android - 如何从 mp3 数组 [Android] 播放音频

Question

在一个测试项目上工作，我目前的目标是使用一个 int 来指定在应用程序初始化时播放哪个 mp3 文件，而无需事先知道配乐的名称(可扩展性问题，因为我希望能够添加新的mp3 文件添加到随机化池中，而无需在将来触及代码)。但是，当我运行下面的代码时，出现了如下描述的错误:

error: no suitable method found for create(MainActivity,String) method MediaPlayer.create(Context,Uri) is not applicable (argument mismatch; String cannot be converted to Uri) 方法 MediaPlayer.create(Context,int) 不适用适用(参数不匹配；String 无法转换为 int)

import android.content.res.AssetManager;
import android.content.res.Resources;
import android.media.MediaPlayer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.RecyclerView;
import android.widget.ArrayAdapter;

import java.io.InputStream;
import java.lang.reflect.Field;
import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    RecyclerView bensonRecycler;
    ArrayList<String> arrayList;

    ArrayAdapter bensonAdapter;
    MediaPlayer bensonPlayer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        AssetManager assetManager = getAssets();
        String[] audios = assetManager.list("sound");

        int i = 1;

        final MediaPlayer mp = MediaPlayer.create(this, audios[i]);
        mp.start();

    }
}

我的 mp3 文件 Assets 具有以下结构:

所以我的问题是我应该怎么做才能在 mp3 Assets 数组中使用 int i 指定要播放的 mp3 音轨(事先不知道 mp3 音轨的名称)？ (我必须使用一个变量来指定在这个测试项目中播放的配乐)

Answer 1

“assets”文件夹根本不是一个真正的文件夹，而是一个 bundle 的字节流。这就是为什么有 AssetManager .

AssetManager

Provides access to an application's raw asset files; see Resources for the way most applications will want to retrieve their resource data. This class presents a lower-level API that allows you to open and read raw files that have been bundled with the application as a simple stream of bytes.

AssetManager 让我们可以很容易地构建文件的字符串数组，并且我们可以将这些文件视为属于一种小型文件系统。诀窍在于如何将这些文件(实际上是字节流)呈现给 MediaPlayer，以便可以根据声音 Assets 的索引播放声音。

以下代码将播放索引声音。代码中的注释解释了它是如何工作的。为了进行测试，我刚刚构建了一个简单的布局，其中包含三个在索引 0、1 和 2 处播放声音的按钮。

主 Activity

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    private void playSoundFromAssets(int index) {
        try {
            AssetManager assetManager = getAssets();
            String[] audios = assetManager.list(SOUNDFILE_PATH);
            if (audios == null || index >= audios.length) {
                return;
            }
            String soundFilePath = new File(SOUNDFILE_PATH, audios[index]).getPath();
            AssetFileDescriptor afd = getAssets().openFd(soundFilePath);
            final MediaPlayer mp = new MediaPlayer();

            /*  For API 24+, we can just use the AssetFileDescriptor to play the sound. However,
                for API 23-, we can't use the AssetFileDescriptor directly but can retrieve a
                FileDescriptor from it that points to the beginning of our assets. The offset
                and length from the AssetFileDescriptor serve for the FileDescriptor as well.
             */

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                mp.setDataSource(afd);
            } else {
                FileDescriptor fd = afd.getFileDescriptor();
                Log.d("MainActivity", String.format("<<<< %s %d %d", soundFilePath, afd.getStartOffset(), afd.getLength()));
                mp.setDataSource(fd, afd.getStartOffset(), afd.getLength());

                // One might think that mp.setDataSource(fd) would play the sound file we want, but
                // it actually plays all sound files one after another. It seems that fd is a
                // FileDescriptor that points to the beginning of the assets.
            }
            afd.close();
            mp.prepare();
            mp.start();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    public void playSound(View view) {

        switch (view.getId()) {
            case R.id.play0:
                playSoundFromAssets(0);
                break;

            case R.id.play1:
                playSoundFromAssets(1);
                break;

            case R.id.play2:
                playSoundFromAssets(2);
                break;
        }
    }

    private static final String SOUNDFILE_PATH = "sound";
}