/**
  * Return the kind of action being performed.
  * Consider using {@link #getActionMasked} and {@link #getActionIndex} to retrieve
  * the separate masked action and pointer index.
  * @return The action, such as {@link #ACTION_DOWN} or
  * the combination of {@link #ACTION_POINTER_DOWN} with a shifted pointer index.
  */
 public final int getAction() {
  return nativeGetAction(mNativePtr);//注意返回值表達式
 }

 public static final int ACTION_POINTER_INDEX_MASK = 0xff00;
 public static final int ACTION_POINTER_INDEX_SHIFT = 8;
 /**
  * For {@link #ACTION_POINTER_DOWN} or {@link #ACTION_POINTER_UP}
  * as returned by {@link #getActionMasked}, this returns the associated
  * pointer index.
  * The index may be used with {@link #getPointerId(int)},
  * {@link #getX(int)}, {@link #getY(int)}, {@link #getPressure(int)},
  * and {@link #getSize(int)} to get information about the pointer that has
  * gone down or up.
  * @return The index associated with the action.
  */
 public final int getActionIndex() {
 //這個表達式實際就是說取getAction()函數返回值的高8位
  return (nativeGetAction(mNativePtr) & ACTION_POINTER_INDEX_MASK)
    >> ACTION_POINTER_INDEX_SHIFT;
 }

 public static final int ACTION_MASK    = 0xff;
 /**
  * Return the masked action being performed, without pointer index information.
  * Use {@link #getActionIndex} to return the index associated with pointer actions.
  * @return The action, such as {@link #ACTION_DOWN} or {@link #ACTION_POINTER_DOWN}.
  */
 public final int getActionMasked() {
 //這個表達式的意思就是說取getAction()函數的低8位
  return nativeGetAction(mNativePtr) & ACTION_MASK;
 }

//注意函數的注釋第一句的說明，表示，返回的id叫pointer identifier,是和event里面的數據關聯(lián)的
 /**
  * Return the pointer identifier associated with a particular pointer
  * data index in this event. The identifier tells you the actual pointer
  * number associated with the data, accounting for individual pointers
  * going up and down since the start of the current gesture.
  * @param pointerIndex Raw index of pointer to retrieve. Value may be from 0
  * (the first pointer that is down) to {@link #getPointerCount()}-1.
  */
 public final int getPointerId(int pointerIndex) {
  return nativeGetPointerId(mNativePtr, pointerIndex);
 }

 //注意函數的注釋里面第一句，意思是提供一個pointer identifier,返回event中對應數據的index
 //index of data的作用是傳給event.getX()等其他的函數來獲取坐標等信息
 //所以這個函數的名字改成getPointerDataIndex比較合適
 /**
  * Given a pointer identifier, find the index of its data in the event.
  *
  * @param pointerId The identifier of the pointer to be found.
  * @return Returns either the index of the pointer (for use with
  * {@link #getX(int)} et al.), or -1 if there is no data available for
  * that pointer identifier.
  */
 public final int findPointerIndex(int pointerId) {
  return nativeFindPointerIndex(mNativePtr, pointerId);
 }

int index = event.getActionIndex();
int id = event.getPointerId(index);
int pointerIndex = event.findPointerIndex(id);
int x=getX(pointerIndex);
int y=getY(pointerIndex);

WindowManager manager=(WindowManager) getApplicationContext().getSystemService(Context.WINDOW_SERVICE);
DisplayMetrics displayMetrics=new DisplayMetrics();
Display display=manager.getDefaultDisplay();
display.getMetrics(displayMetrics);
screenW=displayMetrics.widthPixels;
screenH=displayMetrics.heightPixels;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

import java.util.HashMap;
import java.util.Map;


public class TouchTraceView extends View
{
 Context mContext;
 private Paint line_paint, text_paint, countPaint;
 int screenW, screenH;
 FactoryApplication app;
 private int paintColor = Color.RED;
 Map<Integer, TouchPoint> pointMap;
 float back_x1, back_y1, back_x2, back_y2;

