小弟有个androi动画的问题，困扰我太久了，求大神解惑

Question

我自定义了一个View，在鼠标点击处画一个小球，利用ObjectAnimator和PropertyValuesHolder设置动画效果，为什么设置x,y属性动画正常，小球能沿着坐标轴一定，设置
alpha和scale就不起作用，这玩意是怎么知道我写的X,Y就是让他在坐标轴上移动，但是他为什么不认识alpha和scale，还是我写的有问题，代码如下

package com.example.mrwuchao.newballtest;

import android.animation.AnimatorSet;
import android.animation.ObjectAnimator;
import android.animation.PropertyValuesHolder;
import android.animation.ValueAnimator;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.RadialGradient;
import android.graphics.Shader;
import android.graphics.drawable.ShapeDrawable;
import android.graphics.drawable.shapes.OvalShape;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.view.animation.AccelerateInterpolator;
import android.widget.Button;
import android.widget.RelativeLayout;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
private float BALL_SIZE = 50f;
ArrayList arrayList = new ArrayList<>();
// Button button;
/*出现问题

• 自定义的View可以使用想x，y属性让小球移动，但是使用alpha或者scale就没有效果

• objectAnimator是怎么识别这些属性的，写出一个字符串，他是怎么判断要执行什么动画的？*/
  @Override
  protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  RelativeLayout re = (RelativeLayout)findViewById(R.id.re);
  re.addView(new MyView(this));
  // button = (Button)findViewById(R.id.button);
  }

  public class MyView extends View implements ValueAnimator.AnimatorUpdateListener {

  public MyView(Context context) {
      super(context);
      setBackgroundColor(Color.WHITE);
  }
  
  @Override
  public boolean onTouchEvent(MotionEvent event) {
      if (event.getAction() != MotionEvent.ACTION_DOWN &&
              event.getAction() != MotionEvent.ACTION_MOVE) {
          return false;
      }
      ShapeHolder newBall = addBall(event.getX(), event.getY());
  
      PropertyValuesHolder p1 = PropertyValuesHolder.ofFloat("x",event.getX(),600f,200f);
      PropertyValuesHolder p2 = PropertyValuesHolder.ofFloat("alpha",1.0f,0f);
      ObjectAnimator oa = ObjectAnimator.ofPropertyValuesHolder(newBall,p1,p2);
      oa.setDuration(2000);
     // oa.setInterpolator(new AccelerateInterpolator());
      oa.addUpdateListener(this);
      oa.start();
  

  // ObjectAnimator oa = ObjectAnimator.ofFloat(newBall,"x",600f);
  // ObjectAnimator ob = ObjectAnimator.ofFloat(newBall,"y",600f);
  //

// ob.addUpdateListener(this);
//
// AnimatorSet animatorSet = new AnimatorSet();
// animatorSet.play(oa).with(ob);
// animatorSet.setDuration(2000);
// animatorSet.start();
return true;
}

    private ShapeHolder addBall(float x, float y) {
        OvalShape circle = new OvalShape();
        circle.resize(BALL_SIZE, BALL_SIZE);
        ShapeDrawable drawable = new ShapeDrawable(circle);
        ShapeHolder shapeHolder = new ShapeHolder(drawable);
        shapeHolder.setX(x-BALL_SIZE/2);
        shapeHolder.setY(y-BALL_SIZE/2);
        shapeHolder.setAlpha(1.0f);
        Paint paint = drawable.getPaint();
        int[] color = new int[]{Color.GRAY, Color.BLUE};
        RadialGradient gradient = new RadialGradient(40f, 15f, BALL_SIZE, color, null, Shader.TileMode.CLAMP);
        paint.setShader(gradient);
        //paint.setColor(Color.RED);
        shapeHolder.setPaint(paint);
        arrayList.add(shapeHolder);
        return shapeHolder;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        for (ShapeHolder sh : arrayList) {
            canvas.save();
            canvas.translate(sh.getX(),sh.getY());
            sh.getD().draw(canvas);
            canvas.restore();
        }
    }

    @Override
    public void onAnimationUpdate(ValueAnimator animation) {
        this.invalidate();
    }
}

}

包装类
package com.example.mrwuchao.newballtest;

import android.graphics.Paint;
import android.graphics.drawable.ShapeDrawable;

/**

• Created by Mr.wuchao on 2016/6/8.
  */
  public class ShapeHolder {

  public void setDrawable(ShapeDrawable drawable) {
  this.d = drawable;
  }

  ShapeDrawable d;
  Paint paint;

  float alpha = 1.0f;
  float x,y;

  public ShapeHolder(ShapeDrawable drawable) {
  this.d = drawable;
  }

  public ShapeDrawable getD() {
  return d;
  }

  public void setD(ShapeDrawable d) {
  this.d = d;
  }
  public float getAlpha() {
  return alpha;
  }

  public void setAlpha(float alpha) {
  this.alpha = alpha;
  }
  public float getX() {
  return x;
  }

  public void setX(float x) {
  this.x = x;
  }

  public Paint getPaint() {
  return paint;
  }

  public void setPaint(Paint paint) {
  this.paint = paint;
  }
  public float getY() {
  return y;
  }

  public void setY(float y) {
  this.y = y;
  }
  }