class UsbState

UsbState handles the state machine for interacting with the usb camera.

It handles the validation and communication with the USB device

when selecting an available stream, and starting and stopping that stream.

UsbState communicates with the device through a banjo interface to parent usb device.

Since the parent device may operate on a separate dispatcher, UsbState may exhibit one

asynchronous behavior: the request callback passed as an argument to StartStreaming may

be called using a different dispatcher than the one that called StartStreaming.

All other functions of UsbState operate synchronously.

The SetFormat function must be called successfully before a stream may be started or stopped.

In this case, a "stream" is defined as a series of calls to the request callback provided

by the StartStreaming function. Each call contains a usb_request_t, which points at a vmo with a

blob of data. Concatenated together, these data blobs make up the data portion of a single

video frame. The usb_header included in the usb_request_t contains information to indicate which

video frame is being filled.

State machine for UsbState:

STOPPED ---

<SetFormat

> ---> READY

<

------

| STOPPING (temporary)

<StartStreaming

> |

|

<StopStreaming

>

\

|/ |

STREAMING -------^

Here are the allowed state transitions.

Cells marked with ERROR will return an error if attempted, but

will not result in a state transition.

Initial State

| STOPPED | READY | STREAMING | STOPPING

SetFormat | READY | READY | Error! | Error!

StartStreaming| Error! | STREAMING | Error! | Error!

StopStreaming | Error! | Error! | STOPPING | Error!

The STOPPING state is a temporary state that transitions to

READY when stopping is complete.