Re: [Fxruby-users] Message Identifiers, Types, etc again.

[Lyle J. <jl...@cf...>]

Hugh Sasse Staff Elec Eng wrote:

> Just when I thought I was no longer confused about this:
> 
> On Thu, 12 Jun 2003, Hugh Sasse Staff Elec Eng wrote:
> 
>> OK, reading this in conjunction with paragraphs 3 and 4 of
>>
>> http://www.fox-toolkit.org/messages.html
>>
>> it seems that message_id is actually the sender of the message, and
>> message type is actually the message itself (not counting associated
>> data).  I would have called the message itself the message_id,
> 
> Well, it is not as simple as that.  The FXRuby API docs for
> FXShutter, FXShutterItem, FXScrollbar and FXList at least refer to
> Message Identifiers which are constants, and are not names of the
> originating objects.  So should these be called Message Types for
> consistency with other FOX docs, or what?

Message identifiers are not the same things as message types. Message
identifiers *can* be used as a means of identifying the sender of a
message, but that's just one use for them.

Multiple objects can send the same type of message to a single receiver
for different reasons, and so one use for the identifier is to
distinguish between different messages of the same type. If one of your
GUI's buttons sends you a SEL_KEYPRESS message, you might react
differently than you would if a text field sent the same message type.

Following from that, a more general way to think about message
identifiers is simply as a way to distinguish between different
responses to the same type of message. Since you mentioned FXShutter in
your post, let's work with an example based on that widget. Suppose you
have a shutter widget:

     aShutter = FXShutter.new(...)

and you want to provide some ways to open the first shutter item. One of
those options that you're going to offer is a push button, somewhere on
the GUI, so you construct that button:

     openFirstButton = FXButton.new(...)

When a button is clicked, it sends a SEL_COMMAND message to its message
target. When this button (openFirstButton) is clicked, we want it to
send a message to the shutter, so let's make the shutter its message target:

     openFirstButton.setTarget(aShutter)

The ID_OPEN_FIRST message identifier tells the shutter to open its first
item, so let's make that the message identifier (or selector):

     openFirstButton.setSelector(FXShutter::ID_OPEN_FIRST)

So now, when the user clicks this button, it will send a (SEL_COMMAND,
ID_OPEN_FIRST) message to the shutter.

Let's say that you also want to provide a menu command (in a pulldown
menu) that does the same thing. First you'd construct the menu command:

     openFirstMenuCommand = FXMenuCommand.new(...)

and then set its target and identifier, as you did for the button:

     openFirstMenuCommand.setTarget(aShutter)
     openFirstMenuCommand.setSelector(FXShutter::ID_OPEN_FIRST)

Since a menu command sends a SEL_COMMAND message to its target when it
is selected, this has exactly the same result as the button click did.

Note that for this example, the message identifier (ID_OPEN_FIRST)
doesn't distinguish between different senders, it's just identifying
which command we want the shutter to perform.

Hope this helps,

Lyle