Jetpack 之 ViewModel

ViewModel 以注重生命周期的方式存储和管理界面相关的数据。
ViewModel 类让数据可在发生屏幕旋转等配置更改后继续留存。

问题：

1.旋转界面，配置更改，导致数据丢失，onSaveInstanceState() 方法保存 然后从 onCreate() 中的Bundle 或者 onRestoreInstanceState 恢复数据，但IPC对Bundle有1M的限制，如何做到大数据恢复

2.逻辑层会持有UI层的引用（比如：在MVP的Presenter中需要持有IView接口来回调结果给界面），如何避免内存泄漏

ViewModel生命周期长于Activity，因系统配置变更Activity销毁重建，ViewModel对象会保留并关联到新的Activity。而Activity的正常销毁（系统不会重建Activity）时，ViewModel对象是会清除的。

ViewModel包装了基于观察者模式的LiveData（问题2解决）

ViewModel使用：

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class TestViewModelActivity : AppCompatActivity() {

    val TAG = "TestViewModelActivity"
    val myViewModel: MyViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_viewmodel)
        val name = findViewById<TextView>(R.id.name)

        Log.d(TAG, "myViewModel: $myViewModel")
        Log.d(TAG, "onCreate: / " + System.currentTimeMillis())

        myViewModel.getUserData().observe(this, {
            Log.d(TAG, "observe:" + it.toString() + " / " + System.currentTimeMillis())
            name.text = it.toString()
        })
    }

    override fun onStart() {
        super.onStart()
        Log.d(TAG, "onStart: ")
    }

    override fun onResume() {
        super.onResume()
        Log.d(TAG, "onResume: ")
    }

    override fun onPause() {
        super.onPause()
        Log.d(TAG, "onPause: ")
    }

    override fun onStop() {
        super.onStop()
        Log.d(TAG, "onStop: ")
    }

    override fun onDestroy() {
        super.onDestroy()
        Log.d(TAG, "onDestroy: ")
    }

}
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class MyViewModel : ViewModel() {

    private val userData: MutableLiveData<List<String>> by lazy {
        MutableLiveData<List<String>>().also {
            getData()
        }
    }

    private fun getData() {
        Thread {
            Thread.sleep(3000)
            userData.postValue(listOf("Stew", "Helen", "Bob", "Lucy"))
            Thread.sleep(3000)
            userData.postValue(listOf("Stew", "Helen", "Bob"))
        }.start()
    }

    fun getUserData(): LiveData<List<String>> {
        return userData
    }

}
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TestViewModelActivity: myViewModel: com.stew.kotlinjetpack.viewmodel.MyViewModel@7ee52e8
TestViewModelActivity: onCreate: / 1658413519213
TestViewModelActivity: onStart: 
TestViewModelActivity: onResume: 
TestViewModelActivity: observe:[Stew, Helen, Bob, Lucy] / 1658413522218
TestViewModelActivity: observe:[Stew, Helen, Bob] / 1658413525219

旋转屏幕

TestViewModelActivity: onPause: 
TestViewModelActivity: onStop: 
TestViewModelActivity: onDestroy: 
TestViewModelActivity: myViewModel: com.stew.kotlinjetpack.viewmodel.MyViewModel@7ee52e8
TestViewModelActivity: onCreate: / 1658413576399
TestViewModelActivity: onStart: 
TestViewModelActivity: observe:[Stew, Helen, Bob] / 1658413576410
TestViewModelActivity: onResume:

如果在初始化的时候就开始获取数据，必须在子线程异步获取，不然会报错(?????)

旋转屏幕myViewModel不会变化，退出重进会变化

fragment共享viewmodel数据：

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class Test4Fragment : Fragment() {
    private val sharedViewModel: SharedViewModel by activityViewModels()
    
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    view.findViewById<TextView>(R.id.t1).setOnClickListener {
        i++
        sharedViewModel.getData(i.toString())
    }
    }
}

class Test5Fragment : Fragment() {
    val sharedViewModel: SharedViewModel by activityViewModels()
    
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    val t = view.findViewById<TextView>(R.id.t1)
    sharedViewModel.getSharedData().observe(viewLifecycleOwner, {
        t.text = "Test5 data: $it"
    })
    }

