@Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
  setMeasuredDimension(measureWidth(widthMeasureSpec),measureHeight(heightMeasureSpec));
 }

 /**
  * 测量本身的大小，这里只是测量宽度
  * @param widthMeaSpec 传入父View的测量标准
  * @return 测量的宽度
  */
 private int measureWidth(int widthMeaSpec){
  /*定义view的宽度*/
  int width ;
  /*获取当前 View的测量模式*/
  int mode = MeasureSpec.getMode(widthMeaSpec) ;
  /*
  * 获取当前View的测量值，这里得到的只是初步的值，
  * 我们还需根据测量模式来确定我们期望的大小
  * */
  int size = MeasureSpec.getSize(widthMeaSpec) ;
  /*
  * 如果，模式为精确模式
  * 当前View的宽度，就是我们
  * 的size ；
  * */
  if(mode == MeasureSpec.EXACTLY){
   width = size ;
  }else {
   /*否则的话我们就需要结合padding的值来确定*/
   int desire = size + getPaddingLeft() + getPaddingRight() ;
   if(mode == MeasureSpec.AT_MOST){
    width = Math.min(desire,size) ;
   }else {
    width = desire ;
   }
  }
  mViewWidth = width ;
  return width ;
 }

 @Override
 protected void onDraw(Canvas canvas) {
  if(mTouched){
   canvas.drawColor(0x30000000);
  }
  for (int i = 0 ; i < mIndex.length ; i ++){
   mPaint.setColor(0xff000000);
   mPaint.setTextSize(mTextSize * 3.0f / 4.0f);
   mPaint.setTypeface(Typeface.DEFAULT) ;
   mPaint.getTextBounds(mIndex[i],0,mIndex[i].length(),mTextBound);
   float formX = mViewWidth/2.0f - mTextBound.width()/2.0f ;
   float formY = mTextSize*i + mTextSize/2.0f + mTextBound.height()/2.0f ;
   canvas.drawText(mIndex[i],formX,formY,mPaint);
   mPaint.reset();
  }
 }

 @Override
 public boolean onTouchEvent(MotionEvent event) {
  float y = event.getY() ;
  int index = (int) (y / mTextSize);
  if(index >= 0 && index < mIndex.length){
   Log.v("zgy","======index======="+index) ;
   selectItem(index);
  }
  if(event.getAction() == MotionEvent.ACTION_MOVE){
   mTouched = true ;
  }else if (event.getAction() == MotionEvent.ACTION_MOVE){
  }else {
   mFloatView.setVisibility(INVISIBLE);
   mTouched = false ;
  }
  invalidate();
  /*过滤点其他触摸事件*/
  return true;
 }

 /*定义一个回调接口*/
 public interface OnIndexSelectListener{
  /*返回选中的位置，和对应的索引名*/
  void onItemSelect(int position, String value) ;
 }

  float y = event.getY() ;
  int index = (int) (y / mTextSize);
  if(index >= 0 && index < mIndex.length){
   Log.v("zgy","======index======="+index) ;
   selectItem(index);
  }

  /*设置浮动选中的索引*/
  /*获取windowManager*/
  mWindowManager = (WindowManager) getContext().getSystemService(Context.WINDOW_SERVICE);
  /*overly 视图，通过LayoutInflater 获取*/
  mFloatView = LayoutInflater.from(getContext()).inflate(R.layout.overlay_indexview,null) ;
  /*开始让其不可见*/
  mFloatView.setVisibility(INVISIBLE);
  /*转换 高度 和宽度为Sp*/
  mOverlyWidth = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,70,getResources().getDisplayMetrics()) ;
  mOverlyHeight = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,70,getResources().getDisplayMetrics()) ;
  post(new Runnable() {
   @Override
   public void run() {
    WindowManager.LayoutParams layoutParams = new WindowManager.LayoutParams(mOverlyWidth,mOverlyHeight,
      WindowManager.LayoutParams.TYPE_APPLICATION,
      WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE
        | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,
      PixelFormat.TRANSLUCENT) ;
    mWindowManager.addView(mFloatView,layoutParams);
   }
  }) ;

 <moon.wechat.view.IndexView
  android:layout_width="25dp"
  android:layout_height="match_parent"
  android:layout_alignParentRight="true"/>

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
 android:id="@+id/overly_text"
 android:layout_width="70dp"
 android:layout_height="70dp"
 android:text="A"
 android:gravity="center"
 android:background="@drawable/bg_overly_text"
 android:textSize="40sp"
 android:textColor="#ffffffff"
 android:layout_gravity="center">
</TextView>

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
 <item>
  <shape>
   <solid android:color="#88000000"/>
   <corners android:radius="5dp"/>
  </shape>
 </item>
</layer-list>