AndroidManifest.xml

<!-- Declare the permission for body sensor -->
<uses-permission android:name="android.permission.BODY_SENSORS" />

heart_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.wearable.view.WatchViewStub
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/watch_view_stub"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:rectLayout="@layout/rect_heart"
    app:roundLayout="@layout/round_heart"
    tools:context=".MyActivity"
    tools:deviceIds="wear">
</android.support.wearable.view.WatchViewStub>

MyActivity.java

public class MyActivity extends Activity implements SensorEventListener {

    //UI Elements
    private CircledImageView mCircledImageView;
    private TextView mTextView;

    //Sensor and SensorManager
    Sensor mHeartRateSensor;
    SensorManager mSensorManager;

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

        //Sensor and sensor manager
        mSensorManager = ((SensorManager)getSystemService(SENSOR_SERVICE));
        mHeartRateSensor = mSensorManager.getDefaultSensor(Sensor.TYPE_HEART_RATE);

        //View
        final WatchViewStub stub = (WatchViewStub) findViewById(R.id.watch_view_stub);
        stub.setOnLayoutInflatedListener(new WatchViewStub.OnLayoutInflatedListener() {
            @Override
            public void onLayoutInflated(WatchViewStub stub) {
                mCircledImageView = (CircledImageView) stub.findViewById(R.id.circle);
                mTextView = (TextView) stub.findViewById(R.id.value);
            }
        });



    }

    @Override
    protected void onResume() {
        super.onResume();
        //Register the listener
        if (mSensorManager != null){
            mSensorManager.registerListener(this, mHeartRateSensor, SensorManager.SENSOR_DELAY_NORMAL);
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        //Unregister the listener
        if (mSensorManager!=null)
            mSensorManager.unregisterListener(this);
    }

    @Override
    public void onSensorChanged(SensorEvent event) {
        //Update your data. This check is very raw. You should improve it when the sensor is unable to calculate the heart rate
        if (event.sensor.getType() == Sensor.TYPE_HEART_RATE) {
            if ((int)event.values[0]>0) {
                mCircledImageView.setCircleColor(getResources().getColor(R.color.green));
                mTextView.setText("" + (int) event.values[0]);
            }
        }
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {

    }
}

round_heart.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:tools="http://schemas.android.com/tools"
                xmlns:app="http://schemas.android.com/apk/res-auto"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                tools:context=".MyActivity"
                tools:deviceIds="wear_round">


    <android.support.wearable.view.CircledImageView
        android:layout_centerInParent="true"
        android:layout_width="wrap_content"
        app:circle_radius="50dp"
        android:id="@+id/circle"
        app:circle_color="@color/light_grey"
        android:layout_height="wrap_content">

        <TextView
            android:layout_gravity="center"
            android:text="wait...."
            android:textColor="@color/black"
            android:textSize="20sp"
            android:id="@+id/value"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>


    </android.support.wearable.view.CircledImageView>