}

源码分析

ViewModel实例的获取是通过ViewModelProvider类

ViewModelStoreOwner——ViewModel存储器拥有者；
ViewModelStore——ViewModel存储器，用来存ViewModel的地方；
Factory——创建ViewModel实例的工厂（通过传入的class 反射获取ViewModel实例。）

ViewModelStoreOwner是个接口，实现类有Activity/Fragment，也就是说 Activity/Fragment 都是 ViewModel存储器的拥有者

ViewModelStore中，viewModel作为Value存储在HashMap中。

viewModelProvider.get(UserViewModel.class)
先尝试从ViewModelStore获取ViewModel实例，如果没有获取到，就使用Factory创建，然后存入ViewModelStore。

ViewModelStoreOwner是个接口，实现类有Activity/Fragment

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@Override
public ViewModelStore getViewModelStore() {
    if (getApplication() == null) {
        throw new IllegalStateException("Your activity is not yet attached to the "
                + "Application instance. You can't request ViewModel before onCreate call.");
    }
    if (mViewModelStore == null) {
        NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
            // Restore the ViewModelStore from NonConfigurationInstances
            mViewModelStore = nc.viewModelStore;
        }
        if (mViewModelStore == null) {
            mViewModelStore = new ViewModelStore();
        }
    }
    return mViewModelStore;
}

先尝试 从NonConfigurationInstance从获取 ViewModelStore实例，如果NonConfigurationInstance不存在，就new一个mViewModelStore。

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@Override
@Nullable
public final Object onRetainNonConfigurationInstance() {
    Object custom = onRetainCustomNonConfigurationInstance();

    ViewModelStore viewModelStore = mViewModelStore;
    if (viewModelStore == null) {
        // No one called getViewModelStore(), so see if there was an existing
        // ViewModelStore from our last NonConfigurationInstance
        NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
            viewModelStore = nc.viewModelStore;
        }
    }

    if (viewModelStore == null && custom == null) {
        return null;
    }

    NonConfigurationInstances nci = new NonConfigurationInstances();
    nci.custom = custom;
    nci.viewModelStore = viewModelStore;
    return nci;
}

并且还注意到，在onRetainNonConfigurationInstance()方法中 会把mViewModelStore赋值给NonConfigurationInstances，在Activity因配置改变 而正要销毁时，且新Activity会立即创建，那么系统就会调用此方法，也就说，配置改变时 系统把viewModelStore存在了NonConfigurationInstances中。

ComponentActivity静态内部类，非配置实例

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static final class NonConfigurationInstances {
    Object custom;
    ViewModelStore viewModelStore;
}
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//Acticity.java

NonConfigurationInstances mLastNonConfigurationInstances;

//返回onRetainNonConfigurationInstance()返回的实例
public Object getLastNonConfigurationInstance() {
    return mLastNonConfigurationInstances != null ? mLastNonConfigurationInstances.activity : null;
}

static final class NonConfigurationInstances {
        Object activity;
        HashMap<String, Object> children;
        FragmentManagerNonConfig fragments;
        ArrayMap<String, LoaderManager> loaders;
        VoiceInteractor voiceInteractor;
    }


final void attach(Context context, ActivityThread aThread, ...
            NonConfigurationInstances lastNonConfigurationInstances,... ) {
            ...
            mLastNonConfigurationInstances = lastNonConfigurationInstances;
            ...
       }

mLastNonConfigurationInstances是在Activity的attach方法中赋值

lastNonConfigurationInstances是存在 ActivityClientRecord中的一个组件信息

ActivityThread 中的 ActivityClientRecord 是不受 activity 重建的影响，那么ActivityClientRecord中lastNonConfigurationInstances也不受影响，那么其中的Object activity也不受影响，那么ComponentActivity中的NonConfigurationInstances的viewModelStore不受影响，那么viewModel也就不受影响了。(问题1解决)

  • onSaveInstanceState的调用遵循一个重要原则，即当系统“未经你许可”时销毁了你的activity，则onSaveInstanceState会被系统调用

  • 使用ViewModel恢复数据 则 只有在 因配置更改界面销毁重建 的情况。

  • ViewModel是存在内存中，读写速度快

  • onSaveInstanceState是序列化到磁盘中

  • ViewModel，可以存复杂数据，大小限制就是App的可用内存

  • onSaveInstanceState只能存可序列化和反序列化的对象，且大小有限制（一般Bundle限制大小1M）。