Example of Android USB Host Asynchronous Bulk Transfer

Question

I am working with Android USB Host mode and would like to perform an asynchronous bulk transfer. I have so far been successfully using synchronous bulk transfers, but am having a little trouble grasping how the pieces come together for an asynchronous transfer. From the UsbRequest documentation (bold mine):

Requests on bulk endpoints can be sent synchronously via bulkTransfer(UsbEndpoint, byte[], int, int) or asynchronously via queue(ByteBuffer, int) and requestWait() [a UsbDeviceConnection method].

Ok, so does this mean I call queue() from the existing thread of execution and then requestWait() somewhere else in another thread? Where does requestWait() get my logic from to execute when the request completes? Most of the async work I have done has been in languages like Javascript and Python, generally by passing a callback function as an argument. In Java I was expected to perhaps pass an object that implements a specific method as a callback, but I can't see that happening anywhere. Perhaps my mental model of the whole thing is wrong.

Can someone provide an isolated example of sending an asynchronous bulk transfer?

Answer 1

Basically the requestWait() method is going to return once the queued UsbRequest has completed. You can do this on the same thread or on another. Use the setClientData() AND getClientData() methods to determine which request has just completed, assuming that you had more than one outstanding!

You can queue multiple UsbRequests across multiple EndPoints and the consume their completion status by repeatedly calling requestWait() until you have no more outstanding requests.