I'm developping an app that communicates with usb devices when attached to tablet. To avoid the user to accept android to access the device, I set an intent filter :
<intent-filter>
<action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" />
<action android:name="android.hardware.usb.action.USB" />
</intent-filter>
<meta-data android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" android:resource="@xml/usb_device_filter" />
My question is, having this intent filter, the "onCreate" method gets called each time I plug a device and another activity is launched, without the intent filter, it's called only once. Here is the "onCreate" method :
@Override
protected void onCreate(Bundle savedInstanceState) {
try {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_msp430_hid); //define activity layout
setVersionToTitle();
btnSend = (Button) findViewById(R.id.btnSend); //Send button
btnSend.setOnClickListener(this); //Listener for Send Button
btnSend.setEnabled(true);
btnSelectHIDDevice = (Button) findViewById(R.id.btnSelectHIDDevice); //Select HID Device button
btnSelectHIDDevice.setOnClickListener(this); //Listener for Select HID Device button
btnClear = (Button) findViewById(R.id.btnClear); //Clear button
btnClear.setOnClickListener(this); //Listener for Clear button
edtxtHidInput = (EditText) findViewById(R.id.edtxtHidInput); //User editable text area for sending information to attached device
log_txt = (EditText) findViewById(R.id.log_txt); //Text area for displaying information
mLog("Initialized\nPlease select your USB HID device");
getWindow().setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_STATE_ALWAYS_HIDDEN);
mPermissionIntent = PendingIntent.getBroadcast(this, 0, new Intent(ACTION_USB_PERMISSION), 0); //Get USB permission intent for broadcast
IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
filter.addAction(UsbManager.ACTION_USB_DEVICE_ATTACHED);
filter.addAction(UsbManager.ACTION_USB_DEVICE_DETACHED);
registerReceiver(mUsbReceiver, filter); //Register broadcast receiver
edtxtHidInput.setText("Enter Text Here");
uiHandler.postDelayed(runnable, 100); //Start runnable after 100ms
} catch (Exception e) {
Log.e("Init", "Initialization error", e);
}
}
The broadcast receiver :
private final BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {
public void onReceive(Context context, Intent intent) {
String action = intent.getAction();
if (ACTION_USB_PERMISSION.equals(action)) {
synchronized (this) {
setDevice(intent);
}
}
//device attached
if (UsbManager.ACTION_USB_DEVICE_ATTACHED.equals(action)) {
synchronized (this) {
setDevice(intent); //Connect to the selected device
}
if (device == null) {
mLog("device connected");
}
}
//device detached
if (UsbManager.ACTION_USB_DEVICE_DETACHED.equals(action)) {
if (device != null) {
device = null;
//btnSend.setEnabled(false);
}
mLog("device disconnected");
}
}
The "setDevice" method that simply connects to the device :
private void setDevice(Intent intent) {
device = (UsbDevice) intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
if (device != null && intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
mLog("Selected device VID:" + Integer.toHexString(device.getVendorId()) + " PID:" + Integer.toHexString(device.getProductId()));
connection = mUsbManager.openDevice(device); //Connect to device
intf = device.getInterface(0);
Why ?
Apparently, this is the default behavior. Check android:launchMode
here.
Default launchMode is standard
, which means that every time you attach an USB device, a new Activity instance is created. If you set singleTop
, onNewIntent()
is called and your activity is re-used, if it's on the top of the stack. Otherwise it'll create a new one.
You can also use singleTask
or singleInstance
but Google discourages this in most cases. But sometimes it could be the right solution for a problem.
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