【Android】源码分析ViewModel如何实现横竖屏切换时不销毁

港控/mmm° 2022-12-17 15:52 146阅读 0赞

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# ViewModel #

--------------------

ViewModel是Android Jetpack中的重要组件，其优势是具有下图这样的生命周期、不会因为屏幕旋转等Activity配置变化而销毁，是实现MVVM架构中UI状态管理的重要基础。  
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class HogeActivity : AppCompatActivity { 
    
      override fun onCreate(savedInstanceState: Bundle?) { 
        super.onCreate(savedInstanceState)
        Log.d(TAG, "onCreate")
    
        val activity: FragmentActivity = this
        val factory: ViewModelProvider.Factory = ViewModelProvider.NewInstanceFactory()
    
        // Activity由于横竖品切换销毁重建，此处的viewModel 仍然是重建前的实例
        val viewModel = ViewModelProvider(activity, factory).get(FooViewModel::class.java)
        // 如果直接new实例则会创建新的ViewModel实例
    	// val viewModel = FooViewModel()
    
        Log.d(TAG, " - Activity :${ this.hashCode()}")
        Log.d(TAG, " - ViewModel:${ viewModel.hashCode()}")
      }
    }

上面代码在横竖屏切换时的log如下：

#Activity初次启动
    onCreate
      - Activity :132818886
      - ViewModel:249530701
    onStart
    onResume
    
    #屏幕旋转
    onPause
    onStop
    onRetainNonConfigurationInstance
    onDestroy
    onCreate
      - Activity :103312713  #Activity实例不同
      - ViewModel:249530701  #ViewModel实例相同
    onStart
    onResume

下面代码是保证屏幕切换时ViewModel不销毁的关键，我们依次为入口看一下源码

val viewModel = ViewModelProvider(activity, factory).get(FooViewModel::class.java)

# ViewModelProvider #

--------------------

ViewModelProvider源码很简单，分别持有一个`ViewModelProvider.Factory`和`ViewModelStore`实例

package androidx.lifecycle;
    
    public class ViewModelProvider { 
    
        public interface Factory { 
            @NonNull
            <T extends ViewModel> T create(@NonNull Class<T> modelClass);
        }
    
        private final Factory mFactory;
        private final ViewModelStore mViewModelStore;
    
        public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) { 
            this(owner.getViewModelStore(), factory);
        }
    
        public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) { 
            mFactory = factory;
            this.mViewModelStore = store;
        }
    
        ...
    }

`get()`返回ViewModel实例

package androidx.lifecycle;
    
    public class ViewModelProvider { 
        ...
    
        public <T extends ViewModel> T get(@NonNull Class<T> modelClass) { 
            String canonicalName = modelClass.getCanonicalName();
    
            ...
    
            return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
        }
    
        public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) { 
            ViewModel viewModel = mViewModelStore.get(key);
    
            if (modelClass.isInstance(viewModel)) { 
                //noinspection unchecked
                return (T) viewModel;
            } else { 
                //noinspection StatementWithEmptyBody
                if (viewModel != null) { 
                    // TODO: log a warning.
                }
            }
    
            viewModel = mFactory.create(modelClass);
            mViewModelStore.put(key, viewModel);
            //noinspection unchecked
            return (T) viewModel;
        }
    
        ...
    }

逻辑非常清晰：

1.  ViewModelProvider通过ViewModelStore获取ViewModel
2.  若获取失败，则通过ViewModelProvider.Factory创建ViewModel

# ViewModelStore #

--------------------

package androidx.lifecycle;
    
    public class ViewModelStore { 
    
        private final HashMap<String, ViewModel> mMap = new HashMap<>();
    
        final void put(String key, ViewModel viewModel) { 
            ViewModel oldViewModel = mMap.put(key, viewModel);
            if (oldViewModel != null) { 
                oldViewModel.onCleared();
            }
        }
    
        final ViewModel get(String key) { 
            return mMap.get(key);
        }
    
        public final void clear() { 
            for (ViewModel vm : mMap.values()) { 
                vm.onCleared();
            }
            mMap.clear();
        }
    }

可见，ViewModelStore就是一个对`Map`的封装。

val viewModel = ViewModelProvider(activity, factory).get(FooViewModel::class.java)

上面代码ViewModelProvider()构造参数1中传入的`FragmentActivity`(基类是`ComponentActivity`)实际上是`ViewModelStoreOwner`的一个实现。

package androidx.lifecycle;
    
    public interface ViewModelStoreOwner { 
        @NonNull
        ViewModelStore getViewModelStore();
    }

ViewModelProvider中的ViewModelStore正是来自ViewModelStoreOwner。

public class ViewModelProvider { 
    
        private final ViewModelStore mViewModelStore;
    
        public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) { 
            this(owner.getViewModelStore(), factory);
        }
    
        public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) { 
            this.mViewModelStore = store;
        }

# FragmentActivity\#getViewModelStore() #

--------------------

`FragmentActivity`实现了`ViewModelStoreOwner`的`getViewModelStore`方法

package androidx.fragment.app;
    
    public class FragmentActivity extends ComponentActivity implements ViewModelStoreOwner ... { 
    
        private ViewModelStore mViewModelStore;
    
        @NonNull
        @Override
        public ViewModelStore getViewModelStore() { 
            ...
    
