The LA Fox Developer Newsletter

October 1999

Communications (Con’t from page 3)

y=x

we only have one object, but two references to the same object.

As a result we can call a Print() method of an object based on

the class “Foo” by either of these lines of code:

x.Print()

y.Print()

This holds true not only for conventional memory variables, but

for object properties (which are simply variables scoped to the

object) as well.

In the next example I include a new method, TalkToMeO, that

will allow an object to “register itself with another object as the

recipient of messages from that object. TalkToMe() simply calls

the SetTarget() method, but passes the keyword THIS as the

function argument. The object is actually passing itself by

reference to another object, to enable that object to communi-

cate back. The following lines will instantiate the Messager

class in example six and demonstrate its capabilities.

SET PROCEDURE TO MsgEx6.PRG

Create Stan

oStanleyObject=CREATEOBJECT(”Messager”)

Create OIlie, target of his messages is Stan

oOliverObject=CREATEOBJECT(”Messager”,oStanleyobject)

a Set target of Stan’s messages to Ollie

oStanleyObject.SetTarget(oOhverObject)

oStanleyObject.name = “Stanley”

oOliverObject.name = “Oliver”

a Tell Ollie to say hello to Stan

oStanleyObject.SendMessage()

a Tell Stan to say hello to OHie

oOliverObject.SendMessage()

a Create Charlie, target of his messages is Stan

oCharlieObjectCREATEOBJECT(”Messager”,oStanleyObject)

Tell Stan to talk to Charlie

oCharlieObject.TalkToMe()

oCharlieObject.Name = “Charlie”

a Have Stan say hello to charlie

oCharlieObject.SendMessage()

Listing 6. MsgEr6.PRG.

a PROGRAM MsgEx6.PRG

*

Example 6

Demonstrates how an object can “address”

a a message to another object by sto~1ng a

*

reference to the target object’s variable name

as a property of the sending object

*

The sending object can then use normal

*

object syntax to communicate with the target

object.

a Also demonstrates how an object can “register”

a.

itself with another, so that that object can

send messages back.

DEFINE CLASS Messager AS custom

PROTECTED oTargetObject

oTargetObject = .NULL.

FUNCTION Init

LPARAMETERS toObject

This.SetTarget(toObject)

ENDFUNC

FUNCTION SendMessage()

IF

TYPE(”This.oTargetObject”) = “0”

AND I ISNULL(This.oTargetObject)

This.oTargetObject.SayHello()

ENDIF

ENDFUNC

FUNCTION SayHello

=MessageBox(”Hi there” +;

This.oTargetObject.Name +;

“I- My name is “+ This.Name,O,;

“Messaging Example 6”)

ENDFUNC

FUNCTION SetTarget

LPARAMETERS toObject

IF

TYPE(”to0bject”) = “0” AND!;

ISNULL(toObject)

This.oTargetObject = toObject

ELSE

This.olargetObject = .NULL.

ENDIF

ENDFUNC

FUNCTION TaIkToMe

IF

TYPE(”This.oTargetObject”) = “0”

AND I ISNULL(This.oTargetObject)

This.oTargetObject.SetTarget(This)

ENDIF

ENDFUNC

ENDDEF1NE

This example shows 1) that it is possible to establish or change

an object as a messaging recipient at run-time, 2) that it is

possible for an object to determine who it is receiving a mes-

sage from, and therefore 3) to set up a two-way communication

between two objects. In many cases, a formal two-way “conver-

sation” is not necessary, as an object’s methods can return

values (like any other function) to the calling object, which is

still another way that information can be passed between

objects. This is not really “messaging”, but is simpler than

setting up two-way messaging capabilities, and may be all that

is needed for recipient-to-sender communications. However,

there may be situations in which it may be necessary to set up

more complex “conversations”.

Another technique incorporated in this version of the Messager

class is checking for the existence of the target object before

sending a message to that object. Note that I use both TYPE()

and ISNULLO. This is because it is possible to destroy an

object without destroying its reference. In such a case the

object reference variable will still have a TYPE() of”O”, but its

value will be .NULL. To see this in operation, Create a class AS

CUSTOM, and include a ReleaseO method. This method will

have a single line:

RELEASE This

Instantiate the object using CREATEOBJECTO, then call its

Release() method. The object will be destroyed, but

(Con’t page 5)

Page 4