I'm new to Android Studio and Java and I'm trying to display a dynamic battery percentage on multiple activities, but it is only showing on the main screen. I am also trying to do the same thing with displaying time. I am trying to get it work with my second activity (insulin.xml) with a TextView with an ID of lblBatteryInsulin.

mainActivity.Java

package com.example.insulinpump;

import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.support.wearable.activity.WearableActivity;
import android.util.Log;
import android.view.View;
import android.os.BatteryManager;
import android.content.BroadcastReceiver;

import android.widget.Button;
import android.widget.TextView;

import java.text.SimpleDateFormat;
import java.util.Calendar;

public class MainActivity extends WearableActivity {
    private static final String TAG = "MainActivity.java";

    // insulin calculations 
    int insulin = 0;
    int glucose = -1;

    // insulin.xml, activity_main.xml:
    TextView txtGlucose, lblGlucose;
    Button btnBasal, btnBolus;
    private TextView txtTime;
    private TextView battery; //Battery

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // insulin.xml, activity_main.xml:
        lblGlucose = (TextView) findViewById(R.id.lblGlucoseInsulin);

        // Enables Always-on
        setAmbientEnabled();
        txtTime = findViewById(R.id.lblTimeLog);

        Calendar calendar = Calendar.getInstance();
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("hh:mm a");
        String dateTime = simpleDateFormat.format(calendar.getTime());
        txtTime.setText(dateTime);

        battery =(TextView)findViewById(R.id.lblBatteryPercent);
        this.registerReceiver(this.batterylevelReciever, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
    }

    // Battery Percentage
    private BroadcastReceiver batterylevelReciever = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            int level = intent.getIntExtra(BatteryManager.EXTRA_LEVEL, 0 );
            battery.setText(String.valueOf(level)+ "%");
        }
    };

    // all xml:
    // Display functions:
    public void goHome(android.view.View View) {
        setContentView(R.layout.activity_main);
    }

    public void insulinView(android.view.View View) {
        setContentView(R.layout.insulin);
        txtGlucose = (TextView) findViewById(R.id.lblGlucoseInsulin);
        btnBasal = (Button) findViewById(R.id.btnBasal);
        btnBolus = (Button) findViewById(R.id.btnBolus);

        btnBasal.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view)
            {
                calculate(true);
            }
        });
        btnBolus.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view)
            {
                calculate(false);
            }
        });

        //Time
        txtTime = findViewById(R.id.lblTimeLog);

        Calendar calendar = Calendar.getInstance();
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("hh:mm a");
        String dateTime = simpleDateFormat.format(calendar.getTime());
        txtTime.setText(dateTime);

    }

I have not done this on Android but even on the mobile systems I did I was sure to not call for the battery level, state and more too often. Let's read what Google writes about this:

Generally speaking, the impact of constantly monitoring the battery level has a greater impact on the battery than your app's normal behavior, so it's good practice to only monitor significant changes in battery level—specifically when the device enters or exits a low battery state.

Unless your activity stays in that activity for say 15 or more minutes then I'd display the last known battery level in those activities and then update when we go back to main. Constantly checking the battery constantly creates more power use so use it once in a while. I've seen code where they read it many times a second.

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