To build a DDE server application that does not use default processing:

If you follow this procedure, the DdeServerManager will not handle any requests from DDE clients. Methods must be written and callbacks hooked in order to provide service. This technique of providing DDE server services takes more work but allows for more flexibility.

Suppose, for example, that a user wants an application to make data available through DDE but there is so much data, or the data is so large, that it must be stored on secondary storage. When a DDE client requests data, the server must read the data from secondary storage and return it to the client. To do this the application must hook the DdeNrequestCallback callback and write a method to handle requests for data from a DDE client.

This example builds a DDE server that provides the time only in the String format.

First, create a class called DdeTimeServer2 that is a subclass of Object. It has the two instance variables described in Table 44.

Thus, our new class looks like this:

Second, create a class method new to create a new server. Also send the initialize method to the new object in order to set it up.

Third, create the initialize method. In the initialize method you first create the DdeServerManager and name it Timer. Next you hook up to the callbacks and turn the timer on.

At the end of initialization, turn the timer on by sending the timerOn: message, which in turn sends the timerSet message. These methods are implemented below. The timer uses the Common Widgets timer functionality so that when the timer goes off, the message timerProc: is sent to the application.

Next, create the callback methods that implement the behavior of this server. First you implement the callback for the DdeNinitiateCallback. The initiate:clientData:callData: method checks that the application name and the topic to which the client is attempting to connect match what the server is looking for. If they do match, then the application sends the notifyClientOfSupportFor: message to the DdeServer and sets the return value of the DdeCallbackData object to true. To reject the connect attempt, set the return value of the DdeCallbackData object to false. Notice that you are not attempting to handle an empty string for the application name or the topic name. The code should be embellished to handle this possible situation.

The code that follows is for the DdeNhotlinkCallback method. The method checks the item name and the format. It sets the DdeCallbackData object to true if the item is supported and false otherwise.

The method for handling the warmlink and coldlink has the same form as the method for the hotlink. Instead of implementing the code shown below, the method could test the reason value of the DdeCallbackData object (by way of the reason message) and print out different messages based on the value that is returned.

The next callback you implement is DdeNrequestCallback. This callback is run when the client requests data from the server. As in previous examples, you check whether the item and the format are what you are looking for, then send the data to the client by sending the message sendItem:value:format: to the DdeServer that was passed to the callback method. The return value of the DdeCallbackData object should be set to true if the data is sent to the client and false if the data is not sent to the client.

The last callback you implement is the DdeNterminationCallback. This method should not attempt to disconnect. It only prints to the Transcript that the server dropped a connection.

Now all initialization has been completed and all the callbacks have been set up. The main work of the server is now contained in the method that is run whenever the timer goes off. When the timer goes off and the timerProc: method is run, the one supported item is updated. As the item is updated the DdeServerManager looks for any hot- or warmlinks to the data and updates the clients that are linked to that piece of data.

The message: method that has been used throughout the example follows:

The final method that you need to implement is the free method. This method stops the timer and frees up the DdeServerManager.

You now have a DDE server that provides the current time in String format.

An improvement to the code might be to support the empty application name (server name) and an empty topic name as well as both the date and time in other formats.