CActive Class Reference

#include <e32base.h>

Link against: euser_v5.lib

class CActive : public CBase

Inherits from

Detailed Description

The core class of the active object abstraction.

It encapsulates both the issuing of a request to an asynchronous service provider and the handling of completed requests. An application can have one or more active objects whose processing is controlled by an active scheduler.

Member Attribute Documentation


The request status associated with an asynchronous request.

This is passed as a parameter to all asynchronous service providers.

The active scheduler uses this to check whether the active object's request has completed.

The function can use the completion code to judge the success or otherwise of the request.

Member Enumeration Documentation

Enum TPriority

Defines standard priorities for active objects.


A low priority, useful for active objects representing background processing.


A priority higher than EPriorityIdle but lower than EPriorityStandard.


Most active objects will have this priority.


A priority higher than EPriorityStandard; useful for active objects handling user input.


A priority higher than EPriorityUserInput.

Constructor & Destructor Documentation

CActive ( TInt )


Constructs the active object with the specified priority.

Derived classes must define and implement a constructor through which the priority can be specified. A typical implementation calls this active object constructor through a constructor initialization list.

aPriorityAn integer specifying the priority of this active object. CActive::TPriority defines a standard set of priorities.

~CActive ( )


Frees resources prior to destruction.

Specifically, it removes this active object from the active scheduler's list of active objects.

Typically, a derived class calls Cancel() in its destructor.

See also: CActive::Cancel

Panic Codes
E32USER-CBase40 if the active object has an outstanding request when the destructor is called,

Member Function Documentation

Cancel ( )

IMPORT_C voidCancel()

Cancels the wait for completion of an outstanding request.

If there is no request outstanding, then the function does nothing.

If there is an outstanding request, the function:

1. calls the active object's DoCancel() function, provided by the derived class to implement cancellation of the request.

2. waits for the cancelled request to complete; this must complete as fast as possible.

3. marks the active object's request as complete (i.e. the request is no longer outstanding).

See also: CActive::DoCancel CActive::IsActive CActive::~CActive User::WaitForRequest

Deque ( )

IMPORT_C voidDeque()

Removes the active object from the active scheduler's list of active objects.

Before being removed from the active scheduler's list, the function cancels any outstanding request.

See also: CActive::Cancel

DoCancel ( )

voidDoCancel()[protected, pure virtual]

Implements cancellation of an outstanding request.

This function is called as part of the active object's Cancel().

It must call the appropriate cancel function offered by the active object's asynchronous service provider. The asynchronous service provider's cancel is expected to act immediately.

DoCancel() must not wait for event completion; this is handled by Cancel().

See also: CActive::Cancel

Extension_ ( TUint, TAny *&, TAny * )

IMPORT_C TIntExtension_(TUintaExtensionId,
TAny *&a0,
TAny *a1
)[protected, virtual]

Reimplemented from CBase::Extension_(TUint,TAny *&,TAny *)

Extension function

IsActive ( )

TBool IsActive()const [inline]

Determines whether the active object has a request outstanding.

A request is outstanding when:

1. it has been issued

2. it has not been cancelled

3. it servicing has not yet begun.

Return Value
True, if a request is outstanding; false, otherwise.

IsAdded ( )

TBool IsAdded()const [inline]

Determines whether the active object has been added to the active scheduler's list of active objects.

If the active object has not been added to a scheduler, it cannot handle the completion of any request. No request should be issued until the active object has been added to a scheduler because completion of that request generates what appears to be a stray signal.

Use the active object function Deque() to remove the active object from the scheduler.

See also: CActive::Deque

Return Value
True, if the active object has been added to an active scheduler; false, otherwise.

Priority ( )

TInt Priority()const [inline]

Gets the priority of the active object.

Return Value
The active object's priority value.

RunError ( TInt )

IMPORT_C TIntRunError(TIntaError)[protected, virtual]

Handles a leave occurring in the request completion event handler RunL().

The active scheduler calls this function if this active object's RunL() function leaves. This gives this active object the opportunity to perform any necessary cleanup.

A derived class implementation should handle the leave and return KErrNone. Returning any other value results in the active scheduler function CActiveScheduler::Error() being called.

The default implementation simply returns the leave code.

Note that if the active scheduler is to handle the error, a suitably derived CActiveScheduler::Error() function must be supplied.

See also: CActiveScheduler::Error

aErrorThe leave code
Return Value
The default implementation returns aError. A derived class implementation should return KErrNone, if it handles the leave; otherwise it should return any suitable value to cause the handling of the error to be propagated back to the active scheduler.

RunL ( )

voidRunL()[protected, pure virtual]

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.

Before calling this active object's RunL() function, the active scheduler has:

1. decided that this is the highest priority active object with a completed request

2. marked this active object's request as complete (i.e. the request is no longer outstanding)

RunL() runs under a trap harness in the active scheduler. If it leaves, then the active scheduler calls RunError() to handle the leave.

Note that once the active scheduler's Start() function has been called, all user code is run under one of the program's active object's RunL() or RunError() functions.

See also: CActiveScheduler::Start CActiveScheduler::Error CActiveScheduler::WaitForAnyRequest TRAPD

SetActive ( )

IMPORT_C voidSetActive()[protected]

Indicates that the active object has issued a request and that it is now outstanding.

Derived classes must call this function after issuing a request.

A request is automatically marked as complete (i.e. it is no longer outstanding) by:

1. the active scheduler, immediately before it calls the active object's RunL() function.


2. the active object within the implementation of the Cancel() function.

E32USER-CBase 46 panics may occur if an active object is set active but no request is made on its TRequestStatus, or vice-versa. This panic happens no earlier than the next time that the active scheduler assesses which objects are ready to run, and may happen much later. This panic is termed a 'stray event' because it indicates that some entity has sent an event to the active scheduler thread, but this thread is not in a state ready to handle it.

See also: CActive::IsActive CActive::RunL CActive::Cancel

Panic Codes
E32USER-CBase42 if this active object is already active
E32USER-CBase49 if this active object has not been added to the active scheduler.

SetPriority ( TInt )

IMPORT_C voidSetPriority(TIntaPriority)

Sets the priority of the active object.

aPriorityAn integer specifying the new priority of this active object. CActive::TPriority defines a standard set of priorities.
Panic Codes
E32USER-CBase50 if this function is called while a request is outstanding.