三个timer相关的类之一 - CPeriodic

 

class CPeriodic : public CTimer;

Description:

- Periodic timer active object.

- This class generates regular timer events and handles them with a callback function. The callback is specified as a parameter to Start().(周期性的产生定时时间)

- The callback may not be called immediately after the signal from the timer request has been generated, for the following reasons:

1. the RunL() of another active object may be running at the time of the signal

2. other active objects may have a higher priority than the CPeriodic

- If timing accuracy is important to your application, you can minimise the first problem by ensuring all RunL()s complete quickly, and can eliminate the second by giving the CPeriodic a higher priority than any other active object. Although it is generally recommended that timer-related active objects have a high priority, this will not address the problem of CPeriodic timers running behind, because active object scheduling is not pre-emptive. (如果需要精确定时的话，需要提高CPeriodic优先级或是使得RunL()尽可能的短)

- After a timer signal generated by a CPeriodic, the next signal is requested just before running the callback, and this request can be delayed for the same reasons that running the callback can be delayed. Therefore, a large number N of periods may add up to somewhat more than N times the requested period time. If absolute precision is required in tracking time, do not rely on counting the number of times the callback is called: read the value of the system clock every time you need it.

Construction and destruction:

static IMPORT_C CPeriodic *NewL(TInt aPriority);

- Allocates and constructs a CPeriodic object - leaving.

- Specify a high priority so the callback function is scheduled as soon as possible after the timer events complete.

protected: IMPORT_C CPeriodic(TInt aPriority);

- Classes derived from CPeriodic must define and provide a constructor through which the priority of the active object can be passed. Such a constructor can call CPeriodic's constructor in its constructor initialisation list.

Member functions:

void Start(TTimeIntervalMicroSeconds32 aDelay, TTimeIntervalMicroSeconds32 anInterval, TCallBack aCallBack);

- Starts generating periodic events.

- The event calls the protected RunL() function, which in turn calls the function specified by aCallBack. The first event is generated after aDelay microseconds; subsequent events are generated regularly thereafter at intervals of anInterval microseconds.( 时间到达后，AS会调用RunL()，其又会调用这个callback函数)

- The TCallBack contains a function pointer and a TAny* pointer. The function will be repeatedly called with the pointer as a parameter.

- Once started, periodic events are generated until the CPeriodic object is destroyed.

protected: virtual IMPORT_C void RunL();

- Handles an active object's request completion event.

- A derived class must provide an implementation to handle the completed request. If appropriate, it may issue another request.

- The function is called by the active scheduler when a request completion event occurs, i.e. after the active scheduler's WaitForAnyRequest() function completes.

Example:

// IMPLEMENTATION SPECIFIC CONSTANTS

const TInt KPeriodicTimerInterval5Sec(5000000);

class CMyClass : public CBase

{

    ....

private: // New functions

    /**

    * The call back function.

    * /param aAny A pointer to this class.

    */

    static TInt PeriodicTimerCallBack(TAny* aAny);

 

    /**

    * Notice that this is a sample fuction.

    */

    void SomeFunction();

 

private:   // Member data

    /**

    * The periodic timer.

    * Owned by CMyClass

    */

    CPeriodic* iPeriodicTimer;

};

 

CMyClass::~CMyClass()

{

    if (iPeriodicTimer)

    {

       // Calling Cancel without checking if the timer is active is safe

       iPeriodicTimer->Cancel();

    }

    delete iPeriodicTimer;

}

void CMyClass::ConstructL()

{

    // Initialize the periodic timer.

    iPeriodicTimer = CPeriodic::NewL(CActive::EPriorityIdle);

    // Start the periodic timer, when ever the time elapsed

    // the PeriodicTimerCallBack() will get called.

    // Notice: The timer will periodically hit the PeriodicTimerCallBack

    // until you cancel the timer by calling iPeriodicTimer->Cancel().

    iPeriodicTimer->Start(KPeriodicTimerInterval5Sec, KPeriodicTimerInterval5Sec,     TCallBack(PeriodicTimerCallBack, this));

}

TInt CMyClass::PeriodicTimerCallBack(TAny* aAny)

{

    CMyClass* self = static_cast<CMyClass*>( aAny );

   

    // TODO: The below call is a sample function,

    // you may change it to your requirement.

    self->SomeFunction();

   

    // Cancel the timer when the callback should not be called again.

    // Call: self->iPeriodicTimer->Cancel();

   

    return KErrNone; // Return value ignored by CPeriodic

}

void CMyClass::SomeFunction()

{

    // TODO: Your code!

}

CPeriodic 继承了CTimer所以做了一些必要的工作，比如调用基类的默认构造函数来设置优先级，调用AS的add方法等等。