jetpack库 理解SavedStateHandle

作用

和ViewModel协作，用于从Activity销毁重建中恢复ViewModel的相关状态数据,我们知道当Activity被安卓系统销毁和重建的时候会调用onSaveInstanceState和onRestoreInstanceState方法，这个方法对于ViewModel来说是无法感知的，SavedStateHandle填补了这一空白,使得ViewModel可以通过此来保存一些状态数据

描述

SavedStateHandle是一个key-value映射，用于通过set()和get()方法向已保存的状态中检索和写入数据

如何使用?

1. 将SavedStateHandle作为ViewModel的构造参数
2. ViewModel内部通过SavedStateHandle.getLiveData方法来生成一个 LiveData 对象,LiveData中的数据就是我们想要持久化保存的数据，如果Activity重建，这个LiveData中就会是重建的数据

支持的类型

理论上来说Bundle可保存的类型都支持

原理分析

• SavedStateRegistryOwner

用来表示这个类拥有数据重建的能力，用来提供SavedStateRegistry

• SavedStateRegistryController

SavedStateRegistry的控制类

• SavedStateRegistry

数据存储和恢复

• SavedStateProvider

将要保存的数据通过Bundle封装起来

基本关系

SavedStateRegistryOwner → SavedStateRegistryController → SavedStateRegistry → SavedStateProvider

SavedStateRegistryController可以理解成是SavedStateRegistry的包装类

代码流程分析

保存数据

1.png

在Activity调用onSaveInstanceState的时候，调用了SavedStateRegistryController的performSave方法,该方法如下

@MainThread
    fun performSave(outBundle: Bundle) {
        savedStateRegistry.performSave(outBundle)
    }

直接调用了SavedStateRegistry的performSave方法

    @MainThread
    @Suppress("INACCESSIBLE_TYPE")
    fun performSave(outBundle: Bundle) {
        val components = Bundle() // 创建一个Bundle
                // 上一次重建保留的状态先保留
        if (restoredState != null) {
            components.putAll(restoredState)
        }
        val it: Iterator<Map.Entry<String, SavedStateProvider>> =
            this.components.iteratorWithAdditions()
        // 可以看到，这里基本上是遍历components，然后调用saveState方法，放到我们创建的bundle中
        while (it.hasNext()) {
            val (key, value) = it.next()
            components.putBundle(key, value.saveState())
        }
                // 最后将这部分数据保存到outBundle中
        if (!components.isEmpty) {
            outBundle.putBundle(SAVED_COMPONENTS_KEY, components)
        }
    }

重建数据

重建数据是在onCreate中重建的

@OptIn(markerClass = BuildCompat.PrereleaseSdkCheck.class)
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        // Restore the Saved State first so that it is available to
        // OnContextAvailableListener instances
        mSavedStateRegistryController.performRestore(savedInstanceState);
        mContextAwareHelper.dispatchOnContextAvailable(this);
        super.onCreate(savedInstanceState);
        ReportFragment.injectIfNeededIn(this);
        if (BuildCompat.isAtLeastT()) {
            mOnBackPressedDispatcher.setOnBackInvokedDispatcher(
                    Api33Impl.getOnBackInvokedDispatcher(this)
            );
        }
        if (mContentLayoutId != 0) {
            setContentView(mContentLayoutId);
        }
    }

调用了Controller的performRestore方法

@MainThread
    fun performRestore(savedState: Bundle?) {
        // To support backward compatibility with libraries that do not explicitly
        // call performAttach(), we make sure that work is done here
        if (!attached) {
            performAttach()
        }
        val lifecycle = owner.lifecycle
        check(!lifecycle.currentState.isAtLeast(Lifecycle.State.STARTED)) {
            ("performRestore cannot be called when owner is ${lifecycle.currentState}")
        }
        savedStateRegistry.performRestore(savedState)
    }

最终调用了SavedStateRegistry的performRestore方法

internal fun performRestore(savedState: Bundle?) {
        check(attached) {
            ("You must call performAttach() before calling " +
                "performRestore(Bundle).")
        }
        check(!isRestored) { "SavedStateRegistry was already restored." }
        restoredState = savedState?.getBundle(SAVED_COMPONENTS_KEY)

        isRestored = true
    }

将之前保存的值序列化到restoredState中

上诉SavedStateRegistry机制实现了对于数据的保存，需要使用这一套机制需要实现SavedStateProvider,然后通过SavedRegistry的registerSavedStateProvider将自己注册到里面去就可以了

那么这套机制是如何被ViewModel使用的呢，Activity和Fragment实现了HasDefaultViewModelProviderFactory接口,如Activity

public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
        if (mDefaultFactory == null) {
            mDefaultFactory = new SavedStateViewModelFactory(
                    getApplication(),
                    this,
                    getIntent() != null ? getIntent().getExtras() : null);
        }
        return mDefaultFactory;
    }

在SavedStateViewModelFactory创建ViewModel中的时候，如果ViewModel的构造函数中有SavedStateHandle,通过

LegacySavedStateHandleController.create(
            savedStateRegistry!!, lifecycle, key, defaultArgs
        )

创建SavedStateHandleController进而获取SavedStateHandle.

fun create(
        registry: SavedStateRegistry,
        lifecycle: Lifecycle,
        key: String?,
        defaultArgs: Bundle?
    ): SavedStateHandleController {
                // 从registry获取和这个ViewModel有关的bundle
        val restoredState = registry.consumeRestoredStateForKey(key!!)
                // 这个方法是吧restoreState中保存的key和value拆分出来，保存成map   
        val handle = createHandle(restoredState, defaultArgs)
        val controller = SavedStateHandleController(key, handle)
        controller.attachToLifecycle(registry, lifecycle)
        tryToAddRecreator(registry, lifecycle)
        return controller
    }

最后创建ViewModel成功，此时SavedHandle也有了保存的数据了,SavedStateHandleController创建之后，会把创建的SavedStateProvider注册到StateRegistry中去，实现了数据的保存