3];
    POINT ptTemp[3][5];
    iAngle[0] = (pst->wHour * 30) % 360 + pst->wMinute / 2; //时针每小时30度。
    iAngle[1] = pst->wMinute * 6; //每分钟，分针走6度。
    iAngle[2] = pst->wSecond * 6; //每秒，秒针走6度。

    memcpy(ptTemp, pt, sizeof(pt));
    for (int i = fChange ? 0 : 2; i < 3; i++) //判断是否只画秒针
    {
        RotatePoint(ptTemp[i], 5, iAngle[i]);
        Polyline(hdc, ptTemp[i], 5);
    }
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    HDC         hdc;
    PAINTSTRUCT ps;
    static SYSTEMTIME stPrevious;
    BOOL fChange;
    SYSTEMTIME st;
    static int cxClient, cyClient;

    switch (message)
    {
    case WM_CREATE:
        SetTimer(hwnd, ID_TIMER, 1000, NULL);
        GetLocalTime(&st);
        stPrevious = st;
        return 0;
    case WM_SIZE:
        cxClient = LOWORD(lParam);
        cyClient = HIWORD(lParam);
        return 0;
    case WM_TIMER:
        GetLocalTime(&st);

        //小时和分钟是否改变
        fChange = st.wHour != stPrevious.wHour ||
            st.wMinute != stPrevious.wMinute;

        hdc = GetDC(hwnd);
        SetIsotropic(hdc, cxClient, cyClient);
        //将旧的时分秒针用白色擦掉
        SelectObject(hdc, GetStockObject(WHITE_PEN));
        DrawHands(hdc, &stPrevious, fChange);
        //画新的时分秒针
        SelectObject(hdc, GetStockObject(BLACK_PEN));
        DrawHands(hdc, &st, TRUE);
        ReleaseDC(hwnd, hdc);
        stPrevious = st;
        return 0;
    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);
        SetIsotropic(hdc, cxClient, cyClient);
        DrawClock(hdc);
        DrawHands(hdc, &stPrevious, TRUE);

        EndPaint(hwnd, &ps);
        return 0;

    case WM_DESTROY:
        KillTimer(hwnd, ID_TIMER);
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, message, wParam, lParam);
}

8.4 在状态报告上使用计时器

（1）窗口固定大小：其大小从自定义的FindWindowSize函数求得。窗口风格WS_BORDER。

（2）创建屏幕DC，程序可以从屏幕任何地方获得当前鼠标指针位置的像素颜色。

【WhatClr程序】

效果图

/*------------------------------------------------------------
WHATCLR.C -- Displays Color Under Cursor
(c) Charles Petzold, 1998
------------------------------------------------------------*/
#include <windows.h>
#define ID_TIMER 1
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
void FindWindowSize(int*, int*);
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    PSTR szCmdLine, int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("WhatClr");
    HWND         hwnd;
    MSG          msg;
    WNDCLASS     wndclass;
    int cxWindow, cyWindow;
    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc;
    wndclass.cbClsExtra = 0;
    wndclass.cbWndExtra = 0;
    wndclass.hInstance = hInstance;
    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName = NULL;
    wndclass.lpszClassName = szAppName;
    if (!RegisterClass(&wndclass))
    {
        MessageBox(NULL, TEXT("This program requires Windows NT!"),
            szAppName, MB_ICONERROR);
        return 0;
    }

    FindWindowSize(&cxWindow, &cyWindow);
    hwnd = CreateWindow(szAppName,                  // window class name
        TEXT("What Color"), // window caption
        WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU | WS_BORDER,        // window style
        CW_USEDEFAULT,              // initial x position
        CW_USEDEFAULT,              // initial y position
        cxWindow,                   // initial x size
        cyWindow,              // initial y size
        NULL,                       // parent window handle
        NULL,                       // window menu handle
        hInstance,                  // program instance handle
        NULL);                     // creation parameters

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}
void FindWindowSize(int* pcxWindow, int* pcyWindow)
{
    HDC hdcScreen;
    TEXTMETRIC tm;
    hdcScreen = CreateIC(TEXT("DISPLAY"), NULL, NULL, NULL); //屏幕hdc
    GetTextMetrics(hdcScreen, &tm);
    DeleteDC(hdcScreen);
    *pcxWindow = 2 * GetSystemMetrics(SM_CXBORDER) + 12 * tm.tmAveCharWidth; //宽度=2个边框加12个字符的宽度
    *pcyWindow = 2 * GetSystemMetrics(SM_CYBORDER) + GetSystemMetrics(SM_CYCAPTION) + 2 * tm.tmHeight;;
}
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static HDC hdcScreen;
    static COLORREF cr, crLast;
    POINT pt;
    TCHAR szBuffer[16];
    HDC         hdc;
    PAINTSTRUCT ps;
    RECT        rect;

    switch (message)
    {
    case WM_CREATE:
        hdcScreen = CreateDC(TEXT("DISPLAY"), NULL, NULL, NULL);
        SetTimer(hwnd, ID_TIMER, 100, NULL);
        return 0;
    case WM_TIMER:
        GetCursorPos(&pt);
        cr = GetPixel(hdcScreen, pt.x, pt.y);
        if (cr != crLast)
        {
            crLast = cr;
            InvalidateRect(hwnd, NULL, TRUE);
        }
        return 0;
    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);

        GetClientRect(hwnd, &rect);
        wsprintf(szBuffer, TEXT(" %02X %02X %02X "), GetRValue(cr), GetGValue(cr), GetBValue(cr));
        DrawText(hdc, szBuffer, -1, &rect,
            DT_SINGLELINE | DT_CENTER | DT_VCENTER);

        EndPaint(hwnd, &ps);
        return 0;

    case WM_DESTROY:
        DeleteDC(hdcScreen);
        KillTimer(hwnd, ID_TIMER);
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, message, wParam, lParam);
}