Android开发高级进阶——传感器

   <p>Android系统提供了对传感器的支持，如果手机设备的硬件提供了这些传感器，Android应用可以通过传感器来获取设备的外界条件，包括手机设备的运行状态、当前摆放方向、外界的磁场、温度和压力等。Android系统提供了驱动程序去管理这些传感器硬件，当传感器感知到外部环境发生改变时，Android系统负责管理这些传感器数据。</p>    <h2><strong>一. Android中11中常见的传感器</strong></h2>    <ol>     <li> <p>加速度传感器：SENSOR_TYPE_ACCELEROMETER</p> </li>     <li> <p>磁力传感器：SENSOR_TYPE_FIELD</p> </li>     <li> <p>方向传感器：SENSOR_TYPE_ORIENTATION</p> </li>     <li> <p>陀螺仪传感器：SENSOR_TYPE_GYROSCOPE</p> </li>     <li> <p>光线感应传感器：SENSOR_TYPE_LIGHT</p> </li>     <li> <p>压力传感器：SENSOR_TYPE_PRESSURE</p> </li>     <li> <p>温度传感器：SENSOR_TYPE_TEMPERATURE</p> </li>     <li> <p>接近传感器：SENSOR_TYPE_PROXIMITY</p> </li>     <li> <p>重力传感器：SENSOR_TYPE_GRAVITY</p> </li>     <li> <p>线性加速度传感器：SENSOR_TYPE_LINEAR_ACCELERATION</p> </li>     <li> <p>旋转矢量传感器：SENSOR_TYPE_ROTATION_VECTOR</p> </li>    </ol>    <h2><strong>二. 使用传感器</strong></h2>    <p>使用传感器的步骤分为5步：</p>    <ol>     <li> <p>获取SensorManager对象<br> 调用Context的getSystemService(Context.SENSOR_SERVICE)方法获取SensorManager对象，SensorManager对象代表系统的传感器管理服务。</p> </li>     <li> <p>获取Sensor对象<br> 调用SensorManager的getDefaultSensor(int type)方法获取指定类型的传感器。</p> </li>     <li> <p>注册Sensor对象<br> 在Activity的onResume()方法中调用SensorManager的registerListener()方法为指定的传感器注册监听器，程序通过实现监听器即可获取传感器传来的数据。</p> </li>     <li> <p>重写onAccuracyChanged，onSensorChanged方法<br> 当传感器的精度和数据发送变化时，在这两个方法中做相应的操作。</p> </li>     <li> <p>注销Sensor对象<br> 在Activity的onPause()方法中调用SensorManager的unregisterListener()方法注销指定的传感器监听器。</p> </li>    </ol>    <p>SensorManager提供的注册传感器的方法为registerListener(SensorEventListener listener, Sensor sensor, int rate)，该方法的三个参数说明如下：</p>    <ul>     <li> <p>listener：监听传感器事件的监听器。该监听器需要实现SensorEventListener接口。</p> </li>     <li> <p>sensor：传感器对象。</p> </li>     <li> <p>rate：指定获取传感器数据的频率。rate有以下几个频率值：</p>      <ul>       <li> <p>SensorManager.SENSOR_DELAY_FASTEST：最快。延迟最小，只有特别依赖于传感器数据的应用推荐采用这种频率，这种模式可能造成手机电量大量消耗。</p> </li>       <li> <p>SensorManager.SENSOR_DELAY_GAME：适合游戏的频率。一般有实时性要求的应用适合使用这种频率。</p> </li>       <li> <p>SensorManager.SENSOR_DELAY_NORMAL：正常频率。一般对实时性要求不是特别高的应用适合使用这种频率。</p> </li>       <li> <p>SensorManager.SENSOR_DELAY_UI：适合普通用户界面的频率。这种模式比较省电，而且系统开销也很小，但延迟较大。</p> </li>      </ul> </li>    </ul>    <h2><strong>三. 读取传感器数据</strong></h2>    <p>在onSensorChanged(SensorEvent event)方法中有一个参数event，通过event可以获取传感器的类型以及传感器的数据。</p>    <ul>     <li> <p>获取传感器的类型：event.sensor.getType()</p> </li>     <li> <p>获取传感器的数据：event.values[i]，i为0,1,2...，不同传感器，event.values[i]对应的数据不同，下面以加速度传感器为例，解释values[i]的含义。</p> </li>    </ul>    <pre>  <code class="language-java">* <h4>{@link android.hardware.Sensor#TYPE_ACCELEROMETER       * Sensor.TYPE_ACCELEROMETER}:</h4> All values are in SI units (m/s^2)       * <ul>       * <li> values[0]: Acceleration minus Gx on the x-axis </li>       * <li> values[1]: Acceleration minus Gy on the y-axis </li>       * <li> values[2]: Acceleration minus Gz on the z-axis </li>       * </ul>       * <p>       * A sensor of this type measures the acceleration applied to the device       * (<b>Ad</b>). Conceptually, it does so by measuring forces applied to the       * sensor itself (<b>Fs</b>) using the relation:       * </p>       * <b><center>Ad = - ∑Fs / mass</center></b>       * <p>       * In particular, the force of gravity is always influencing the measured       * acceleration:       * </p>       * <b><center>Ad = -g - ∑F / mass</center></b>       * <p>       * For this reason, when the device is sitting on a table (and obviously not       * accelerating), the accelerometer reads a magnitude of <b>g</b> = 9.81       * m/s^2       * </p>       * <p>       * Similarly, when the device is in free-fall and therefore dangerously       * accelerating towards to ground at 9.81 m/s^2, its accelerometer reads a       * magnitude of 0 m/s^2.       * </p>       * <p>       * It should be apparent that in order to measure the real acceleration of       * the device, the contribution of the force of gravity must be eliminated.       * This can be achieved by applying a <i>high-pass</i> filter. Conversely, a       * <i>low-pass</i> filter can be used to isolate the force of gravity.       * </p>       * <pre class="prettyprint">       *     public void onSensorChanged(SensorEvent event)       *     {       *          // alpha is calculated as t / (t + dT)       *          // with t, the low-pass filter's time-constant       *          // and dT, the event delivery rate       *          final float alpha = 0.8;       *          gravity[0] = alpha * gravity[0] + (1 - alpha) * event.values[0];       *          gravity[1] = alpha * gravity[1] + (1 - alpha) * event.values[1];       *          gravity[2] = alpha * gravity[2] + (1 - alpha) * event.values[2];       *          linear_acceleration[0] = event.values[0] - gravity[0];       *          linear_acceleration[1] = event.values[1] - gravity[1];       *          linear_acceleration[2] = event.values[2] - gravity[2];       *     }       * </pre>       * <p>       * <u>Examples</u>:       * <ul>       * <li>When the device lies flat on a table and is pushed on its left side       * toward the right, the x acceleration value is positive.</li>       * <li>When the device lies flat on a table, the acceleration value is       * +9.81, which correspond to the acceleration of the device (0 m/s^2) minus       * the force of gravity (-9.81 m/s^2).</li>       * <li>When the device lies flat on a table and is pushed toward the sky       * with an acceleration of A m/s^2, the acceleration value is equal to       * A+9.81 which correspond to the acceleration of the device (+A m/s^2)       * minus the force of gravity (-9.81 m/s^2).</li>       * </ul></code></pre>    <p>从加速度传感器源代码中可以看出，values[0]表示x轴上的加速度，values[1]表示y轴上的加速度，values[2]表示z轴上的加速度。</p>    <h2><strong>四. 针对是否有传感器功能优化</strong></h2>    <p>因为并非所有手机都支持所有传感器，不用系统引入的传感器不同，所以在使用之前有必要判断一下，、从而提高性能。</p>    <p>判断是否有传感器有两种方法：</p>    <ol>     <li> <p>运行时检测</p> <pre>  <code class="language-java">SensorManager sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);  Sensor sensor = sensorManager.getDefaultSensor(Sensor.TYPE_ORIENTATION);  if (sensor != null){           //传感器存在  }else{           //传感器不存在  }</code></pre> </li>     <li> <p>使用Android Market过滤器来限定目标设备必须带有指定传感器配置。</p> <pre>  <code class="language-java"><use-feature         name = "android.hardware.sensor.orientation"        android:required = "true"/></code></pre> </li>    </ol>    <h2><strong>五. 方向传感器小Demo</strong></h2>    <p>利用方向传感器，界面中的图片向手机旋转的反方向旋转。代码如下：</p>    <pre>  <code class="language-java">public class MainActivity extends AppCompatActivity implements SensorEventListener{        private ImageView mIvSensor;      private Sensor mSensor;      private SensorManager mSensorManager;      private float mDegress = 0f;        @Override      protected void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);          setContentView(R.layout.activity_main);            mIvSensor = (ImageView) findViewById(R.id.iv_sensor);            mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);          mSensor = mSensorManager.getDefaultSensor(Sensor.TYPE_ORIENTATION);        }        @Override      protected void onResume() {          super.onResume();          mSensorManager.registerListener(this, mSensor, SensorManager.SENSOR_DELAY_UI); //rate suitable for the user interface      }        @Override      protected void onPause() {          super.onPause();          mSensorManager.unregisterListener(this);      }        @Override      public void onSensorChanged(SensorEvent event) {          if (event.sensor.getType() == Sensor.TYPE_ORIENTATION){              float degree = - event.values[0];              RotateAnimation rotateAnimation = new RotateAnimation(mDegress, degree, Animation.RELATIVE_TO_SELF, 0.5f, Animation.RELATIVE_TO_SELF, 0.5f);              rotateAnimation.setDuration(100);              mIvSensor.startAnimation(rotateAnimation);              mDegress = degree;          }      }        @Override      public void onAccuracyChanged(Sensor sensor, int accuracy) {          //TODO:当传感器精度发生变化时      }  }</code></pre>    <p>演示效果：</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/fea32089af02bcbd9a1f7d585a08196a.gif"></p>    <p style="text-align:center">orientation_sensor_demo.gif</p>    <h2><strong>六. 注意</strong></h2>    <ol>     <li> <p>别忘记注销。</p> </li>     <li> <p>不要阻塞onSensorChanged方法。</p> </li>     <li> <p>避免使用过时的方法或传感器类型。</p> </li>     <li> <p>在使用前先验证传感器是否存在。</p> </li>     <li> <p>谨慎选择传感器延时。</p> </li>    </ol>    <p> </p>    <p>来自：http://www.jianshu.com/p/2e15b4e8dab2</p>    <p> </p>