fix: RN warnings about EventListeners (react-native-sensors#440)

* fix RN warning about event listeners

* stop emitting events if there are no listeners on Android

Author: andresilveirah

android/src/main/java/com/sensors/RNSensor.java

@@ -31,6 +31,8 @@ public class RNSensor extends ReactContextBaseJavaModule implements SensorEventL
   private float[] orientation = new float[3];
   private float[] quaternion = new float[4];
 
+  private Boolean isBeingObserved = false;
+
   public RNSensor(ReactApplicationContext reactContext, String sensorName, int sensorType) {
     super(reactContext);
     this.reactContext = reactContext;
@@ -94,6 +96,10 @@ private void sendEvent(String eventName, @Nullable WritableMap params) {
 
   @Override
   public void onSensorChanged(SensorEvent sensorEvent) {
+    if(!isBeingObserved) {
+      return; // avoid all the computation if there are no observers
+    }
+
     int currentType = sensorEvent.sensor.getType();
     if(currentType != this.sensorType) { // not for the current Sensor
       return;
@@ -148,4 +154,19 @@ public void onSensorChanged(SensorEvent sensorEvent) {
   @Override
   public void onAccuracyChanged(Sensor sensor, int accuracy) {
   }
+
+  // this is called by RN when the first listener is registered
+  // not implementing this method will cause a warning on RN 0.65 onwards
+  @ReactMethod
+  public void addListener(String eventName) {
+    isBeingObserved = true;
+  }
+
+  // this is called by RN when the last listener is deregistered
+  // not implementing this method will cause a warning on RN 0.65 onwards
+  @ReactMethod
+  public void removeListeners(Integer count) {
+    isBeingObserved = false;
+    stopUpdates(); // maybe only calling `stopUpdates()` is enough
+  }
 }