BroadcastReceiver fires after the activity is already created

问题

I want to use a BroadcastReceiver to get permission to communicate with a USB device. I am trying to implement it the same way it is done on android website http://developer.android.com/guide/topics/usb/host.html It all works, kind of. But the broadcastReceiver is fireing only after the main activity is created. Which means I am able to communicate with the device only after close the app and open it again (when I don't unregister the broadcastReceiver, when I do I can't communicate at all). What can be the reason? My code is like this:

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) 
            {
                device = (UsbDevice) intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
                if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) 
                {
                    if(device != null)
                    {

           //things I do when the permission is granted             

                    }

                } 
                else 
                {
                    devMessage = "permission denied for device ";
                }
            }
        }
    }
};

The part of the code where I register it:

public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main);

    mUsbManager = (UsbManager) getSystemService(Context.USB_SERVICE);       

    mPermissionIntent = PendingIntent.getBroadcast(this, 0, new Intent(ACTION_USB_PERMISSION), 0);
    filter = new IntentFilter(ACTION_USB_PERMISSION);
    registerReceiver(mUsbReceiver, filter);

    HashMap<String, UsbDevice> deviceList = mUsbManager.getDeviceList();
    Iterator<UsbDevice> deviceIterator = deviceList.values().iterator();
        while(deviceIterator.hasNext())
        {
            device = deviceIterator.next();
            mUsbManager.requestPermission(device, mPermissionIntent);             
        }

            // ...      

    if(device!=null)
    {
      // ...
    }
    else
    {
      // ...
    }
    tv.setText(devMessage);
    }

Does anyone know why is this happening, what I might be doing wrong?

回答1:

You're registering your broadcast receiver in you activity. That means that before you run that activity, you cannot receive broadcasts.

You should probably look at registering a reciever-tag in in you AndroidManifest.xml. This is the docs for the receiver-tag. This allows you to register receivers without starting your activity.

This part is important:

The <application> element has its own enabled attribute that applies to all application components, including broadcast receivers. The <application> and <receiver> attributes must both be "true" for the broadcast receiver to be enabled. If either is "false", it is disabled; it cannot be instantiated.

来源：https://stackoverflow.com/questions/10450473/broadcastreceiver-fires-after-the-activity-is-already-created

标签

android

usb

broadcastreceiver