 public TouchTraceView(Context context, AttributeSet attr)
 {
  super(context, attr);
  mContext = context;
  app = ;//作用僅僅是獲取默認屏幕的長和寬
  this.screenH = app.screenH;
  this.screenW = app.screenW;
  pointMap = new HashMap<>();
  initPaint();
 }

 private void initPaint()
 {
  line_paint = new Paint();
  line_paint.setAntiAlias(true);
  line_paint.setColor(paintColor);
  text_paint = new Paint();
  text_paint.setAntiAlias(true);
  text_paint.setColor(Color.BLUE);
  text_paint.setTextSize(30);
  countPaint = new Paint();
  countPaint.setAntiAlias(true);
  countPaint.setColor(Color.GREEN);
  countPaint.setTextSize(60);
 }

 @Override
 protected void onDraw(Canvas canvas)
 {
  super.onDraw(canvas);
  int num = pointMap.size();
  if (num == 0)
  {
   clearDraw(canvas);
   return;
  }
  for (Map.Entry<Integer, TouchPoint> entry : pointMap.entrySet())
  {
   TouchPoint point = entry.getValue();
   canvas.drawLine(0, point.y, getWidth(), point.y, line_paint);
   canvas.drawLine(point.x, 0, point.x, getHeight(), line_paint);
   if (num == 1)
   {
    canvas.drawText(" (" + point.x + "," + point.y + ")", screenW / 2, screenH / 2, text_paint);
   } else
   {
    canvas.drawText(String.valueOf(pointMap.size()), screenW / 2, screenH / 2, countPaint);
   }
  }
 }


 @Override
 public boolean onTouchEvent(MotionEvent event)
 {
  int index = event.getActionIndex();
  int id = event.getPointerId(index);
  int pointerIndex = event.findPointerIndex(id);
  int pointerCount = event.getPointerCount();
  int historySize = event.getHistorySize();
  switch (event.getActionMasked())
  {
   case MotionEvent.ACTION_POINTER_DOWN:
    pointMap.put(pointerIndex, new TouchPoint(event.getX(pointerIndex), event.getY(pointerIndex)));
    break;
   case MotionEvent.ACTION_POINTER_UP:
    pointMap.remove(pointerIndex);
    break;
   case MotionEvent.ACTION_MOVE:
    for (int h = 0; h < historySize; h++)
    {
     for (int p = 0; p < pointerCount; p++)
     {
      pointMap.put(p, new TouchPoint(event.getHistoricalX(p, h), event.getHistoricalY(p, h)));
     }
    }
    for (int p = 0; p < pointerCount; p++)
    {
     pointMap.put(p, new TouchPoint(event.getX(p), event.getY(p)));
    }

    break;
   case MotionEvent.ACTION_DOWN:
    pointMap.put(0, new TouchPoint(event.getX(pointerIndex), event.getY(pointerIndex)));
    back_x1 = event.getX();
    back_y1 = event.getY();
    break;
   case MotionEvent.ACTION_UP:
    back_x2 = event.getX();
    back_y2 = event.getY();
    if (Math.abs(back_x1 - back_x2) > screenW / 2 && Math.abs(back_y1 - back_y2) > screenH / 2)
    {
     callOnClick();
    }
    pointMap.clear();
    break;
   default:
    break;
  }
  if (event.getPointerCount() == 0) pointMap.clear();
  invalidate();
  return true;
 }

 class TouchPoint
 {
  public float x = 0;
  public float y = 0;

  TouchPoint(float x, float y)
  {
   this.x = x;
   this.y = y;
  }
 }

 void clearDraw(Canvas canvas)
 {
  Paint paint = new Paint();
  paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
  canvas.drawPaint(paint);
  paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC));
  canvas.drawColor(Color.WHITE);
 }
}