Android asynchronous USB reads to ByteBuffer, but cannot determine size

I am reading bytes from an FTDI-based USB device on the Android 3.1 OS (on a Motorola Xoom) using the recently-added USB API for Android: http://developer.android.com/guide/topics/usb/host.html

However, I'm running into a problem when doing Asynchronous reads.

        ByteBuffer bb = ByteBuffer.allocate(128);
        UsbRequest request = new UsbRequest();
        request.initialize(deviceConnection, deviceInterface.getEndpoint(0));
        boolean sent = request.queue(bb, 128);
        request = deviceConnection.requestWait();
        if(request.getEndpoint() == deviceInterface.getEndpoint(0)) {
            // Send a message to the Activity with the read array
            Message msg = usbTest.handleAsyncMessage.obtainMessage();
            msg.obj = bb.array()
            usbTest.handleAsyncMessage.sendMessage(msg);
        }

This function reads the bytes properly from the device. However, it does not tell me how much is read. bb.position() always returns 0 and bb.remaining() always returns 128. I cannot ensure that my read will return 128 bytes (may reads, such as requesting the device's status, only send back around 12 bytes), and I have no way of determining where the read ends and the garbage in the B开发者_高级运维yteBuffer starts. I've tried rewinding the ByteBuffer but the mark doesn't get set by the read. Is there any way to determine exactly how many bytes are read?

Thanks

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I'm unsure this is your problem, but it looks like you have to do something special when dealing with endpoint 0. From the javadocs of UsbRequest:

Requests on endpoint zero are not supported by this class; use controlTransfer(int, int, int, int, byte[], int, int) for endpoint zero requests instead.

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Apparently you can't. A shortcoming of Android's USB support. :(

See How can I determine the length of received bytes of UsbRequest.queue(..) method?