Android 中磁场传感器的使用详解

public class PointerView extends View implements SensorEventListener {
    private SensorManager sensorManager;
    private Bitmap pointer;
    private float[] allValue;
    public PointerView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        sensorManager = (SensorManager) context.getSystemService(Context.SENSOR_SERVICE);
        //为磁场传感器注册监听器
        sensorManager.registerListener(this, sensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD), sensorManager.SENSOR_DELAY_GAME);
        //指定要绘制的指针位图
        pointer = BitmapFactory.decodeResource(getResources(), R.drawable.magnetic);
    }
    //当传感器的值发生变化时，触发
    @Override
    public void onSensorChanged(SensorEvent event) {
        if (event.sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD) {
            //获取磁场传感器的值
            float[] value = event.values;
            allValue = value;
            //刷新界面
            postInvalidate();
        }
    }
    //当传感器的精度改变时，触发
    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {
    }
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //根据x轴 y轴的磁场强度绘制指针
        if (allValue != null) {
            //x轴的磁场强度
            float x = allValue[0];
            //y轴的磁场强度
            float y = allValue[1];
            canvas.save();
            //重置绘图对象前面应该先调用canvas.save();
            //否则会报Underflow in restore - more restores than saves错误
            canvas.restore();
            //设置旋转的中心点,为屏幕的中心点
            canvas.translate(getWidth() / 2, getHeight() / 2);
            if (y == 0 && x > 0) { //表示正东方
                canvas.rotate(90);
            } else if (y == 0 && x < 0) {//表示正西方
                canvas.rotate(270);
            } else {
                //根据x y轴的磁场强度来计算布局的旋转角度,用三角函数计算旋转角度
                if (y >= 0) {
                    canvas.rotate((float) (Math.tanh(x / y) * 90));
                } else {
                    canvas.rotate((float) (Math.tanh(x / y) * 180));
                }
            }
            //绘制指针
            canvas.drawBitmap(pointer, -pointer.getWidth() / 2, -pointer.getHeight() / 2, new Paint());
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    tools:context=".MagneticSensorActivity">
    <com.example.alertdialog.custom.PointerView
        android:id="@+id/pointerView"
        android:layout_width="300dp"
        android:layout_height="300dp" />
</LinearLayout>