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Every request issued by an active object must complete exactly once It can complete normally or complete early as a result of an error or a call to Cancel() Let s rst examine what happens in a call to CActive::Cancel() and return to the other completion scenarios later CActive::Cancel() rst determines if there is an outstanding request and, if so, it calls the DoCancel() method, a pure virtual function in CActive, implemented by the derived class (which should not override the non-virtual base class Cancel() method) DoCancel() does not need to check if there is an outstanding request; if there is no outstanding request, Cancel() does not call it The encapsulated asynchronous service provider should provide a method to cancel an outstanding request and DoCancel() should call this method DoCancel() can include other processing, but it should not leave or allocate resources and it should not carry out any lengthy operations This is because Cancel() is itself a synchronous function which does not return until both DoCancel() has returned and the original asynchronous request has completed That is, having called DoCancel(), CActive::Cancel() then calls User::WaitForRequest(), passing in a reference to its iStatus member variable It is blocked until the asynchronous service provider posts a result (KErrCancel) into it, which should happen immediately, as described above The cancellation event is thus handled by the Cancel() method of the active object rather than by the active scheduler Finally, Cancel() resets the iActive member of the active object to re ect that there is no longer an asynchronous request outstanding The Cancel() method of the CActive base class performs all this generic cancellation code When implementing a derived active object class, you only need to implement DoCancel() to call the appropriate cancellation function on the asynchronous service provider and perform any cleanup necessary You most certainly should not call User::WaitForRequest(), since this will upset the thread semaphore count Internally, the active object must not call the protected DoCancel() method to cancel a request; it should call CActive::Cancel(), which invokes DoCancel() and handles the resulting cancellation event When an active object request is cancelled by a call to Cancel(), the RunL() event handler does not run This means that any post-cancellation cleanup must be performed in DoCancel() rather than in RunL()
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At this point, we can summarize the ways in which a request issued from an active object to an asynchronous service provider can complete: The request is issued to the asynchronous service provider by the active object Some time later, the asynchronous service provider calls User::RequestComplete() which generates a completion event and passes back a completion result The active scheduler detects the completion event, resumes the thread and initiates event handling on the highest priority active object that has iActive set to ETrue and iStatus set to a value other than KRequestPending This is a normal case, as described in the walkthrough above, although the completion result may not re ect a successful outcome The asynchronous request cannot begin, for example if invalid parameters are passed in or insuf cient resources are available The asynchronous service provider should de ne a function that neither leaves nor returns an error code (it should typically return void) Thus, under these circumstances, the request should complete immediately, posting an appropriate error into the TRequestStatus object passed into the request function The request is issued to the asynchronous service provider and Cancel() is called on the active object before the request has completed The active object calls the appropriate cancellation function on the asynchronous service provider, which should terminate the request immediately The asynchronous service provider should complete the request with KErrCancel as quickly as possible, because CActive::Cancel() blocks until completion occurs The request is issued to the asynchronous service provider and Cancel() is called on the active object some time after the request has completed This occurs when the completion event has occurred but is yet to be processed by the active scheduler The request appears to be outstanding to the active object framework, if not to the asynchronous service provider, which simply ignores the cancellation call CActive::Cancel() discards the normal completion result
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