Android实现拖动进度条改变图片透明度

layout文件：

 <?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"
     android:paddingBottom="@dimen/activity_vertical_margin"
     android:paddingLeft="@dimen/activity_horizontal_margin"
     android:paddingRight="@dimen/activity_horizontal_margin"
     android:paddingTop="@dimen/activity_vertical_margin"
     tools:context="com.hanqi.testapp2.PractiseActivity"
     android:orientation="vertical">

     <SeekBar
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:id="@+id/sb_1"
         android:max="255"    //设置最大进度为255
         android:progress="0"/>  //设置初始进度为0
     <ImageView
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:src="@drawable/f4"
         android:id="@+id/iv_1"/>
 </LinearLayout>

java类代码：

 package com.hanqi.testapp2;

 import android.support.v7.app.AppCompatActivity;
 import android.os.Bundle;
 import android.widget.ImageView;
 import android.widget.SeekBar;

 public class PractiseActivity extends AppCompatActivity {

     SeekBar sb_1;
     ImageView iv_1;
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_practise);
         sb_1 =(SeekBar)findViewById(R.id.sb_1);
         iv_1 =(ImageView)findViewById(R.id.iv_1);
         iv_1.setImageAlpha(0);  //此步骤是设置一开始图片就为透明状态，不设置图片会在开始运行的时候显示
         sb_1.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
             @Override
             public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {

                 iv_1.setImageAlpha(progress);
             }

             @Override
             public void onStartTrackingTouch(SeekBar seekBar) {

             }

             @Override
             public void onStopTrackingTouch(SeekBar seekBar) {

             }
         });
     }
 }

效果为：