我使用服务打开USB附件的连接。我使用绑定到我从UsbManager的openAccessory（）方法获得的FileDescriptor的输入流。只要我不开始新的活动，一切都很好。出于某种原因，当我开始一个新活动时，我得到IOException（文件描述符已关闭）。任何想法为什么会这样？我不想工作，以便输入流保持跨活动的连接。
您必须将USB连接放在服务中（不是绑定服务，而是前台或后台），并将接收到的数据与广播接收器进行通信。
让我通过完整的服务帮助您更多地阅读附件模式中的USB。读取的数据放在缓冲区中。
public final class UsbService extends Service {
//Member attributes
// USB
private static final String ACTION_USB_PERMISSION = "com.examples.accessory.controller.action.USB_PERMISSION";
private Context mMainContext;
LocalBroadcastManager mBroadcastManager = null;
private UsbManager mUsbManager;
private PendingIntent mPermissionIntent;
private UsbAccessory mAccessory;
private ParcelFileDescriptor mFileDescriptor;
private FileInputStream mInputStream;
private FileOutputStream mOutputStream;
private boolean mPermissionRequestPendding = false;
private Thread mAccessMonitor;
private Thread mUsbListenner;
private ConcurrentLinkedQueue<String> usbDataBuffer;
/* ************************************* */
/* BROADCAST RECEIVERS */
/* ************************************* */
private final BroadcastReceiver mUsbPermissionReceiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
usbPermissionMonitor(intent);
}
};
/* ************************************* */
/* RUNNABLES */
/* ************************************* */
// Running thread listenning to USB port for received messages
private Runnable UsbListenner = new Runnable() {
public void run() {
while(1)
readUsbAccessory();
}
};
// Running thread listenning to USB accessories
private Runnable AccessoryMonitor = new Runnable() {
public void run() {
android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_BACKGROUND);
while(mIsServiceRunning){
try {
getAccessory();
sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
};
/* ************************************* */
/* SERVICE FUNCTIONS */
/* ************************************* */
public UsbService(){}
@Override
public void onCreate() {
mMainContext = getApplicationContext();
mBroadcastManager = LocalBroadcastManager.getInstance(mMainContext);
mBroadcastManager.registerReceiver(usbServiceReceiver, usbServiceFilter);
initUsb();
}
public IBinder onBind(Intent intent){
return null;
}
@Override
public void onRebind(Intent intent) {
super.onRebind(intent);
}
@Override
public boolean onUnbind(Intent intent) {
super.onUnbind(intent);
return true;
}
// Destroy
@Override
public void onDestroy() {
closeAccessory();
mBroadcastManager.unregisterReceiver(mUsbPermissionReceiver);
mBroadcastManager.unregisterReceiver(usbServiceReceiver);
}
@Override
public int onStartCommand(Intent intent, int flags, int startId){
return START_NOT_STICKY;
}
/* ************************************* */
/* USB CONTROL FUNCTIONS */
/* ************************************* */
private void initUsb()
{
usbDataBuffer = new ConcurrentLinkedQueue<>();
mAccessMonitor= new Thread(AccessoryMonitor);
mUsbListenner = new Thread(UsbListenner);
mUsbManager = (UsbManager)getSystemService(Context.USB_SERVICE);
mPermissionIntent = PendingIntent.getBroadcast(this, 0, new Intent(ACTION_USB_PERMISSION), 0);
IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
filter.addAction(UsbManager.ACTION_USB_ACCESSORY_DETACHED);
filter.addAction(UsbManager.EXTRA_ACCESSORY);
mBroadcastManager.registerReceiver(mUsbPermissionReceiver, filter);
mAccessMonitor.start();
}
private void getAccessory()
{
if (mFileDescriptor != null) {
return;
}
UsbAccessory[] accessories = mUsbManager.getAccessoryList();
UsbAccessory accessory = (accessories == null ? null : accessories[0]);
if (accessory != null)
{
if (mUsbManager.hasPermission(accessory))
{
//"has permission";
if(!mPermissionRequestPendding) {
openAccessory(accessory);
}
}
else
{
//"no permission";
synchronized (mUsbPermissionReceiver)
{
if(!mPermissionRequestPendding) {
mUsbManager.requestPermission(accessory, mPermissionIntent);
mPermissionRequestPendding = true;
}
}
}
}
}
private void openAccessory(UsbAccessory accessory)
{
if(accessory != null) {
mFileDescriptor = mUsbManager.openAccessory(accessory);
if (mFileDescriptor != null) {
mAccessory = accessory;
FileDescriptor fd = mFileDescriptor.getFileDescriptor();
mInputStream = new FileInputStream(fd);
mOutputStream = new FileOutputStream(fd);
mUsbListenner.start();
mPermissionRequestPendding = false;
}
}
}
private void closeAccessory() {
try {
if (mFileDescriptor != null) {
mFileDescriptor.close();
}
}
catch (IOException e) {
}
finally {
mAccessory = null;
mFileDescriptor = null;
}
}
public void sendDataUsb(String msg)
{
if (mFileDescriptor != null) {
byte[] bytes = msg.getBytes();
try {
mOutputStream.write(bytes);
} catch (IOException e) {
e.printStackTrace();
}
}
}
/*
* Runnable block that will poll the accessory data stream
* for regular updates, adds each message to a buffer.
*/
private void readUsbAccessory() {
if (mFileDescriptor != null) {
int ret = 0;
byte[] buffer = new byte[16384];
while (mFileDescriptor != null) {
try {
ret = mInputStream.read(buffer);
if (ret > 0) {
usbDataBuffer.add(new String(buffer, 0, ret));
}
} catch (IOException e) {
break;
}
}
}
}
/*
* This receiver monitors for the event of a user granting permission to use
* the attached accessory. If the user has checked to always allow, this will
* be generated following attachment without further user interaction.
*/
private void usbPermissionMonitor(Intent intent)
{
String action = intent.getAction();
if (ACTION_USB_PERMISSION.equals(action)) {
synchronized (this) {
final UsbAccessory accessory = intent.getParcelableExtra(UsbManager.EXTRA_ACCESSORY);
if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
openAccessory(accessory);
mPermissionRequestPendding = false;
} else {
//permission refused
mUsbManager.requestPermission(accessory, mPermissionIntent);
}
}
} else if (UsbManager.ACTION_USB_ACCESSORY_DETACHED.equals(action)) {
UsbAccessory accessory = intent.getParcelableExtra(UsbManager.EXTRA_ACCESSORY);
if (accessory != null && accessory.equals(mAccessory)) {
Toast.makeText(this, "Cable detach !", Toast.LENGTH_LONG).show();
closeAccessory();
mUsbActive = false;
}
}
}
}