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:

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

创建ViewModel

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

在布局文件中使用,

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<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,

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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

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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:

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//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,

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//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,

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//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