Android實現爆炸式菜單按鈕彈出效果

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent"
 android:layout_height="match_parent" >
 <ImageButton
  android:id="@+id/button"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_centerInParent="true"
  android:layout_margin="10dp"
  android:src="@drawable/im" 
  android:background="@android:color/transparent"/>
 <ImageButton
  android:id="@+id/button1"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_centerInParent="true"
  android:layout_margin="10dp"
  android:visibility="invisible" 
  android:src="@drawable/i"
  android:background="@android:color/transparent"/>
 <ImageButton
  android:id="@+id/button2"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_centerInParent="true"
  android:layout_margin="10dp"
  android:src="@drawable/ii"
  android:visibility="invisible" 
  android:background="@android:color/transparent"/>
 <ImageButton
  android:id="@+id/button3"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_centerInParent="true"
  android:layout_margin="10dp"
  android:src="@drawable/iii"
  android:visibility="invisible"
  android:background="@android:color/transparent"/>
 <ImageButton
  android:id="@+id/button4"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_centerInParent="true"
  android:layout_margin="10dp"
  android:src="@drawable/iiii"
  android:visibility="invisible"
  android:background="@android:color/transparent" />

</RelativeLayout>

package com.example.boombuttons;
import java.util.ArrayList;
public class MainActivity extends Activity implements OnClickListener{
 // 中心按鈕
 private ImageButton button;
 // 四個子按鈕
 private ImageButton button1;
 private ImageButton button2; 
 private ImageButton button3; 
 private ImageButton button4;
 // 子按鈕列表
 private List<ImageButton> buttonItems = new ArrayList<ImageButton>(3);
 // 標識當前按鈕彈出與否，1代表已經未彈出，-1代表已彈出
 private int flag = 1;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  // 實例化按鈕并設立監聽
  button = (ImageButton)findViewById(R.id.button);
  button.setOnClickListener(this);
  button1 = (ImageButton)findViewById(R.id.button1);
  button2 = (ImageButton)findViewById(R.id.button2);
  button3 = (ImageButton)findViewById(R.id.button3);
  button4 = (ImageButton)findViewById(R.id.button4);
  // 將子按鈕們加入列表中
  buttonItems.add(button1);
  buttonItems.add(button2);
  buttonItems.add(button3);
  buttonItems.add(button4); 
 }
 /**
  * 按鈕移動動畫
  * @params 子按鈕列表
  * @params 彈出時圓形半徑radius
  */
 public void buttonAnimation(List<ImageButton> buttonList,int radius){
  for(int i=0;i<buttonList.size();i++){
   ObjectAnimator objAnimatorX;
   ObjectAnimator objAnimatorY;
   ObjectAnimator objAnimatorRotate;
   // 將按鈕設為可見
   buttonList.get(i).setVisibility(View.VISIBLE);
   // 按鈕在X、Y方向的移動距離
   float distanceX = (float) (flag*radius*(Math.cos(Util.getAngle(buttonList.size(),i))));
   float distanceY = -(float) (flag*radius*(Math.sin(Util.getAngle(buttonList.size(),i))));
   // X方向移動
   objAnimatorX = ObjectAnimator.ofFloat(buttonList.get(i), "x", buttonList.get(i).getX(),buttonList.get(i).getX()+distanceX);
   objAnimatorX.setDuration(200);
   objAnimatorX.setStartDelay(100);
   objAnimatorX.start();
   // Y方向移動
   objAnimatorY = ObjectAnimator.ofFloat(buttonList.get(i), "y", buttonList.get(i).getY(),buttonList.get(i).getY()+distanceY);
   objAnimatorY.setDuration(200);
   objAnimatorY.setStartDelay(100);
   objAnimatorY.start();
   // 按鈕旋轉
   objAnimatorRotate = ObjectAnimator.ofFloat(buttonList.get(i), "rotation", 0, 360);
   objAnimatorRotate.setDuration(200);
   objAnimatorY.setStartDelay(100);
   objAnimatorRotate.start();
   if(flag==-1){
    objAnimatorX.addListener(new AnimatorListener() {
     @Override
     public void onAnimationStart(Animator animation) {
      // TODO Auto-generated method stub
     }
     @Override
     public void onAnimationRepeat(Animator animation) {
      // TODO Auto-generated method stub
     }
     @Override
     public void onAnimationEnd(Animator animation) {
      // TODO Auto-generated method stub
      // 將按鈕設為可見
      for (int i = 0; i < buttonItems.size(); i++) {
       buttonItems.get(i).setVisibility(View.INVISIBLE);
      }
     }
     @Override
     public void onAnimationCancel(Animator animation) {
      // TODO Auto-generated method stub
     }
    });
   }
  }
 }
}

public class Util {
 /**
  * 返回每個按鈕應該出現的角度(弧度單位)
  * @param index
  * @return double 角度(弧度單位)
  */
 public static double getAngle(int total,int index){
  return Math.toRadians(90/(total-1)*index+90);
 }
}