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There are actually two types of CxMessages.  The first, CXM_IEVENT,
corresponds to an input event and travels through the Commodities
Exchange network.  The other type, CXM_COMMAND, carries a command to
a commodity.  A CXM_COMMAND normally comes from the controller
program and is used to pass user commands on to a commodity.  A
commodity receives these commands through an Exec message port that
the commodity sets up before it calls CxBroker.  The NewBroker's
nb_Port field points to this message port.  A c ommodity can tell the
difference between the two types of CxMessages by calling the
CxMsgType() function.

    ULONG CxMsgType(CxMsg *cxm);
    UBYTE *CxMsgData(CxMsg *cxm);
    LONG  CxMsgID(CxMsg *cxm);

A CxMessage not only has a type, it can also have a data pointer as
well as an ID associated with it.  The data associated with a
CXM_IEVENT is an InputEvent structure.  By using the CxMsgData()
function, a commodity can obtain a pointer to the correspond ing
InputEvent of a CXM_IEVENT.  Commodities Exchange does not give an ID
to the CXM_IEVENT CxMessages it introduces to the Commodities network,
but certain CxObjects can assign an ID to a CXM_IEVENT CxMessage.

CXM_COMMAND CxMessages generally do not use the data pointer.
Instead, they use an ID.  They use this ID to specify the command
passed to a commodity.  CxMsgID() extracts the ID from a CxMessage.

    oldactivationvalue = LONG ActivateCxObj(CxObj *co,
                           long newactivationvalue);

After successfully completing the initial set up and activating the
broker using ActivateCxObj(), a commodity can begin its input
processing loop.  The example Broker.c receives input from one source,
the controller program.  The controller program sends a CxMessage each
time the user clicks its enable, disable, or kill gadgets.  Using the
CxMsgID() function, the commodity finds out what the command is and
executes it.

Notice that Broker.c uses Ctrl-C as a break key.  This is a change
from 1990 Atlanta DevCon Notes on Commodities Exchange which said to
use Ctrl-E.  The break key for any commodity should be Ctrl-C.

The commands that a commodity can receive from the controller program
(as defined in <libraries/commodities.h>) are:

    CXCMD_DISABLE     /* please disable yourself       */
    CXCMD_ENABLE      /* please enable yourself        */
    CXCMD_KILL        /* go away for good              */
    CXCMD_APPEAR      /* open your window, if you can  */
    CXCMD_DISAPPEAR   /* hide your window              */

The CXCMD_DISABLE, CXCMD_ENABLE, and CXCMD_KILL commands correspond
to the similarly named controller program gadgets, "Disable",
"Enable", and "Kill"; CXCMD_APPEAR and CXCMD_DISAPPEAR correspond to
the controller program gadgets, "Show" and "Hi de".  These gadgets
are ghosted in Broker.c because it has no window (It doesn't make
much sense to give the user a chance to click the "Show" and "Hide"
gadgets).  In order to do this, Broker.c has to tell Commodities
Exchange to ghost these gadge ts.  When CxBroker() sets up a broker,
it looks at the NewBroker.nb_Flags field to see if the COF_SHOW_HIDE
bit (from <libraries/commodities.h>) is set.  If it is, the "Show"
and "Hide" gadgets for this broker will be selectable. Otherwise they
are g hosted and disabled.

Shutting down a commodity is easy.  After replying to all CxMessages
waiting at the broker's message port, a commodity can delete its
CxObjects.  DeleteCxObj() removes a single CxObject from the
Commodities network.

    void DeleteCxObj(CxObj *co);

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