android studio Koala简单的音乐播放器

### 创建 Android 音乐播放器应用

#### 准备工作
为了构建一个简单的音乐播放器，在开始编码之前，需确保已安装并配置好最新版的 Android Studio 和必要的 SDK 工具。

#### 添加权限声明
在 `AndroidManifest.xml` 文件中添加读取存储卡上的音频文件所需权限：

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

#### 布局设计
定义主活动布局文件 `activity_main.xml` 如下所示：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <!-- 显示当前歌曲名称 -->
    <TextView
        android:id="@+id/songTitleLabel"
        android:textColor="#000"
        android:textSize="24sp"
        android:gravity="center_horizontal"
        android:paddingTop="24dp"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"/>

    <!-- 控制按钮区域 -->
    <Button
        android:id="@+id/playBtn"
        android:text="Play"
        android:layout_gravity="center"
        android:layout_marginTop="20dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</LinearLayout>

#### 主要逻辑实现
编辑 MainActivity.java 或者 Kotlin 版本的 MainActivity.kt 来处理媒体控制逻辑。这里给出 Java 实现的例子：

package com.example.musicplayer;

import androidx.appcompat.app.AppCompatActivity;
import android.Manifest;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import java.util.ArrayList;
import android.media.MediaPlayer;
import android.net.Uri;
import android.provider.MediaStore;
import android.database.Cursor;
import pub.devrel.easypermissions.AfterPermissionGranted;
import pub.devrel.easypermissions.EasyPermissions;

public class MainActivity extends AppCompatActivity implements EasyPermissions.PermissionCallbacks {

    private MediaPlayer mediaPlayer;
    private TextView songTitleLabel;
    private Button playBtn;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 初始化组件引用
        songTitleLabel = findViewById(R.id.songTitleLabel);
        playBtn = findViewById(R.id.playBtn);

        requestStoragePermission();
        
        setupMediaPlayerControls();
    }

    private void setupMediaPlayerControls() {
        playBtn.setOnClickListener(new View.OnClickListener(){
            public void onClick(View v){
                togglePlayback();
            }
        });
    }

    private void togglePlayback() {
        if (mediaPlayer != null && mediaPlayer.isPlaying()) {
            pauseMusic();
        } else {
            startPlayingFirstSong();
        }
    }

    private void startPlayingFirstSong() {
        ArrayList<String> songsList = getSongsFromDevice();

        if (!songsList.isEmpty()){
            String firstSongPath = songsList.get(0);  // 获取第一个音轨路径
            
            try{
                mediaPlayer = new MediaPlayer();
                mediaPlayer.setDataSource(firstSongPath);
                mediaPlayer.prepare();                
                mediaPlayer.start();
                updateUIForPlayingState(songsList.get(0));
            } catch(Exception e){
                e.printStackTrace();
            }
            
            setOnCompletionListener();
        }
    }

    private void setOnCompletionListener(){
        mediaPlayer.setOnCompletionListener(new OnCompletionListener(){[^1]
            @Override
            public void onCompletion(MediaPlayer mp) {
                stopMedia(mp);
                updateUIForStoppedState();
            }
        });
    }

    private void updateUIForPlayingState(String title){
        playBtn.setText("Pause");
        songTitleLabel.setText(title);
    }

    private void updateUIForStoppedState(){
        playBtn.setText("Play");
        songTitleLabel.setText("");
    }

    private void pauseMusic(){
        if(mediaPlayer!=null){
            mediaPlayer.pause();
            updateUIForPausedState();
        }
    }

    private void updateUIForPausedState(){
        playBtn.setText("Resume");
    }

    private void stopMedia(MediaPlayer mPlayer){
        if(mPlayer!=null){
            mPlayer.stop();
            mPlayer.release();
            mPlayer=null;
        }
    }

    private ArrayList<String> getSongsFromDevice(){
        final ArrayList<String> tempAudioList=new ArrayList<>();
        Uri uri= MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
        Cursor cursor=getContentResolver().query(uri,null,null,null,
          MediaStore.Audio.AudioColumns.DISPLAY_NAME+" ASC");

        if(cursor!=null&&cursor.moveToFirst()){
           do{
              int audioIndex=cursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DATA);
              tempAudioList.add(cursor.getString(audioIndex));              
           }while (cursor.moveToNext());
           
           cursor.close();
        }
        return tempAudioList;
    }

    @AfterPermissionGranted(REQUEST_CODE_ASK_PERMISSIONS)
    private void requestStoragePermission(){
       String[] perms={Manifest.permission.READ_EXTERNAL_STORAGE};
       
       if(EasyPermissions.hasPermissions(this,perms)){
         // Already have permission, do the thing!
       }else{
         // Do not have permissions, request them now
         EasyPermissions.requestPermissions(this,"Need to read storage", REQUEST_CODE_ASK_PERMISSIONS, perms);
       }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode,String[] permissions,int[] grantResults){
      super.