            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;
        }
    
        static final class NonConfigurationInstances { 
            Object custom;
            ViewModelStore viewModelStore;
            FragmentManagerNonConfig fragments;
        }
    
        ...
    }

通过`getLastNonConfigurationInstance()` 获取 `NonConfigurationInstances` 实例，从而得到真正的`viewModelStore`，`getLastNonConfigurationInstance()`又是什么？

# Activity\#getLastNonConfigurationInstance() #

--------------------

package android.app;
    
    public class Activity extends ContextThemeWrapper implements ... { 
    
        /* package */ NonConfigurationInstances mLastNonConfigurationInstances;
    
        @Nullable
        public Object getLastNonConfigurationInstance() { 
            return mLastNonConfigurationInstances != null
                    ? mLastNonConfigurationInstances.activity : null;
        }

> Retrieve the non-configuration instance data that was previously returned by onRetainNonConfigurationInstance(). This will be available from the initial onCreate(Bundle) and onStart() calls to the new instance, allowing you to extract any useful dynamic state from the previous instance.

通过官方文档我们知道，屏幕旋转前通过`onRetainNonConfigurationInstance()`返回的Activity实例，屏幕旋转后可以通过`getLastNonConfigurationInstance()`获取，因此屏幕旋转前后不销毁的关键就在`onRetainNonConfigurationInstance`

# Activity\#onRetainNonConfigurationInstance() #

--------------------

屏幕旋转时，`onRetainNonConfigurationInstance()`在`onStop`和`onDestroy`之间调用

package android.app;
    
    public class Activity extends ContextThemeWrapper implements ... { 
    
        public Object onRetainNonConfigurationInstance() { 
            return null;
        }
    
        ...
    }

`onRetainNonConfigurationInstance`在Activity中只有空实现，在`FragmentActivity`中被重写

package androidx.fragment.app;
    
    public class FragmentActivity extends ComponentActivity implements ViewModelStoreOwner, ... { 
    
        @Override
        public final Object onRetainNonConfigurationInstance() { 
            Object custom = onRetainCustomNonConfigurationInstance();
    
            FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();
    
            if (fragments == null && mViewModelStore == null && custom == null) { 
                return null;
            }
    
            NonConfigurationInstances nci = new NonConfigurationInstances();
            nci.custom = custom;
            nci.viewModelStore = mViewModelStore;
            nci.fragments = fragments;
            return nci;
        }
    
        static final class NonConfigurationInstances { 
            Object custom;
            ViewModelStore viewModelStore;
            FragmentManagerNonConfig fragments;
        }
    
        ...
    }

FragmentActivity 通过 onRetainNonConfigurationInstance() 返回 了存放ViewModelStore的NonConfigurationInstances 实例。  
值得一提的是`onRetainNonConfigurationInstance`提供了一个hook时机：`onRetainCustomNonConfigurationInstance`，允许我们像ViewModel一样使得自定义对象不被销毁

NonConfigurationInstances会在`attach`中由系统传递给新重建的Activity：

final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor,
            Window window, ActivityConfigCallback activityConfigCallback, IBinder assistToken)

然后在`onCreate`中，通过`getLastNonConfigurationInstance()`获取NonConfigurationInstances中的ViewModelStore

package androidx.fragment.app;
    
    public class FragmentActivity extends ComponentActivity implements ViewModelStoreOwner ... { 
    
        private ViewModelStore mViewModelStore;
    
        @SuppressWarnings("deprecation")
        @Override
        protected void onCreate(@Nullable Bundle savedInstanceState) { 
            mFragments.attachHost(null /*parent*/);
    
            super.onCreate(savedInstanceState);
    
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null && nc.viewModelStore != null && mViewModelStore == null) { 
                mViewModelStore = nc.viewModelStore;
            }
            ...
        }
    }

# Recap #

--------------------

## Activity首次启动 ##

*  FragmentActivity\#onCreate()被调用
    
     *  此时 FragmentActivity 的 mViewModelStore 尚为 null
 *  HogeActivity的onCreate() 被调用
    
     *  ViewModelProvider 实例创建
     *  FragmentActivity\#getViewModelStore() 被调用，mViewModelStore被创建并赋值

## 发生屏幕旋转 ##

*  FragmentActivity\#onRetainNonConfigurationInstance() 被调用
    
     *  持有mViewModelStore 的NonConfigurationInstances 实例被返回

## Activity重建 ##

*  FragmentActivity\#onCreate() 被调用
    
     *  从Activity\#getLastNonConfigurationInstance() 获取 NonConfigurationInstances 实例
     *  NonConfigurationInstances 中保存了屏幕旋转前的 FragmentActivity 的 mViewModelStore，将其赋值给重建后的FragmentActivity 的 mViewModelStore
 *  HogeActivity\#onCreate() 被调用
    
     *  通过ViewModelProvider\#get() 获取 ViewModel 实例

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