diff -r 43e37759235e -r 51a74ef9ed63 Symbian3/SDK/Source/GUID-C73318E7-44E2-5F42-BDB0-AAE9BD72BF5C.dita --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/Symbian3/SDK/Source/GUID-C73318E7-44E2-5F42-BDB0-AAE9BD72BF5C.dita Wed Mar 31 11:11:55 2010 +0100 @@ -0,0 +1,63 @@ + + + + + +Using +TCallBackThis document describes the TCallBack class. +

The class encapsulates a pointer to a function which takes an argument +of type TAny* and returns a TInt. The class +is generally useful but, in particular, simplifies the programming interface +of the CIdle and CPeriodic classes.

+

Given a suitable function and a pointer to an object, a callback is constructed +simply. The function must be a non-member function or a static member of a +class. For example:

+TInt Foo(TAny *); // a non-member function +X* pX=new X; // a class X object +

or, as a static member of class X:

+TInt X::Foo(TAny *); // a static function of class X +X* pX=new X; // a class X object +

A callback function returns a true value to indicate whether it should +be called again. This is important when used with the CIdle and CPeriodic classes. +The following code fragment shows the programming paradigm:

+TCallBack cb(Foo,pX); // construction of the callback +for (;;) + { + if (!cb.CallBack()) // invoke callback until it returns + { // a false value + break; + } + } +

Calling cb.CallBack() results in a call to the callback +function Foo() passing it the pointer pX.

+

A common requirement is for the callback function to be a non-static member +of a class. This can be implemented by passing, to the callback function, +a pointer to an instance of the class of which it is a static member. For +example:

+class X + { + static X* NewL(); + static TInt Foo(TAny* pX); +private: + TInt DoFoo(); + ... + } +

where the static function Foo() is implemented as:

+static TInt X::Foo(TAny* pX) + { + return ((X*)pX)->DoFoo(); + } +

Typically, create an instance of class X and, at some +later stage, create the callback:

+... +X* pX = X::NewL(); +... +TCallBack cb(Foo,pX); +... + \ No newline at end of file