Android 进度条改变图片透明度

 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     tools:context="com.example.administrator.jsq.MainActivity"
     android:orientation="vertical">
     <!--图片透明-->
     <SeekBar
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:max="250"
         android:progress="0"
         android:id="@+id/sar"/>
     <ImageView
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:src="@drawable/f8"
         android:id="@+id/tp"
         />
 </LinearLayout>

 package com.example.administrator.jsq;

 import android.media.Image;
 import android.support.v7.app.AppCompatActivity;
 import android.os.Bundle;
 import android.util.Log;
 import android.view.View;
 import android.widget.ImageView;
 import android.widget.SeekBar;

 public class MainActivity extends AppCompatActivity {
     SeekBar sar;
     ImageView tp;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         sar = (SeekBar)findViewById(R.id.sar);
         tp = (ImageView)findViewById(R.id.tp);
         tp.setImageAlpha(0);
         sar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
             @Override
             public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                 tp.setImageAlpha(progress);
             }

             @Override
             public void onStartTrackingTouch(SeekBar seekBar) {
                 Log.e("TAG", "进度条开始拖动");
             }

             @Override
             public void onStopTrackingTouch(SeekBar seekBar) {
                 Log.e("TAG","进度条停止拖动");
             }
         });
     }
 }

 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.example.administrator.jsq">

     <application
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:supportsRtl="true"
         android:theme="@style/AppTheme">
         <activity android:name=".MainActivity">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
     </application>

 </manifest>