怎样获得高精度计时器
在Managed DirectX Kick Start第六章中作者用了一个Utility类来获取游戏中所需的精确时间间隔，但并没有对这个类是怎么实现的进行阐述，我在这里就对这个类的设计进行补充说明，顺便也了解一下在C#中如何调用Win32 API函数以及大师设计类的思想。
这个类保存在Utility.cs文件中，它实现了一个能精确到1毫秒的计时器。当然.NET中没有这样的函数，因此我们需要使用非托管的Win32函数，在这之前别忘了在using从句中加入
其实在DirectX的实例框架程序中也有个实现高精度时钟的类，它位于SDK Root/Samples/Managed/Common/dxmutmisc.cs或wdxmutmisc.cs(.NET2.0)中其名为FrameworkTimer，在理解了上面的Ulitity.Timer后相信理解FrameworkTimer也会变得简单许多吧
这个类保存在Utility.cs文件中，它实现了一个能精确到1毫秒的计时器。当然.NET中没有这样的函数，因此我们需要使用非托管的Win32函数，在这之前别忘了在using从句中加入
using
System.Runtime.InteropServices;
那么在让我们来看看我们要用到函数的原型
[System.Security.SuppressUnmanagedCodeSecurity] [DllImport(
"
kernel32
"
)]
private
static
extern
bool
QueryPerformanceFrequency(
ref
long
PerformanceFrequency);
2.BOOL QueryPerformanceCounter( [out] LARGE_INTEGER *lpPerformanceCount );
[System.Security.SuppressUnmanagedCodeSecurity] [DllImport(
"
kernel32
"
)]
private
static
extern
bool
QueryPerformanceCounter(
ref
long
PerformanceCount);
3.DWORD timeGetTime(VOID); //当我们的系统不支持高精度时使用它，以使程序在不同机器上运行时获得统一的时间系统。 该函数位于winmm.lib，它返回Windows开机时到当前以毫秒为单位时间值。它在C#中的声明如下：
[System.Security.SuppressUnmanagedCodeSecurity] [DllImport(
"
winmm.dll
"
)]
public
static
extern
int
timeGetTime();
public
enum
DirectXTimer 
{  Reset, //重置时钟 Start, //开启时钟 Stop, //暂停或停止时钟 Advance, //使时钟提前0.1秒 GetAbsoluteTime, //获得开机到当前的时间 GetApplicationTime, //获得最近一次时钟开启到当前的时间间隔 GetElapsedTime //获得两次调用间的时间间隔  }
;
这样做的好处是在下面的设计中只需要构造一个函数，使代码看起来更见解明了 接着让我们来好好理解一下所需要的静态成员变量，它们中大部分的含义从名字上就可以看出 我在这里只对一些含义不那么明了的量做一些解释。
//
以下以time结尾都是时刻值
private
static
bool
isTimerInitialized
=
false
;
private
static
bool
m_bUsingQPF
=
false
;
private
static
bool
m_bTimerStopped
=
true
;
private
static
long
m_llQPFTicksPerSec
=
0
;
private
static
long
m_llStopTime
=
0
;
private
static
long
m_llLastElapsedTime
=
0
;
private
static
long
m_llBaseTime
=
0
;
//
高精度时钟启动的时刻
//
以下是不使用高精度时钟时的相应变量
private
static
double
m_fLastElapsedTime
=
0.0
;
private
static
double
m_fBaseTime
=
0.0
;
private
static
double
m_fStopTime
=
0.0
;
public
static
float
Timer(DirectXTimer command)
{ if (!isTimerInitialized)  { isTimerInitialized = true; // Use QueryPerformanceFrequency() to get frequency of timer. If QPF is // not supported, we will timeGetTime() which returns milliseconds. long qwTicksPerSec = 0; m_bUsingQPF = QueryPerformanceFrequency(ref qwTicksPerSec); if (m_bUsingQPF) m_llQPFTicksPerSec = qwTicksPerSec;  } if (m_bUsingQPF) { double time; double fElapsedTime; long qwTime = 0; if (m_llStopTime != 0 && command != DirectXTimer.Start && command != DirectXTimer.GetAbsoluteTime) qwTime = m_llStopTime; //大部分正确使用的情况下，这个if条件不会被足 //这样做可增强程序的健壮性，以应付特殊情况 else QueryPerformanceCounter(ref qwTime);//取得该函被调用时的计数值 // Return the elapsed time if (command == DirectXTimer.GetElapsedTime) { fElapsedTime = (double) (qwTime - m_llLastElapsedTime) / (double) m_llQPFTicksPerSec; m_llLastElapsedTime = qwTime; return (float)fElapsedTime; } // Return the current time if (command == DirectXTimer.GetApplicationTime) { double fAppTime = (double) (qwTime - m_llBaseTime) / (double) m_llQPFTicksPerSec; return (float)fAppTime; } // Reset the timer if (command == DirectXTimer.Reset) { m_llBaseTime = qwTime; m_llLastElapsedTime = qwTime; m_llStopTime = 0; m_bTimerStopped = false; return 0.0f; } // Start the timer if (command == DirectXTimer.Start) { if (m_bTimerStopped) m_llBaseTime += qwTime - m_llStopTime; m_llStopTime = 0; m_llLastElapsedTime = qwTime; m_bTimerStopped = false; return 0.0f; } // Stop the timer if (command == DirectXTimer.Stop) { //这个判断看上去很多于但可以处理错误的调用，初学者应该从中得到启示 if (!m_bTimerStopped) { m_llStopTime = qwTime; m_llLastElapsedTime = qwTime; m_bTimerStopped = true; } return 0.0f; } // Advance the timer by 1/10th second if (command == DirectXTimer.Advance) { m_llStopTime += m_llQPFTicksPerSec/10; return 0.0f; } if (command == DirectXTimer.GetAbsoluteTime) { time = qwTime / (double) m_llQPFTicksPerSec; return (float)time; } return -1.0f; // Invalid command specified } //这里开始是没有使用高精度时钟的代码，实现方法和上面几乎一样 else { // Get the time using timeGetTime() double time; double fElapsedTime; // Get either the current time or the stop time, depending // on whether we're stopped and what command was sent if (m_fStopTime != 0.0 && command != DirectXTimer.Start && command != DirectXTimer.GetAbsoluteTime) time = m_fStopTime; else time = timeGetTime() * 0.001; // Return the elapsed time if (command == DirectXTimer.GetElapsedTime) { fElapsedTime = (double) (time - m_fLastElapsedTime); m_fLastElapsedTime = time; return (float) fElapsedTime; } // Return the current time if (command == DirectXTimer.GetApplicationTime) { return (float) (time - m_fBaseTime); } // Reset the timer if (command == DirectXTimer.Reset) { m_fBaseTime = time; m_fLastElapsedTime = time; m_fStopTime = 0; m_bTimerStopped = false; return 0.0f; } // Start the timer if (command == DirectXTimer.Start) { if (m_bTimerStopped) m_fBaseTime += time - m_fStopTime; m_fStopTime = 0.0f; m_fLastElapsedTime = time; m_bTimerStopped = false; return 0.0f; } // Stop the timer if (command == DirectXTimer.Stop) { if (!m_bTimerStopped) { m_fStopTime = time; m_fLastElapsedTime = time; m_bTimerStopped = true; } return 0.0f; } // Advance the timer by 1/10th second if (command == DirectXTimer.Advance) { m_fStopTime += 0.1f; return 0.0f; } if (command == DirectXTimer.GetAbsoluteTime) { return (float) time; } return -1.0f; // Invalid command specified } }
|
其实在DirectX的实例框架程序中也有个实现高精度时钟的类，它位于SDK Root/Samples/Managed/Common/dxmutmisc.cs或wdxmutmisc.cs(.NET2.0)中其名为FrameworkTimer，在理解了上面的Ulitity.Timer后相信理解FrameworkTimer也会变得简单许多吧