Weirdness Passing Messages
dburgun at dsl.pipex.com
Fri Mar 24 07:36:03 EST 2006
On 23 Mar 2006, at 22:00, J. Landman Gay wrote:
> This isn't to say it shouldn't be fixed, only that I can see how it
> would get mucked up. The way the sample scripts are written is
> fairly convoluted; I would never have thought to do the job this
> way. I suspect that is why this problem has never been reported in
> all these years, since as Richard mentioned, it really takes a very
> specific setup to reproduce this problem.
> A much better approach would be to move all the handlers into the
> stack script and take advantage of the natural message hierarchy,
> rather than relying on all those "sends".
This is how I used to do it and the problem with that is that you
need to change a lot of scripts etc. in order to build a new app or
to extend an existing app. What I am trying to achieve is the ability
to "drop" groups into a stack (via paste) and have them just work
with no code fiddling! You just saw a demo of the problem not the
real app. I have developed a message passing library stack that
allows this to happen. Since I was going do donate this stack to
RevOnline once I have tested and improved it, I thought I'd explain
how it works and what I am hoping to achieve now (since we are on the
subject!) so as to get some input and ideas.
I have a Stack called "StackISM" (Inter-Stack Messaging), this is
accessed from the main stack by use of the "start using" statement.
When a Stack instantiates "StackISM" the initialization routine (in
StackISM) walks thru the objects (all objects in all cards) of the
calling stack looking for a Handler called "ISM_Initialize", if found
it calls it.
The "ISM_initialize" handler in the object of the calling stack does
any object specific initialization and may then call
"ISM_ListenForMessages(theMessageID)" if it wants to receive message
of type "theMessageID". It may call "ISM_ListenForMessages" multiple
times, once for each message it wants to receive.
When an event happens in an object (e.g. mouseUp in a button) that
needs action to be performed by other objects, "ISM_PutMessage
(theMessageID,theSubCode,theMessageData)" is called. This will send a
message(s) to all "listening" objects.
As a "real" example of this in action, take the following case. I
have a simple Group that just gets a Folder from the user. The
intention is that the folder will be used as a base by other groups/
objects. I have another Group that lists the files in a folder in a
field, and yet another group that does something with a file selected
in the "File List" Group.
Here's how it works:
GroupFolder: ISM_Initialize - Sets the Folder to empty and calls
ISM_PutMessage("ISM_FolderSelected", kFolderKind, FolderPathName
GroupFolder:ButtonChooseFolder:mouseUp - Gets the folder selected by
the user and calls ISM_PutMessage("ISM_FolderSelected", kFolderKind,
GroupFiles:FieldFileList: ISM_Initialize - calls "ISM_LIstenForMessage
theMessageKind, theFolderPathName) - Gets a list of the files in the
specified folder and puts them into "me" (hence the problem that
started this thread!).
GroupFiles:FieldFileList:mouseUp - Gets line selected (file name)
from "me" and then calls ISM_PutMessage("ISM_FileSelected",
GroupProcessFile:FieldFileContents: ISM_Initialize - calls
theMessageKind, theFilePathName) - Put the contents of the file into
This is the basic mode of operation, I will do some better
documentation when I release it to RevOnline.
Incidentally, the above works just fine regardless of the stack/card/
object using "put <something> into me". The problem I described
yesterday was brought to light because when a new folder is selected,
I wanted to select the first line (file) in the list, so inside
ISM_FolderSelect(), I did a ISMPutMessage("ISM_FileSelected,
kFileKind, line 1 of me), when this happened, "me" in the receiving
handler got Beswick'ed!
The really good thing about this is, say I wanted to extended this
app, so it had two base folders (maybe to to give a list of the files
in two folders and then compare them) and two file lists. All I need
do is to paste GroupFolder twice and GroupFiles twice and change a
custom property in the second folder and second file Group and hey
presto it works! No changing object names! No fiddling around duping
The custom property of each object contains the "kind", this is used
to identify the origin of the Message and Message Listeners only get
the "kind" they are listening for!
Although this may be thought of as convoluted at the atomic level, if
you take a system-wide view of it, it's really much less convoluted
than doing it using separate scripts in a lots of different objects
and hard-coding object names.
All the Best
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