Using +CAsyncOneShot

diff -r 43e37759235e -r 51a74ef9ed63 Symbian3/SDK/Source/GUID-837EF355-9154-506B-AE4F-4290FC9DBDC9.dita --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Symbian3/SDK/Source/GUID-837EF355-9154-506B-AE4F-4290FC9DBDC9.dita Wed Mar 31 11:11:55 2010 +0100 @@ -0,0 +1,68 @@ + + + + + +Using +CAsyncOneShotThis document descrbes the use of the CAsyncOneShot class +in scheduling. +

An instance of a class derived from the CAsyncOneShot class +can be used to perform one-off processing when no higher-priority active objects +are ready to run.

While it has many uses, a common usage is to use it to unload a library. +A typical CAsyncOneShot derived class is defined as:

+class CLibUnloader : public CAsyncOneShot + { +public: + static CLibUnloader* NewL(RLibrary& aLibrary); + inline void Unload(); +protected: + CLibUnloader(); + ~CLibUnloader(); + virtual void RunL(); +private: + RLibrary iLib; + }; +

The static NewL() function takes a reference to an existing +open RLibrary object and creates a new CLibUnloader object:

+CLibUnloader* CLibUnloader::NewL(RLibrary& aLibrary) + { + CLibUnloader* u=new(ELeave)CLibUnloader; + u->iLib=aLibrary; + return u; + } +

The remaining functions are implemented as follows:

Constructor sets a low +active object priority
CLibUnloader::CLibUnloader() + :CAsyncOneShot(E_a_very_low_priority) + {}
Calling Unload() activates +the active object; the active scheduler will call its RunL() function +as soon as there are no other higher priority active objects ready to run.
void CLibUnloader::Unload() + { + Call(); + }
The active scheduler +calls RunL(), which, in this example, simply deletes the +active object. This causes the destructor to be called which closes the library. +It also has the effect of removing the active object from the active scheduler +through the CActive base class destructor.
void CLibUnloader::RunL() + { + delete this; + } CLibUnloader::~CLibUnloader() + { + Cancel(); + iLib.Close(); + }

In other uses of CAsyncOneShot, RunL() could +do more complex processing, and indeed could re-queue the active object with +another call to CAsyncOneShot::Call().

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