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<concept id="GUID-2161BD64-889B-5EAB-B023-1162FE9619DB" xml:lang="en"><title>How
to use multiple active objects</title><shortdesc>This document illustrates how to use one active object to control
another.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>The following example code fragments show how to construct a program with
two active objects, where one controls the initiation and cancellation of
the other.</p>
<p>In these examples:</p>
<p><codeph>CActiveConsole</codeph> is an active object and contains a pointer
to a <codeph>CConsoleBase</codeph> object which is a service provider. Through
this service provider, the active object provides the facility to request
a character from the keyboard.</p>
<p><codeph>RunL()</codeph> delegates the handling of completed requests to
the pure virtual function <codeph>ProcessKeyPress()</codeph>, which must be
provided by a derived class.</p>
<p><codeph>CMessageKeyProcessor</codeph> is further derived from <codeph>CActiveConsole</codeph> and
contains a pointer to another active object <codeph>CExampleActiveObject</codeph>,
which requires input from the keyboard.</p>
<p>Depending on the input character, the <codeph>CMessageKeyProcessor</codeph> active
object does one of the following:</p>
<ul>
<li id="GUID-9151F698-3D4F-5C3D-A758-194018FC053C"><p>issues a request to
the <codeph>CExampleActiveObject</codeph> active object</p> </li>
<li id="GUID-FBF8743F-0147-502B-BBE8-824DB727F791"><p>cancels any outstanding
request to the <codeph>CExampleActiveObject</codeph> active object</p> </li>
<li id="GUID-2E714DE6-AAE1-5926-8026-9B6E1C09C5C7"><p>does nothing</p> </li>
</ul>
<p>The implementation of the <codeph>CExampleActiveObject</codeph> active
object is not relevant to the example and is not shown.</p>
<p>The following diagram shows the relationship between the classes.</p>
<fig id="GUID-AF998428-D142-56F8-8967-AF66AF9119F8">
<image href="GUID-BEDF6765-4BF4-532F-A2C7-F052FE8A9CA2_d0e237990_href.png" placement="inline"/>
</fig>
<section id="GUID-5E2A6378-546D-46C5-8081-6425A96FAAFF"><title>Encapsulating the service provider</title> <p>The class <codeph>CActiveConsole</codeph> encapsulates
the provision of basic keyboard services. Its <codeph>iConsole</codeph> data
member is a pointer to the service provider, the <codeph>CConsoleBase</codeph> object.</p> <p>The
active object class is defined as:</p> <codeblock id="GUID-DC98E432-1927-55F5-94A3-68D2493B75E8" xml:space="preserve">class CActiveConsole : public CActive
    {
public:
    CActiveConsole(CConsoleBase* aConsole);
    void ConstructL();
    ~CActiveConsole();
    void RequestCharacter();
    void RunL();
    void DoCancel();
    virtual void ProcessKeyPress(TChar aChar)=0;
protected:
    CConsoleBase* iConsole; 
    };</codeblock> <p>The class constructor takes a pointer to a <codeph>CConsoleBase</codeph> object
as its single argument and initializes its <codeph>iConsole</codeph> data
member to this value:</p> <codeblock id="GUID-2C5C49DB-AE88-5F84-82D7-BA5680A286C8" xml:space="preserve">CActiveConsole::CActiveConsole(CConsoleBase* aConsole)
    : iConsole(aConsole)
    {}</codeblock> <p>The <codeph>ConstructL()</codeph> function adds the
active object to the active scheduler:</p> <codeblock id="GUID-CE947B19-9664-5774-ABFB-17E79A112131" xml:space="preserve">void CActiveConsole::ConstructL()
    {
    CActiveScheduler::Add(this);
    }</codeblock> <p>The destructor cancels any outstanding request before
destroying the active object:</p> <codeblock id="GUID-0120EE92-2BD8-5B2E-841A-DC366B3BED12" xml:space="preserve">CActiveConsole::~CActiveConsole()
    {
    Cancel();
    }</codeblock> <p><codeph>DoCancel()</codeph> is implemented to cancel
the request to <codeph>iConsole</codeph>.</p> <p>The <codeph>RequestCharacter()</codeph> function
makes a request for a key press to the service provider by calling <codeph>iConsole-&gt;Read(iStatus)</codeph> and
setting the active request flag:</p> <codeblock id="GUID-EF731B8D-7001-503F-8167-7A10697D5202" xml:space="preserve">void CActiveConsole::RequestCharacter()
    {
    iConsole-&gt;Read(iStatus);
    SetActive();
    }</codeblock> <p>The <codeph>RunL()</codeph> function makes a call to
the <codeph>ProcessKeyPress()</codeph> function. This is a pure virtual function
that derived classes must implement to handle the key press and to reissue
the request:</p> <codeblock id="GUID-DF9F07BB-2099-5310-8921-3A290580B4AF" xml:space="preserve">void CActiveConsole::RunL()
    {
    ProcessKeyPress(TChar(iConsole-&gt;KeyCode()));
    }</codeblock> </section>
<section id="GUID-FFD3321F-3A46-4C40-A779-39C142DC79C3"><title>Further deriving from the active object</title> <p>The class <codeph>CMessageKeyProcessor</codeph> is
a concrete class, derived from <codeph>CActiveConsole</codeph>. It provides
an implementation for the<codeph>ProcessKeyPress()</codeph> function and can
issue or cancel requests to a<codeph>CExampleActiveObject</codeph> active
object.</p> <p>This active object class is defined as:</p> <codeblock id="GUID-C5F51E0D-517C-523D-BADA-4B3CDCA53073" xml:space="preserve">class CMessageKeyProcessor : public CActiveConsole
    {
public:
    ...
