ViewModelSaveState

记得加上依赖 module的gradle

    implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
    implementation "androidx.lifecycle:lifecycle-viewmodel:2.2.0"
    implementation 'androidx.lifecycle:lifecycle-viewmodel-savedstate:2.3.0-alpha03'

ViewModel类

public class MyViewModel extends ViewModel {
    private final SavedStateHandle handle;
    public MyViewModel(SavedStateHandle handle) {

        this.handle = handle;
    }

    public MutableLiveData<Integer> getNumber() {
        if(!handle.contains(MainActivity.KEY_NUMBER)) {
            handle.set(MainActivity.KEY_NUMBER,0);
        }
        return handle.getLiveData(MainActivity.KEY_NUMBER);

    public void add() {
        getNumber().setValue(getNumber().getValue() + 1);
    }
}

//private MutableLiveData<Integer> number;
//        number.setValue(number.getValue() + 1);

//        淘汰
//        if(number == null) {
//            number = new MutableLiveData<>();
//            number.setValue(0);
//        }
//        return number;
    }

public class MainActivity extends AppCompatActivity {
    ActivityMainBinding binding;
    MyViewModel myViewModel;
    public final static String KEY_NUMBER = "my_number";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = DataBindingUtil.setContentView(this,R.layout.activity_main);
        myViewModel = new ViewModelProvider(this).get(MyViewModel.class);

        binding.setData(myViewModel);
        binding.setLifecycleOwner(this);
    }

//        if(savedInstanceState != null) {
//            myViewModel.getNumber().setValue(savedInstanceState.getInt(KEY_NUMBER));
//        }
//    @Override
//    protected void onSaveInstanceState(@NonNull Bundle outState) {
//        super.onSaveInstanceState(outState);
//        outState.putInt(KEY_NUMBER,myViewModel.getNumber().getValue());
//    }
}