java - 如何从另一个类更新在 Activity 中找到的 Textview

Question

MainActivity类

public class MainActivity extends Activity {
    TextView textupdate;

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    textupdate = findViewById(R.id.textView1);
    MediaPlayerclass mpclass = MediaPlayerclass(getApplicationContext());
    mpclass.MediaPlayer_Start(R.raw.track1);

}

public void mUpdateCountv() {

    System.out.println("textupdate "textupdate); //Returns null from MediaPlayerclass
    textupdate.setText("MpCompleted");

}

媒体播放器类

public class MediaPlayerclass {
    private Context mContext = null;
    private int mResId = 0;
    MainActivity mainactivity;
    public MediaPlayer mCurrentPlayer = null;

public MediaPlayerclass(Context context) {
    this.mContext = context;
    mainactivity = new MainActivity();
}

public void MediaPlayer_Start(int resId) {
    this.mResId = resId;
    mCurrentPlayer = MediaPlayer.create(mContext, mResId);
    mCurrentPlayer.setOnCompletionListener(onCompletionListener);
    mCurrentPlayer.start();
}

public MediaPlayer.OnCompletionListener onCompletionListener = new MediaPlayer.OnCompletionListener() {
    @Override
    public void onCompletion(MediaPlayer mediaPlayer) {
        mCurrentPlayer.release();

        //Textview Update
        mainactivity.mUpdateCountv();
    }
};

日志

05-01 13:00:48.258: E/AndroidRuntime(5599): FATAL EXCEPTION: main

05-01 13:00:48.258: E/AndroidRuntime(5599): Process: com.example.media, PID: 5599

05-01 13:00:48.258: E/AndroidRuntime(5599): java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference

05-01 13:00:48.258: E/AndroidRuntime(5599): at com.example.media.MainActivity.mUpdateCountv(MainActivity.java:665)

05-01 13:00:48.258: E/AndroidRuntime(5599): at com.example.media.MediaPlayerclass$1.onCompletion(MediaPlayerclass.java:41)

05-01 13:00:48.258: E/AndroidRuntime(5599): at android.media.MediaPlayer$EventHandler.handleMessage(MediaPlayer.java:2538)

当我从 mediaplayerclass 调用 mUpdateCountv 方法时，textupdate 始终为 null，因此我无法从 MediaPlayerclass 类更新我的 textview。我的代码有什么问题吗？提前谢谢。

Answer 1

在 MediaPlayer 类构造函数中执行以下操作:

mainActivity = (MainActivity) context;

并在您的 MainActivity 中传递 getActivity() 而不是 getApplicationContext()

您正在创建一个新的 Activity，但您需要旧的实例。您可以将其用作 Context 对象(因为 Activity 是 Context 的子类)，如果需要调用 update 方法，则将其强制转换为 MainActivity。