新增qwt源码版本

qwtdemo/qwt/qwt_system_clock.h

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+/* -*- mode: C++ ; c-file-style: "stroustrup" -*- *****************************
+ * Qwt Widget Library
+ * Copyright (C) 1997   Josef Wilgen
+ * Copyright (C) 2002   Uwe Rathmann
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the Qwt License, Version 1.0
+ *****************************************************************************/
+
+#ifndef QWT_SYSTEM_CLOCK_H
+#define QWT_SYSTEM_CLOCK_H
+
+#include "qwt_global.h"
+
+/*!
+  \brief QwtSystemClock provides high resolution clock time functions.
+
+  Sometimes the resolution offered by QTime ( millisecond ) is not accurate
+  enough for implementing time measurements ( f.e. sampling ).
+  QwtSystemClock offers a subset of the QTime functionality using higher
+  resolution timers ( if possible ).
+
+  Precision and time intervals are multiples of milliseconds (ms).
+
+  \note The implementation uses high-resolution performance counter on Windows,
+        mach_absolute_time() on the Mac or POSIX timers on other systems. 
+        If none is available it falls back on QTimer.
+*/
+
+class QWT_EXPORT QwtSystemClock
+{
+public:
+    QwtSystemClock();
+    virtual ~QwtSystemClock();
+
+    bool isNull() const;
+
+    void start();
+    double restart();
+    double elapsed() const;
+
+private:
+    class PrivateData;
+    PrivateData *d_data;
+};
+
+#endif