在重新启动活动之前，TextView为空

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putString("textViewText", textView.getText().toString());
}

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
    String textViewText = savedInstanceState.getString("textViewText");
    textView.setText(textViewText);
}

public class MyViewModel extends ViewModel {
    private MutableLiveData<String> textViewText = new MutableLiveData<>();

    public void setTextViewText(String text) {
        textViewText.setValue(text);
    }

    public LiveData<String> getTextViewText() {
        return textViewText;
    }
}

// 在活动中使用ViewModel
public class MyActivity extends AppCompatActivity {
    private MyViewModel viewModel;

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

        viewModel = ViewModelProviders.of(this).get(MyViewModel.class);

        final TextView textView = findViewById(R.id.textView);

        viewModel.getTextViewText().observe(this, new Observer<String>() {
            @Override
            public void onChanged(@Nullable String text) {
                textView.setText(text);
            }
        });
    }

    // 在活动中设置TextView的文本内容
    private void setTextViewText(String text) {
        viewModel.setTextViewText(text);
    }
}