Call function from different activity

Question

I am developing an app that lets the user connect to a BLE device, and then after reading 20 values from the BLE peripheral, it moves to a new activity/screen where I make an image out of the 20 values that I read. I want to then be able to read a new set of 20 values but without leaving the screen that is showing the image. How can I achieve this?

In the activity where I read BLE characteristics, I have this function:

public class BluetoothLeService extends Service {
    private final static String TAG = BluetoothLeService.class.getSimpleName();
    ....
    public int read_counter = 0;
    public int measurement_arr[] = new int[20];


    // Implements callback methods for GATT events that the app cares about.  For example,
    // connection change and services discovered.
    private final BluetoothGattCallback mGattCallback = new BluetoothGattCallback() {

    ....
        @Override
        public void onCharacteristicRead(BluetoothGatt gatt,
                                         BluetoothGattCharacteristic characteristic,
                                         int status) {
            if (status == BluetoothGatt.GATT_SUCCESS) { // IF THE READ OPERATION WAS SUCCESSFUL.
                broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic);
            }
        }

     public void readCharacteristic(BluetoothGattCharacteristic characteristic) {
        if (mBluetoothAdapter == null || mBluetoothGatt == null) {
            Log.w(TAG, "BluetoothAdapter not initialized");
            return;
        }
        mBluetoothGatt.readCharacteristic(characteristic);
    }
    ....
    private void broadcastUpdate(final String action,
                                 final BluetoothGattCharacteristic characteristic) {
            final Intent intent = new Intent(action);

            final byte[] data = characteristic.getValue(); // data is presented as a byte array over BLE characteristics.

            measurement_arr[read_counter] = (int) data[0]; // Read data byte.

            // Use hex formatting.
            if (data != null && data.length > 0) {
                final StringBuilder stringBuilder = new StringBuilder(data.length);
                for(byte byteChar : data)
                    stringBuilder.append(String.format("%02X ", byteChar));
                intent.putExtra(EXTRA_DATA, new String(data) + "\n" + stringBuilder.toString()); // THIS IS WHERE DATA IS BEING READ.
            }
            read_counter += 1; // after value read, increment the count.
            if (read_counter < 20){ // Receive 20 packets of data.
                Log.d("MEASUREMENT", "Reading new characteristic");
                readCharacteristic(characteristic);
            }
            else {
                Log.d("Finished BLE read", "Moving to next activity.");
                read_counter = 0;
                // Go to next activity where we show image. 
                Intent show_image = new Intent(this, Reconstruction.class);
                show_image.putExtra("myArr", measurement_arr); // Go to acivity where we reconstruct image.
                show_image.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                startActivity(show_image);
            }
        sendBroadcast(intent);
    }

My 2nd activity, which displays the image using a Canvas then has the following code.

public class Reconstruction extends AppCompatActivity {
    public int data[] = new int[20];
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(new MyView(this));
        // Get Bundle object that contain the array
        Bundle extras = getIntent().getExtras();
        // Extract the array from the Bundle object
        data = extras.getIntArray("myArr");
        // Output the array
        for(int item:data){
            Log.i("2nd_ACTIVITY", String.valueOf(item));
        }
    }

    public class MyView extends View
    {
        Paint paint = null;
        public MyView(Context context)
        {
            super(context);
            paint = new Paint();
        }

        @Override
        protected void onDraw(Canvas canvas)
        {
            ... make image

            ...After making the image, how do I go back to the readCharacteristic function from the previous activity, but without changing what the display is showing?
        }
    }
}

I have read online some things about passing the context of my second activity to the first, but I don't quite understand this, and I am also not quite sure how Context works.

Answer 1

Don't use a link to the activity inside another activity. It can lead to memory leak.

Regarding your question, you can use Broadcast receivers to communicate between activities or use a Service which will communicate with your BLE device

Simple usage of Broadcast Receiver:

class YourActivity extends Activity {
    private BroadcastReceiver mMyReceiver;

    protected void onCreate(Bundle data) {
        ...
        mMyReceiver = new MyReceiver();
    }

    public void onStart() {
        registerReceiver(mMyReceiver, new IntentFilter("your action"));
    }

    public void onStop() {
        unregisterReceiver(mMyReceiver);
    }

    public void onDestroy() {
        mMyReceiver = null;
    }

    // Inner class has a link to YourActivity instance
    private class MyReceiver extends BroadcastReceiver {
        public void onReceive(Intent intent) {
            // procces messages here
        }
    }
}

// Calling:
context.sendBroadcast(new Intent("your action"));