Android应用中上下滑动调节亮度功能的原理是什么

import android.app.Activity;  
import android.os.Bundle;  
import android.view.LayoutInflater;  
import android.view.MotionEvent;  
import android.view.View;  
import android.view.WindowManager;  
import android.widget.TextView;  
public class MainActivity extends Activity {  
  private float startY = 0;//手指按下时的Y坐标  
  private float startX = 0;//手指按下时的Y坐标  
  private TextView brightnessTextView;  
  private View mVideoView;  
  @Override  
  protected void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    mVideoView = LayoutInflater.from(this).inflate(R.layout.activity_main, null);  
    setContentView(mVideoView);  
    brightnessTextView = (TextView) mVideoView.findViewById(R.id.text);  
  }  
  /* 
  * 设置屏幕亮度 
  * 0 最暗 
  * 1 最亮 
  */  
  public void setBrightness(float brightness) {  
    WindowManager.LayoutParams lp = getWindow().getAttributes();  
    lp.screenBrightness = lp.screenBrightness + brightness / 255.0f;  
    if (lp.screenBrightness > 1) {  
      lp.screenBrightness = 1;  
    } else if (lp.screenBrightness < 0.1) {  
      lp.screenBrightness = (float) 0.1;  
    }  
    getWindow().setAttributes(lp);  
    float sb = lp.screenBrightness;  
    brightnessTextView.setText((int) Math.ceil(sb * 100) + "%");  
  }  
  @Override  
  public boolean onTouchEvent(MotionEvent event) {  
    int screenWidth = mVideoView.getWidth();  
    switch (event.getAction()) {  
      case MotionEvent.ACTION_DOWN:  
        startX = event.getX();  
        startY = event.getY();  
        break;  
      case MotionEvent.ACTION_MOVE:  
        float endY = event.getY();  
        float distanceY = startY - endY;  
        if (startX > screenWidth / 2) {  
          //右边  
          //在这里处理音量  
        } else {  
          //屏幕左半部分上滑，亮度变大，下滑，亮度变小  
          final double FLING_MIN_DISTANCE = 0.5;  
          final double FLING_MIN_VELOCITY = 0.5;  
          if (distanceY > FLING_MIN_DISTANCE && Math.abs(distanceY) > FLING_MIN_VELOCITY) {  
            setBrightness(10);  
          }  
          if (distanceY < FLING_MIN_DISTANCE && Math.abs(distanceY) > FLING_MIN_VELOCITY) {  
            setBrightness(-10);  
          }  
        }  
        break;  
    }  
    return super.onTouchEvent(event);  
  }  
}  

<&#63;xml version="1.0" encoding="utf-8"&#63;>  
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"  
  android:id="@+id/activity_main"  
  android:layout_width="match_parent"  
  android:layout_height="match_parent"  
  android:background="@mipmap/ic_launcher">  
  
  <TextView  
    android:id="@+id/text"  
    android:layout_width="150dp"  
    android:layout_height="50dp"  
    android:layout_gravity="center"  
    android:background="@android:color/holo_orange_light"  
    android:gravity="center"  
    android:textColor="@android:color/white" />  
</FrameLayout>