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Project work within an OBP course on CTU FEL, Prague

Distributed chat in Smalltalk compatible with Java version

Oto Válek

CTU FEL Prague, summer term 2001/2002, classes on Tuesday 9:15 am

1. Summary

This is a Smalltalk (Visual Works) application, implementing a simple distributed chat. Therefore, no central server is needed for user communication. The chat is compatible with existing Java version, using a subset of its communication protocol.

Chat on Windows

2. Quick user reference

First use the Setup button to choose your nickname. You may wish to change the default value of port (6060). Either press Connect to connect to other user (enter his hostname or IP) or just wait for an incoming connection.
During conversation, observe the list of other users on the right.
At the end, use the Disconnect button. You may now join an other user or stop accepting incoming connection by pressing Close. Closing the main window does that automatically.

3. Running the application

The application is stored in a image file. To run it on your system with VisualWorks, use 
visual chat.im
You may also load parcel chat.pcl or source and execute 
Chat open

4. Application architecture overview

The application consists of 5 classes in Valeko category. Each non-trivial method has a short description of its purpose as a comment. TCP socket communication is provided by classes in NetClients parcels. Distribution image has been created with the RuntimePackager wizard.

Chat Main application class. Maintains references to other objects using "lazy initialization". Holds a list of open interfaces so that it may change their window titles and add lines to the main text area.
The changeRequest method is overriden to detect window closing and close listening port. Contains several methods, that Sender and Receiver use to update user interface.
Receiver Starts a server listening on the given port, which receives and processes messages in the background thread (forkAt: method of a block). There's a method for each protocol message. Each protocol message received is passed to all methods, which process only the particular one.
Sender Handles outgoing TCP connections - attaches application to the network and send protocol messages to other users from the HostList and the MESSAGE message also to himself. TCP errors, such as timeouts, are caught and result in the removal of the remote user from the HostList.
HostList Implements a user list using a Dictionary collection accessed by nickname and containing HostItem items. It uses dependency mechanism to notify the Chat class of any changes, allowing it to update the list in dialog.
Also stores information about my identity (nick, IP, port). Its contents are accessed using methods returning various ordered collections.
HostItem Holds information about one user : nickname, IP and port. It is able to print its contents to a stream.

5. Communication protocol

Communication protocol is character and line based, like many othes TCP using protocols (HTTP, TCP). Every command is handled in separate TCP connection and is followed by newline character. Unless otherwise noted, command reception is acknowledged by OK response followed by newline character. A list of commands follows:
GET Request for the user list. Command is sent by a new member before the CONNECT command. Host responds with the list of ITEM <nick> <host> <port> lines terminated by the OK line.
CONNECT <nick> <host> <port> Connection to the network. A new member sends this message to users acquired with GET command and anounces its identification. Others will add the new user to their user lists.
MESSAGE <nick> <host> <port> <text> A text message. A sender sends it to all the users, including himself. If the receiver doesn't know the sender, he stores him to his user list.
FILE <nick> <host> <port> <name> <size> Not implemented and ignored.
DISCONNECT <nick> Disconnection from the network. A sender sends it to all the users, excluding himself.

6. Files

Attached file: chat.zip - ZIP archive of the image, parcel, source and this manual