Question

目前我正在尝试创建一个媒体播放器，我试图从我创建的活动类调用updateMusic方法，并将其显示到listFragemnt类

但我一直收到这个错误，我不确定如何让它工作我最近在活动中编写了整个应用程序并且它工作得非常好，因此我尝试将其格式化为片段，以便我可以添加一个用于滑动的viewpager，但除了这一点之外

MediaActivity类

 public class MediaActivity extends ListActivity {
    Uri uri = Uri.parse(Environment.getExternalStorageDirectory() + "/");
    public Uri uriMusic = Uri.parse("/storage/emulated/0/MUSIC/");
    public Uri uriVideo = Uri.parse("/storage/emulated/0/VIDEOS/");
    private static final String SD_PATH = new String(Environment.getExternalStorageDirectory().getPath() + "/");
    public static final String MUSIC_PATH = new String("/storage/emulated/0/MUSIC/");
    public static final String VIDEO_PATH = new String("/storage/emulated/0/VIDEOS/");

     public List<String> videos = new ArrayList<String>();
    public List<String> songs = new ArrayList<String>();
    private MediaPlayer mp = new MediaPlayer();
 public void updateMusic() {

        File home = new File(MUSIC_PATH);
        if (home.listFiles(new mp3Filter()).length > 0) {
            for (File file : home.listFiles(new mp3Filter())) {
                songs.add(file.getName());
            }
            ArrayAdapter<String> songList = new ArrayAdapter<String>(this, R.layout.song_item, songs);
            setListAdapter(songList);


        }
    }

然后这是CategorySong类，它扩展了片段以显示列表

    public class CategorySongs extends ListFragment {
    MediaActivity mediaActivity;

    @Override
    public View onCreateView(LayoutInflater inflater,
                             @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

        container.removeAllViews();

        mediaActivity = new MediaActivity();
        mediaActivity.updateMusic();
        mediaActivity.buttons();

        return inflater.inflate(R.layout.activity_media, container, false);


    }


}

但我刚收到此错误

07-13 20:54:26.701    7281-7281/com.example.mason.mediaplayer E/MediaPlayer-JNI﹕ QCMediaPlayer mediaplayer NOT present 07-13 20:54:26.711    7281-7281/com.example.mason.mediaplayer E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.mason.mediaplayer, PID: 7281
java.lang.IllegalStateException: System services not available to Activities before onCreate()
        at android.app.Activity.getSystemService(Activity.java:5259)
        at android.widget.ArrayAdapter.init(ArrayAdapter.java:310)
        at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:153)
        at com.example.mason.mediaplayer.MediaActivity.updateMusic(MediaActivity.java:287)
        at com.example.mason.mediaplayer.CategorySongs.onCreateView(CategorySongs.java:28)

Answer 1

这是因为Android OS首先实例化Activity，然后它进行一些额外的设置以提供该Activity的基本Context和环境信息。这些是访问LayoutInflater等系统服务所必需的，这是ArrayAdapter在构建期间尝试访问的内容。

您的代码包含您声明它们的同一行中成员变量的分配。这些赋值在构造时发生（它们也可能在Activity的构造函数中），这是在OS完成Activity的设置之前。您需要将这些作业移至onCreate()或更晚。

onCreate（）之前活动不可用的系统服务？

1 个答案: