Chapter 1

The Start flashing menu item starts the Hello world! text in the center of the display flashing
and Stop flashing stops it. The flashing is implemented by an active object that creates
regular timer events and then handles them. Its event handling changes the visibility of the
Hello World! text and redraws the view.
We'll start by looking at CDelayedHello, a derived active object class that implements Set
Hello and its associated Cancel. Then I'll explain some of the underlying fundamentals,
before taking a closer look at CFlashingHello (which implements Start flashing and
Stop flashing) along with some other active object patterns.
class CDelayedHello : public CActive
{
public:
// Construct/destruct
static CDelayedHello* NewL();
~CDelayedHello();
// Request
void SetHello(TTimeIntervalMicroSeconds32 aDelay);
private:
// Construct/destruct
CDelayedHello();
void ConstructL(CEikonEnv* aEnv);
// From CActive
void RunL();
void DoCancel();
private:
RTimer iTimer; // Has
CEikonEnv* iEnv; // Uses
};
From the class declaration, you can see that CDelayedHello:
� Is-a active object, derived from CActive
� Has-a event generator – an RTimer, whose API we'll see below
� Includes an active-object request function, SetHello() that requests an event
from the RTimer
� Implements RunL(), to handle the event generated when the request completes
� Implements DoCancel(), to cancel any outstanding request.
All active object classes share this pattern. They are derived from CActive and implement
its RunL() and DoCancel() functions. They include an event generator and at least one
request function (Figure 17.2).