ACTIVE OBJECTS UNDER THE HOOD in Java

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ACTIVE OBJECTS UNDER THE HOOD
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If a single request has completed on the thread in the interim, the active scheduler performs lookup and calls the appropriate event handler on that active object If more than one request has completed in that time, the active scheduler calls the event handler for the highest priority active object It follows that, if multiple events are generated in close succession while another event is being handled, those events may not be handled in the sequence in which they occurred because the active object search list is ordered by priority to support responsive event-handling Normally, active object code should be designed so the priority does not matter, otherwise the system can become rather delicate and be thrown off balance by minor changes or additional active objects on the thread However, to be responsive, say for user input, it is sometimes necessary to use a higher priority value Long-running, incremental tasks, on the other hand, should have a lower priority than standard since they are designed to use idle processor time (as I ll describe later in this chapter) It s important to understand that the priority value is only an indication of the order in which the active scheduler performs lookup and eventhandling when multiple events have completed In contrast to the priority values of threads used by the kernel scheduler, it does not represent an ability to pre-empt other active objects Thus, if you assign a particular active object a very high priority, and it completes while a lower-priority active object is handling an event, no pre-emption occurs The RunL() of the lower-priority object runs to completion, regardless of the fact that it is holding up the handler for the higher-priority object On Symbian OS, you cannot use active object priorities to achieve a guaranteed response time; for this you must use the pre-emptive scheduling associated with threads1 , which is described in 10 If you have a large number of active objects in a single thread which complete often, they compete for their event handler to be run by the active scheduler If some of the active objects have high priorities and receive frequent completion events, those with lower priorities wait inde nitely until the active scheduler can call their RunL() methods In effect, it s possible to hang lower-priority active objects by adding them to an active scheduler upon which a number of high-priority active objects are completing
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1 The new hard real-time kernel in Symbian OS 80 can commit to a particular response time On earlier versions of Symbian OS, the kernel has soft real-time capabilities and cannot make such guarantees
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RESPONSIBILITIES OF AN ACTIVE OBJECT
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An active object s priority is only an indication of the order in which the active scheduler performs lookup and event-handling It does not re ect an ability to pre-empt other active objects
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92 Responsibilities of an Active Object
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Figure 91 illustrates the roles and actions of the active scheduler, active object and the asynchronous service provider It extends Figure 81
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Executable Active Object Active Scheduler Asynchronous Service Provider
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Create and Install the Active Scheduler
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Create Active Object (with appropriate priority) and add it to the Active Scheduler
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Issue a request to the Active Object
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Make a request to the Asynchronous Service Provider, passing in iStatus
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Sets iStatus=KRequestPending and starts the service
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Call SetActive()
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Start the Active Scheduler if it is not already started
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Wait Loop CActiveScheduler::Start()
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RunL() handles the completed event and resubmits another request or stops the Active Scheduler
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Calls RunL() on the Active Object with iStatus!=KRequestPen ding and iActive=ETrue
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Service completes and uses RequestComplete() to notify the Active Scheduler (by signalling its thread semaphore) and to post a completion result
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Roles and actions of the active scheduler, an active object and an asynchronous service provider
ACTIVE OBJECTS UNDER THE HOOD
It looks complex, but I ll explain how it all ts together throughout this chapter and you ll probably want to refer back to it later The following list summarizes the responsibilities of an active object: As I described in 8, the priority of an active object must be set on construction The priority generally defaults to EPriorityStandard (=0, from class CActive) or EActivePriorityDefault (=0 if using the TActivePriority enumeration de ned in coemainh for use with application code) This is the standard priority for an active object and should be used unless there is a good reason to set its priority to some other value, for example to EActivePriorityWsEvents (=100) for handling user input responsively An active object provides at least one method for clients to initiate requests to its encapsulated asynchronous service provider The active object always passes its own iStatus object to the asynchronous function, so does not need to include a TRequestStatus reference among the parameters to the request issuer method unless it is acting as a secondary provider of asynchronous services After submitting a request to an asynchronous service provider, the active object must call SetActive() upon itself This sets the iActive ag, which indicates an outstanding request This ag is used by the active scheduler upon receipt of an event and by the base class upon destruction, to determine whether the active object can be removed from the active scheduler An active object should only submit one asynchronous request at a time The active scheduler has no way of managing event-handling for multiple requests associated with one active object An active object should pass its iStatus object to an asynchronous service function It should not reuse that object until the asynchronous function has completed and been handled The active scheduler inspects the TRequestStatus of each active object to determine its completion state and the event-handling code uses the value it contains to ascertain the completion result of the function An active object must implement the pure virtual methods RunL() and DoCancel() declared in the CActive base class Neither method should perform lengthy or complex processing, to avoid holding up event handling in the entire thread This is particularly important in GUI applications where all user interface code runs in the same thread If any single RunL() is particularly lengthy, the user interface will be unresponsive to input and will freeze until that event handler has completed