Control Flow, Errors, and Event Handling

As we saw in Internal Architecture a flow stack is the basis of execution in SpeakRight. In this approach, pushing a flow object onto the stack is similar to making a sub-routine call. Popping a flow object off the stack is similar to returning from a sub-routine.

Execution of a sequence of flow objects is done by a flow object having a list of sub-flow objects. Every time its getNext method is called it returns the next object from the list.

Conditional flow is done by adding logic in getNext, to return one flow object or another based on some condition.

Looping is done by having getNext return its sub-flows more than once, iterating over them multiple times.

Error Handling

Callflows can have errors, such as user errors (failing to say anything recognizable) and application errors (missing files, db error, etc). SpeakRight manages error handling separately from the getFirst/getNext mechanism. getNext handles the error-free case. If an error occurs then of the IFlow error handling methods is called.

IFlow OnNoInput(current, results); //app was expecting input and none was provided by the user
IFlow OnException(current, results); //a generic failure such as exception being thrown.
IFlow OnDisconnect(current, results); //user terminated the interaction (usually by hanging up the phone. how in multimodal?)
IFlow OnHalt(current, results); //system is stopping
IFlow OnValidateFailed(current, results);

Note that a number of things that aren't really errors are handled this way. The goal is to keep the "nexting" logic clean, and handle everything else separately.

Errors are handled in a similar manner as exceptions; a search up the flow stack is done for an error handler. If the current flow doesn't have one, then its parent is tried. It's an runtime error if an error handler is not found.

The outermost flow is usually a class derived from SRApp. SRApp provides error handlers with default behaviour. They play a prompt indicating that a problem has occurred, and transfers the call to an operator.

Catch and Throw

The basic flow of control in a SpeakRight app is nesting of flow objects. These behave like subroutine calls; when the nested flow finishes, the parent flow resumes execution. Sometimes a non-local transfer of control is needed. SpeakRight supports a generic throw and catch approach. A flow can throw a custom flow event, which MUST be caught by a flow above it in the flow stack.

return new ThrowFlowEvent("abc");

and the catch looks like any other error handler

IFlow OnCatch(current, results, thrownEvent);

Note: like all other handlers a flow event can catch its own throw. May seem weird but this lets developers move code around easily.

Some control flow is possible in execute. If you want a flow to branch if a db error happens, then do this. However, in this case a flow object can not catch its own flow event, since that would cause Execute to be called again, and infinite recursion...

Update: See also Optional Sub-Flow Objects

GotoUrlFlow
The GotoUrlFlow flow object is used to redirect the VoiceXML browser to an external URL. It is used to redirect to static VoiceXML pages or to another application.