    CMessageKeyProcessor(CConsoleBase* aConsole, CExampleActiveObject* iExampleObject);
    void ProcessKeyPress(TChar aChar);
private:
    CExampleActiveObject* iExampleObject;
    };</codeblock> <p><b>Notes</b> </p> <ul>
<li id="GUID-2341BA65-3806-5354-9F9D-EB1A8FD3FD61"><p>The first constructor
parameter specifies a <codeph>CConsoleBase</codeph> which will be used to
provide asynchronous keyboard input.</p> </li>
<li id="GUID-E9CAAF35-C404-5418-9720-F560C4CED53D"><p>the second constructor
parameter specifies a <codeph>CExampleActiveObject</codeph> which will be
controlled by this <codeph>CMessageKeyProcessor</codeph>.</p> </li>
</ul> <p>The behaviour of the <codeph>ProcessKeyPress()</codeph> function
depends on the key code value:</p> <codeblock id="GUID-8081A453-4D44-5F2B-BAF3-7A43A1417BEF" xml:space="preserve">void CMessageKeyProcessor::ProcessKeyPress(TChar aChar)
    {
    if (aChar == 'm' || aChar == 'M')
        {
        iExampleObject-&gt;Cancel();
        iExampleObject-&gt;IssueRequest();
        }
    if (aChar == 'c' || aChar == 'C')
        {
        iExampleObject-&gt;Cancel();
        }
    if (aChar != EKeyEscape)
        {
        RequestCharacter();
        }
    else
        {
        iExampleObject-&gt;Cancel();
        CActiveScheduler::Stop();
        }
    }</codeblock> </section>
<section id="GUID-91B9EF85-A595-479D-8D57-40CC427C19E7"><title>Enhanced framework</title> <p>In the code fragment below,
an active scheduler is created to which both a <codeph>CMessageKeyProcessor</codeph> active
object and a<codeph>CExampleActiveObject</codeph> active object are added:</p> <codeblock id="GUID-830AA24C-7781-505A-9DD1-4A042040688D" xml:space="preserve">LOCAL_C void doExampleL()
    {
    CActiveScheduler* exampleScheduler=new (ELeave) CActiveScheduler;
    CleanupStack::PushL(exampleScheduler);
    CActiveScheduler::Install(exampleScheduler);

    CExampleActiveObject* iExampleObject =
            CExampleActiveObject::NewLC();
    CMessageKeyProcessor* keyProcessor=
             CMessageKeyProcessor::NewLC(console, iExampleObject);

    keyProcessor-&gt;RequestCharacter();
    CActiveScheduler::Start();
    CleanupStack::PopAndDestroy(3);
    }</codeblock> <p><b>Notes</b> </p> <ul>
<li id="GUID-1FB2DB58-C282-5B21-A070-DEC020E98059"><p>An instance of the active
scheduler, <codeph>exampleScheduler</codeph> is pushed onto the cleanup stack
and installed as the current active scheduler.</p> </li>
<li id="GUID-3404475D-4044-5DB4-BEDF-5A308B2521CA"><p>An instance of the <codeph>CExampleActiveObject</codeph> active
object is created.</p> </li>
<li id="GUID-F7C3C84D-66E5-5F8D-8213-8DC3864D659D"><p>An instance of the <codeph>CMessageKeyProcessor</codeph> active
object is created and this is in control.</p> </li>
<li id="GUID-DA61DFD2-B057-50C9-8FD1-C53E884A4A0E"><p><codeph>keyProcessor-&gt;RequestCharacter()</codeph> issues
a request for keyboard input.</p> </li>
<li id="GUID-5BFAEEED-051C-5741-BAA0-96AFC3EA5A02"><p><codeph>CActiveScheduler::Start()</codeph> starts
the active scheduler. At least one outstanding request is necessary before
the wait loop is started, otherwise the thread hangs. All further request
issuing and servicing occurs within this function. The wait loop continues
until one of the active objects’ <codeph>RunL()</codeph> calls<codeph>CActiveScheduler::Stop()</codeph>.</p> </li>
<li id="GUID-E3FB7738-DC6C-5472-8CBF-FAAD47C440D5"><p>The active objects and
the active scheduler are popped from the cleanup stack and destroyed.</p> </li>
</ul> </section>
</conbody></concept>