Android-Jetpack笔记-ViewModelSavedState

上篇文章提到，虽然viewModel要比onSaveInstanceState简单，但是viewModel只能在屏幕旋转和语言切换后（即配置变更时）的页面重建维持数据，当页面意外销毁时数据无法恢复（viewModel也会重建），而这点onSaveInstanceState可以做到。关于意外销毁，我们暂且理解成非配置变更引起的销毁重建，比如内存不足等场景。

Jetpack笔记代码

本文源码基于SDK 29

问题复现

引入依赖：

def lifecycle_version = "2.2.0"
//extensions包含Lifecycles、LiveData、ViewModel
implementation "android.arch.lifecycle:extensions:$lifecycle_version"

创建ViewModel，

class CommonViewModel extends ViewModel {
    public MutableLiveData<String> text = new MutableLiveData<>();
}

在布局文件中使用，

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <data>
        <import type="com.holiday.jetpackstudy.viewmodel_livedata.CommonViewModel" />
        <variable
            name="commonViewModel"
            type="CommonViewModel" />
    data>

    <TextView
          android:id="@+id/tv_text"
          android:text="@{commonViewModel.text}"
          android:textSize="@dimen/tv_text_size" />
layout>

在act中使用，

class ViewModelActivity extends BaseActivity {
    CommonViewModel mCommonViewModel;
    String mTime;

    void onCreate(Bundle savedInstanceState) {
        //传this，基于act创建viewModel
        mCommonViewModel = ViewModelProviders.of(this).get(CommonViewModel.class);
        mBinding.setCommonViewModel(mCommonViewModel);
        //观察数据变化
        mCommonViewModel.text.observe(this, new Observer<String>() {
            @Override
            public void onChanged(String s) {
                //更新UI
                mBinding.tvText.setText(s);
            }
        });
        //在页面被意外销毁后，ViewModel会重建
        QrLog.e(String.valueOf(mCommonViewModel.hashCode()));

        if (null == savedInstanceState) {
            mTime = String.valueOf(System.currentTimeMillis() / 1000);
            QrLog.e("onCreate 获取当前时间 = " + mTime);
        } else {
            mTime = savedInstanceState.getString("test");
            QrLog.e("onCreate 恢复上次时间 = " + mTime);
        }
    }

    void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        //在页面被意外销毁时，存储act的创建时间
        outState.putString("test", mTime);
    }
}

在onCreate方法中，新加了savedInstanceState的取值操作，同时重写了onSaveInstanceState方法存储时间，那么如何模拟页面被意外销毁呢，可以在开发者选项中选中不保留活动-用户离开后即销毁每个活动，开启后，运行app，然后按home键引起页面意外销毁，然后回到页面，查看日志：

可见当页面意外销毁时，viewModel并不能很好的维持数据。

解决

如果需要让ViewModel能在页面意外销毁时维持数据，那就需要结合SavedStateHandle使用了，新建一个ViewModel，

class SavedStateViewModel extends ViewModel {
    //需要引用SavedStateHandle
    private SavedStateHandle mHandle;

    public SavedStateViewModel(SavedStateHandle handle) {
        mHandle = handle;

        Object text = mHandle.get("text");
        if (null == text) {
            String time = String.valueOf(System.currentTimeMillis() / 1000);
            mHandle.set("text", time);
            QrLog.e("SavedStateViewModel 初始化数据 = " + time);
        } else {
            QrLog.e("SavedStateViewModel 恢复数据 = " + text);
        }
    }
}

然后在act中加入：

//ViewModelActivity.java
class ViewModelActivity extends BaseActivity {
    SavedStateViewModel mSavedStateViewModel;
    void onCreate(Bundle savedInstanceState) {
        //这边创建时传入了SavedStateViewModelFactory
        mSavedStateViewModel = ViewModelProviders
            .of(this, new SavedStateViewModelFactory(getApplication(), this))
            .get(SavedStateViewModel.class);
        QrLog.e("mSavedStateViewModel hashCode = " + mSavedStateViewModel.hashCode());
    }
}

运行到该页面，点击home键触发意外销毁，然后回到页面，查看日志，

发现虽然mSavedStateViewModel不再是同一个实例，但是数据是可以恢复的。

至于原理，大致的思路就是在SavedStateViewModelFactory中，

//SavedStateViewModelFactory.java
<T extends ViewModel> T create(String key, Class<T> modelClass) {
    //借助SavedStateHandleController存储了SavedStateHandle
    SavedStateHandleController controller = SavedStateHandleController.create(
                mSavedStateRegistry, mLifecycle, key, mDefaultArgs);
    //创建viewmodel时传入SavedStateHandle
    T viewmodel = constructor.newInstance(controller.getHandle());
}

而在SavedStateHandleController中，

//SavedStateHandleController.java
static SavedStateHandleController create(SavedStateRegistry registry, Lifecycle lifecycle,
                                         String key, Bundle defaultArgs) {
    //通过act类名生成的key找到Bundle
    Bundle restoredState = registry.consumeRestoredStateForKey(key);
    //通过Bundle恢复数据，具体实现看下一个方法
    SavedStateHandle handle = SavedStateHandle.createHandle(restoredState, defaultArgs);
    //包装成SavedStateHandleController进行返回
    SavedStateHandleController controller = new SavedStateHandleController(key, handle);
    return controller;
}

//SavedStateHandle.java
static SavedStateHandle createHandle(Bundle restoredState,Bundle defaultState) {
    if (restoredState == null && defaultState == null) {
        return new SavedStateHandle();
    }
    //数据恢复
    Map<String, Object> state = new HashMap<>();
    ArrayList keys = restoredState.getParcelableArrayList(KEYS);
    ArrayList values = restoredState.getParcelableArrayList(VALUES);
    for (int i = 0; i < keys.size(); i++) {
        state.put((String) keys.get(i), values.get(i));
    }
    //虽然SavedStateHandle不再是同一个实例，但是数据都被恢复过来了
    return new SavedStateHandle(state);
}

即本质还是通过Bundle的序列化和反序列化来恢复数据的。

参考

• B站-jetpack课程ViewModelSavedState