ViewModel 源碼解析

時間  2021-08-14
標籤 java android api markdown app ide 函數 oop this spa 欄目 Java 简体版
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ViewModel 概述

ViewModel 類旨在以注重生命週期的方式存儲和管理界面的數據，讓數據能夠在發生屏幕旋轉等配置更改後繼續保留。它還處理界面與應用其餘部分的通訊（例如，調用業務邏輯類）。界面應該可以觀察ViewModel 中的更改，一般經過LiveData 或者 DataBinding 公開此信息，它決不能擁有界面的引用。java

ViewModel 源碼解析

public abstract class ViewModel {
    // Can't use ConcurrentHashMap, because it can lose values on old apis (see b/37042460)
    @Nullable
    private final Map<String, Object> mBagOfTags = new HashMap<>();
    private volatile boolean mCleared = false;

    /** * ViewModel 被清除是會調用這個方法 * 咱們能夠重寫這個方法在裏面作一些回收工做 */
    @SuppressWarnings("WeakerAccess")
    protected void onCleared() {
    }

    @MainThread
    final void clear() {
        mCleared = true;
        if (mBagOfTags != null) {
            synchronized (mBagOfTags) {
                for (Object value : mBagOfTags.values()) {
                    // see comment for the similar call in setTagIfAbsent
                    closeWithRuntimeException(value);
                }
            }
        }
        onCleared();
    }

    
    @SuppressWarnings("unchecked")
    <T> T setTagIfAbsent(String key, T newValue) {
        T previous;
        synchronized (mBagOfTags) {
            previous = (T) mBagOfTags.get(key);
            if (previous == null) {
                mBagOfTags.put(key, newValue);
            }
        }
        T result = previous == null ? newValue : previous;
        if (mCleared) {
            closeWithRuntimeException(result);
        }
        return result;
    }

    /** * Returns the tag associated with this viewmodel and the specified key. */
    @SuppressWarnings({"TypeParameterUnusedInFormals", "unchecked"})
    <T> T getTag(String key) {
        if (mBagOfTags == null) {
            return null;
        }
        synchronized (mBagOfTags) {
            return (T) mBagOfTags.get(key);
        }
    }

    private static void closeWithRuntimeException(Object obj) {
        if (obj instanceof Closeable) {
            try {
                ((Closeable) obj).close();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
    }
}
複製代碼

能夠看到 ViewModel 的源碼其實很簡單，就是維護mBagOfTags 這個HashMap和clear() 這個函數。接着咱們看下clear() 在哪裏被調用android

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);
    }

    Set<String> keys() {
        return new HashSet<>(mMap.keySet());
    }

    /** * Clears internal storage and notifies ViewModels that they are no longer used. */
    public final void clear() {
        for (ViewModel vm : mMap.values()) {
            vm.clear();
        }
        mMap.clear();
    }
}
複製代碼

是在 ViewModelStore 的clear() 裏面被調用了。能夠看到ViewModelStore 這個類就是管理ViewModel。而ViewModelStore 的clear()是在Activity 的構造方法中被調用api

public ComponentActivity() {
        //省略代碼...
        getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    if (!isChangingConfigurations()) {
                        getViewModelStore().clear();
                    }
                }
            }
        });
        //省略代碼...
    }

複製代碼

當Activity onDistroy 的時候且不是更改配置的狀況下就會調用 getViewModelStore().clear() 這樣ViewModel 就會被清除。 接着咱們看下是誰把ViewModel put 進來的。markdown

public class ViewModelProvider {
    private static final String DEFAULT_KEY =
            "androidx.lifecycle.ViewModelProvider.DefaultKey";
    //省略代碼...

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
        String canonicalName = modelClass.getCanonicalName();
        if (canonicalName == null) {
            throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
        }
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
    }

    @SuppressWarnings("unchecked")
    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        ViewModel viewModel = mViewModelStore.get(key);

        if (modelClass.isInstance(viewModel)) {
            if (mFactory instanceof OnRequeryFactory) {
                ((OnRequeryFactory) mFactory).onRequery(viewModel);
            }
            return (T) viewModel;
        } else {
            //noinspection StatementWithEmptyBody
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
        } else {
            viewModel = (mFactory).create(modelClass);
        }
        mViewModelStore.put(key, viewModel);
        return (T) viewModel;
    }
    
    //省略代碼...

}
複製代碼

能夠看到get方法大體就是若是存在就直接返回，若是沒有就建立有個ViewModel保存並返回。這裏用到了mViewModelStore、mFactory 兩個對象，這兩個對象ViewModelProvider 類在構造方法中實例化的，咱們接着看下它的構造方法。app

public class ViewModelProvider {
    //省略代碼...
    private final Factory mFactory;
    private final ViewModelStore mViewModelStore;
    //構造函數1
    public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    }
   
    public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
        this(owner.getViewModelStore(), factory);
    }
   
    public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
        mFactory = factory;
        mViewModelStore = store;
    }
    //省略代碼...
}
複製代碼

咱們一般用的都是「構造函數1」 須要傳入ViewModelStoreOwner 對象，這是一個接口類ide

@SuppressWarnings("WeakerAccess")
public interface ViewModelStoreOwner {
    /** * Returns owned {@link ViewModelStore} * * @return a {@code ViewModelStore} */
    @NonNull
    ViewModelStore getViewModelStore();
}
複製代碼

咱們使用的時候傳入的是Activity/Fragment，因此可知 Activity/Fragment 都實現了這個接口。 能夠看到 「owner instanceof HasDefaultViewModelProviderFactory」 說明同時還可能實現了HasDefaultViewModelProviderFactory 這個接口。函數

public class ComponentActivity extends androidx.core.app.ComponentActivity implements LifecycleOwner, ViewModelStoreOwner, HasDefaultViewModelProviderFactory, SavedStateRegistryOwner, OnBackPressedDispatcherOwner {
複製代碼

果真是這樣的，這裏咱們就先看下HasDefaultViewModelProviderFactory 這個接口oop

public interface HasDefaultViewModelProviderFactory {
    /** * Returns the default {@link ViewModelProvider.Factory} that should be * used when no custom {@code Factory} is provided to the * {@link ViewModelProvider} constructors. * * @return a {@code ViewModelProvider.Factory} */
    @NonNull
    ViewModelProvider.Factory getDefaultViewModelProviderFactory();
}
複製代碼

接這個咱們看下Activity 對ViewModelStoreOwner、HasDefaultViewModelProviderFactory 接口方法的實現this

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

複製代碼

咱們先看 getViewModelStore() 正常狀況下在第一次調用時建立實例，下次調用直接獲取。NonConfigurationInstances 這個類主要做用是，在發生屏幕旋轉等配置更改會將一些須要保存的實例放在這裏面，並傳遞給下一個Activity(恢復的Activity)，這樣就能保證這些實例在這種狀況下無需從新建立；好比ViewModelStorespa

getDefaultViewModelProviderFactory() 第一次調用的時候會建立一個SavedStateViewModelFactory 對象。顧名思義，就是 保存狀態的ViewModel工廠，代碼就不帶你們看了。主要邏輯就是若是不須要 SavedStateHandle 就是經過反射建立一個ViewModel 對象。須要 SavedStateHandle 的狀況你們能夠看下這篇文章 www.jianshu.com/p/731ca4282…

小結

以上咱們從ViewModel 的源碼反向推倒知道了 ViewModel 的建立、恢復以及清除的過程。

所以咱們也知道了，爲何Activity 在發生屏幕旋轉等配置更改時ViewModel 沒有從新建立；ViewModel 的生命週期爲何要比Activity 長一點。還不知道的同窗能夠從新看一遍。

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