Help: reading out sensor data - Android Q&A, Help & Troubleshooting

Hi folks,
well ... first of all: I'm completely new to programming related to Android. I do have basic knowledge in programming with Basic and C. But im all new to Java and programming with the Android ecosystem (I am familiar with the Android ecosystem, though ... I've just never written code for Android).
Now I wanted a simple app to read out and display some sensor data, such as acceleration along y-axis and rotation angle around y-axis.
But I found the whole way you get stuff out of the sensors very confusing ans somehow unnecessarily complicated. However....
I've successfully managed to code an app, that checks whether certain sensors are available or not - and show an according text in a TextView for the user to read.
The next step was, I wanted to code an app that reads the rotation angle and shows the current angle in a TextView for the user. But I haven't managed that.
I found the whole System of reading the rotation data very complex..... why isn't there just a simple function like "get the y-axis rotation angle" and thats it .... I've searched a lot in the Interwebz, tried to understand how that stuff works, and used some code of some examples i thought i understood ... well, I obviously didn't ....
This is what I've got so far:
Code:
package test.sensor4;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.app.Activity;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
//float A[] = new float[9];
//float I[] = new float[9];
float[] mGravity;
float[] mGeomagnetic;
float azimut;
float pitch;
float roll;
Button btn1;
TextView tv1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
btn1 = (Button)findViewById(R.id.button1);
btn1.setOnClickListener(this);
//SensorManager.registerListener(this)
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
public void onSensorChanged(SensorEvent event) {
if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER)
mGravity = event.values;
if (event.sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD)
mGeomagnetic = event.values;
if (mGravity != null && mGeomagnetic != null) {
float A[] = new float[9];
float I[] = new float[9];
boolean success = SensorManager.getRotationMatrix(A, I, mGravity, mGeomagnetic);
if (success) {
float orientation[] = new float[3];
SensorManager.getOrientation(A, orientation);
azimut = orientation[0]; // orientation contains: azimut, pitch and roll
pitch = orientation[1];
roll = orientation[2];
TextView tv1 = (TextView)findViewById(R.id.text1);
tv1.setText(String.valueOf(+roll));
}
}
}
@Override
public void onClick(View e) {
TextView tv2 = (TextView)findViewById(R.id.text2);
tv2.setText(String.valueOf(+pitch));
}
}
what this does is: it doesn't change the text from the first textview at all ... in the second textview it prints "0.0"
I don't understand why .... Maybe you guys can help me.
Thanks in advance.
Best wishes.