在Android中使用BroadcastReceiver时无法获得理想的Toast消息

Question

My android app should recognize, via bluetooth, if there is appropriate arduino server around. If so, app will show toast messages depending of the presence of the desirable arduino server. If it is found it should taost message about that, and if it is not, another message.

Here is my code when user presses button to search for arduino server:

public void onClick(View v) {

            Set<BluetoothDevice> pairedDevices = mBluetoothAdapter.getBondedDevices();
            if (pairedDevices.size() > 0) {
                for (BluetoothDevice device : pairedDevices) {
                    if (device.getName().equals("ARD_SPP")) {
                        sendButton.setVisibility(View.VISIBLE);
                        Toast.makeText(ConnectActivity.this, "Arduino server found, please sign up", Toast.LENGTH_SHORT);
                        b = true;
                        break;
                    }
                }
            }
            mBluetoothAdapter.startDiscovery();
            if (mBluetoothAdapter.startDiscovery()) {
                registerReceiver(discoveryResult, new IntentFilter(BluetoothDevice.ACTION_FOUND));
            }
            mBluetoothAdapter.cancelDiscovery();
            if (b == false)
                Toast.makeText(ConnectActivity.this, "Server not found", Toast.LENGTH_SHORT).show();
            }

And this is code inside BroadcastReceiver:

BroadcastReceiver discoveryResult = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        String deviceName = intent.getStringExtra(BluetoothDevice.EXTRA_NAME);
        if (deviceName.equals("ARD_SPP")) {
            Toast.makeText(ConnectActivity.this, "Arduino server found, please sign up", Toast.LENGTH_SHORT).show();                
            b = true;
        }
    }

(I didn't put part of the code about bluetooth, to make it easier to read, consider that bluetooth is already on)

The problem is that message that server is not found, is shown even if the server is around. So, at first, "server not found" is shown, and after that there is message that server is found. I don't know how to fix that.

I tried with using boolean variable b. I used condition with it in different places inside my code, inside onClick and BroadcastReceiver, but nothing gave appropriate result. Maybe I didn't use BroadcastReceiver in a good way...

Does anyone have idea how I can fix this ? It lools like solution is quite simple but I don't get it.

Answer 1

The discovery process is asynchronous, so you have to wait until you get the results in the BroadcastReceiver you defined. This will happen between the next 12 seconds (it should be pretty fast, but that's the window the BluetoothAdapter specifies for discover).

So basically your concept error is to suppose that this is something synchronous, and code it that way.

You should:

1) Get rid of the mBluetoothAdapter.cancelDiscovery() as this is cancelling the ongoing discovery process.

2) Don't show the Server not found message when b is false (btw change the condition to if (!b)), as the b variable at that point could be just modified by pairedDevices loop, and as far as I understand, you don't want to show if the Arduino is paired but if it's around.

3) To properly manage the messages that you get from the discovery process, you should have this in mind (from the official documentation http://developer.android.com/reference/android/bluetooth/BluetoothAdapter.html ):

Register for ACTION_DISCOVERY_STARTED and ACTION_DISCOVERY_FINISHED intents to determine exactly when the discovery starts and completes. Register for ACTION_FOUND to be notified as remote Bluetooth devices are found.

So basically if ACTION_FOUND is received, you should check if it's the Arduino and display the proper message. If during the whole process you end up receiving ACTION_DISCOVERY_FINISHED without finding it, then you can show the "Not Found" message.

Always read the official documentation first to have a better understanding of what you're doing.

Hope it helps.