The Message Passing Architecture

The Showcase Message Passing Architecture consists of the Message Queue, a Message Dispatcher and the Showcase Interpreters, as illustrated in the following diagram. These operate in the larger context of custom add-ins, and the running Showcase application with loaded scenes and models.

The simplest way to understand the Message Passing Architecture is to follow a Message as it moves through the system:

1. The add-in sends a message to Showcase following the established protocol. For example, to trigger a built-in menu item, the add-in would use the MENU_INVOKE_ITEM Message.
2. The Message is automatically placed into the Showcase Message Queue, along with Messages from other add-ins, and Showcase itself.
3. At the beginning of each re-draw, the Message Dispatcher services the queued Messages, passing them to the Showcase interpreters.

   Showcase makes use of a number of built-in interpreters, responsible for carrying out the actions indicated in the Messages. This is transparent to the add-in developer.

4. Response Messages flow in the other direction: from Showcase to the add-in. They are used to provide an indication of success or failure of the task to the add-in.

   It is the responsibility of your add-in to listen for Response Messages. Since the Message Passing Architecture is asynchronous and frame-based, it should not be assumed that an action has been carried out simply because your add-in has issued a Message.

Note the following:

• Messages are serviced by Showcase in the order received by the Message Queue.
• Messages are broadcast to all interpreters (including those instantiated by your own add-ins).
• Messages do not expire. Messages that cannot be serviced by Showcase in one frame are serviced in the next.
• To view Messages in real-time as they are handled by Showcase, run the version of Showcase that includes a console window (ShowcaseConsole.exe), and load the EchoMessagesCustom.py add-in included in the SDK.

For details see Sending Messages.