Read another variable in Activity that is updated in Mainactivity

Question

I have a function that runs every second. This function sends a request for information via socket to a device. This information is received on a AsyncTask that makes the calculations and updates some variables that are in the MainActivity and are displayed on screen.

The problem is that I also need to display the contents of these variables in another Activity. And that content has to track the changes that the variable receives every second.

I have to use the same variable that’s on and MainActivity I can’t open a new one socket because the first one keeps receiving the data even with another Activity being shown.

Answer 1

That’s not the best approach.

If you have an operation (function) that is executed periodically and the result you want to obtain in more than one location of your application, do not do it in an Activity.
An Activity should not depend on another to perform its function.

Android provides the class Service for such cases.
In a Service operations are performed independently of the other components of the application, however communication between them is possible.

In this case, a possible approach is to use a Bound Service(Tied service):

(...)A linked service allows components (such as activities) to be linked to the service, send requests, receive responses and even establish inter-process communication (IPC)....
_{(android documentation, Tied services)}

Example that uses a service to calculate the square root.

Sqrcalculatorservice.java

public class SqrCalculatorService extends Service {

    public static final int SQR_MSG = 0;
    public static final String SQR_RESULT = "sqrResult";

    public SqrCalculatorService() {
    }

    public static Intent makeIntent(Context context){
        return new Intent(context, SqrCalculatorService.class);
    }
    private final Messenger mCalculateMessenger = new Messenger(new CalculateHandler());

    private static class CalculateHandler extends Handler{

        @Override
        public void handleMessage(Message msg) {

            switch (msg.what) {
                case SQR_MSG:
                    double sqr = calculate(msg.arg1);
                    replyToCaller(msg.replyTo, sqr);
                    break;
                default:
                    super.handleMessage(msg);
            }
        }

        private static double calculate(int arg1) {
            return Math.sqrt(arg1);
        }

        private static void replyToCaller(Messenger msg, double sqr) {
            Message reply = Message.obtain(null, SQR_MSG);
            Bundle data = new Bundle();
            data.putDouble(SQR_RESULT, sqr);
            reply.setData(data);
            try {
                msg.send(reply);
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }
    }

    @Override
    public IBinder onBind(Intent intent) {
        return mCalculateMessenger.getBinder();
    }
}

Mainactivity.java

public class MainActivity extends AppCompatActivity {

    private Messenger mServiceMessenger = null;
    private TextView tvResultado;
    private EditText edValor;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tvResultado = (TextView) findViewById(R.id.tvResultado);
        edValor = (EditText)findViewById(R.id.edValor);
    }

    @Override
    protected void onStart() {
        super.onStart();
        if(mServiceMessenger == null){
            bindService(SqrCalculatorService.makeIntent(this),
                        srvConnection, Context.BIND_AUTO_CREATE);
        }
    }

    @Override
    protected void onStop() {
        unbindService(srvConnection);
        super.onStop();
    }

    private ServiceConnection srvConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName componentName, IBinder iBinder) {

            mServiceMessenger = new Messenger(iBinder);
        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {

            mServiceMessenger = null;
        }
    };

    final Messenger mResultMessenger = new Messenger(new ResultHandler());

    private class ResultHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case SqrCalculatorService.SQR_MSG:
                    Bundle data = msg.getData();
                    displaySQRResult(data);
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
        private void displaySQRResult(Bundle data) {
            double sqr = data.getDouble(SqrCalculatorService.SQR_RESULT);
            tvResultado.setText(Double.toString(sqr));
        }
    }

    public void onButtonClick(View v){
        int value = Integer.parseInt(edValor.getText().toString());
        Message msg = Message.obtain(null, SqrCalculatorService.SQR_MSG, value, 0);
        msg.replyTo = mResultMessenger;
        try {
            mServiceMessenger.send(msg);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin">

    <TextView
        android:id="@+id/lbValor"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Valor a calcular: "/>

    <EditText
        android:id="@+id/edValor"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_toEndOf="@+id/lbValor"
        android:layout_toRightOf="@+id/lbValor"
        android:layout_alignBaseline="@+id/lbValor"/>
    <TextView
        android:id="@+id/lbResultado"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/lbValor"
        android:text="Resultado: "
        android:layout_marginTop="24dp"/>
    <TextView
        android:id="@+id/tvResultado"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_toEndOf="@+id/lbResultado"
        android:layout_toRightOf="@+id/lbResultado"
        android:layout_alignBaseline="@+id/lbResultado"/>

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/lbResultado"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="24dp"
        android:text="Calcular raiz quadrada"
        android:onClick="onButtonClick"/>
</RelativeLayout>

Registration of the service on Androidmanifest.xml

<application>
    ....
    ....
    <service
        android:name=".SqrCalculatorService"
        android:enabled="true"
        android:exported="true">
    </service>
</application>