一酌呆、ViewModel的特點(diǎn):

• ViewModel 可以實(shí)現(xiàn)Activity與fragment之間的數(shù)據(jù)共享，以及同一個(gè)Activity中多個(gè)Fragment之間的數(shù)據(jù)共享
• ViewModel可以感知Activity的生命周期啼辣，actiivty ondestory前躏敢，會調(diào)用viewModel的onCleared()方法 供我們來釋放資源

二、viewModel是如何實(shí)現(xiàn) 數(shù)據(jù)共享和感知生命周期的

我們知道ViewModel 是一個(gè)抽象類岗照，自定義ViewModel是需繼承ViewModel類.

public abstract class ViewModel {
 
    @Nullable
    private final Map<String, Object> mBagOfTags = new HashMap<>();
    private volatile boolean mCleared = false;

    /**
     * This method will be called when this ViewModel is no longer used and will be destroyed.
     * <p>
     * It is useful when ViewModel observes some data and you need to clear this subscription to
     * prevent a leak of this ViewModel.
     */
    @SuppressWarnings("WeakerAccess")
    protected void onCleared() {
    }

    @MainThread
    final void clear() {
        mCleared = true;
       
        //省略代碼...
        onCleared();
    }

TranslateViewModel.kt
如TranslateViewModel 繼承自ViewModel類

class TranslateViewModel constructor() : ViewModel(){
    
}

初始化如TranslateViewModel對象,卻不能直接使用new TranslateViewModel()來初始化該對象蚕钦。
需要調(diào)用ViewModelProviders.of(this).get(TranslateViewModel::class.java) 方法

錯(cuò)誤的初始化方法：

mViewModel = new TranslateViewModel()

正確的初始化方法:

mViewModel = ViewModelProviders.of(this).get(TranslateViewModel::class.java)

2.1 如何實(shí)現(xiàn)數(shù)據(jù)共享的

關(guān)鍵點(diǎn)就在于

 mViewModel = ViewModelProviders.of(this).get(TranslateViewModel::class.java)

2.1.1亭病、ViewModelProviders.of() 返回的是一個(gè)ViewModelProvider

public class ViewModelProviders {
  public static ViewModelProvider of(@NonNull FragmentActivity activity) {
        return of(activity, null);
    }

 public static ViewModelProvider of(@NonNull FragmentActivity activity,
            @Nullable Factory factory) {
        Application application = checkApplication(activity);
        if (factory == null) {
            factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
        }
        return new ViewModelProvider(activity.getViewModelStore(), factory);
    }
}

2.1.2、ViewModelProvider用于提供一個(gè)ViewModel嘶居，構(gòu)建一個(gè)ViewModelProvider,需要兩個(gè)參數(shù)

• ViewModelStore 提供ViewModel的存儲和復(fù)用機(jī)制
• ViewModelProvider.Factory - ViewModel的生產(chǎn)工廠罪帖，用于生產(chǎn)一個(gè)新的ViewModel對象

  public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
        mFactory = factory;
        mViewModelStore = store;
    }

我們看一下ViewModelProvider.get()方法，此方法用戶返回一個(gè)viewModel對象

public class ViewModelProvider {
  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);
    }
    
     public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        //關(guān)鍵代碼 
        ViewModel viewModel = mViewModelStore.get(key);

        if (modelClass.isInstance(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;
    }
}

可以發(fā)現(xiàn)

(1)會先去mViewModelStore 查找,有沒有XXX::class 對應(yīng)的ViewModel對象邮屁，如果有整袁，則直接返回

    ViewModel viewModel = mViewModelStore.get(key);

        if (modelClass.isInstance(viewModel)) {
            return (T) viewModel;
        } else {
            //noinspection StatementWithEmptyBody
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }

(2)如果mViewModelStore中沒有 目標(biāo)ViewModel對象，則直接利用ViewModelProvider.Factory 生產(chǎn)一個(gè)新的ViewModel對象佑吝，同時(shí)保存到mViewModelStore中坐昙，以供下次復(fù)用。

 if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
        } else {
            viewModel = (mFactory).create(modelClass);
        }
        mViewModelStore.put(key, viewModel);

2.1.3芋忿、ViewModelStore 用于存儲復(fù)用ViewModel對象炸客，

同時(shí)FragmentActivity 和Fragment都繼承了ViewModelStoreOwner ,可以獲取到viewModelStore對象。

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);
    }

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

public interface ViewModelStoreOwner {
    /**
     * Returns owned {@link ViewModelStore}
     *
     * @return a {@code ViewModelStore}
     */
    @NonNull
    ViewModelStore getViewModelStore();
}

總結(jié)：
至此原理清楚了戈钢。

• 每個(gè)Activity或者Fragment當(dāng)中持有一個(gè)mViewModelStore嚷量，存儲和復(fù)用activity當(dāng)中用到的ViewModel
• 創(chuàng)建ViewModel對象時(shí),如果傳入的是同一個(gè)Activity，那么ViewModelProvider中的ViewModelStore就是同一個(gè)對象逆趣。
• ViewModelProvider調(diào)用get(XXXViewModel::class) ,獲取ViewModel時(shí)，通過Activity的ViewModelStore實(shí)現(xiàn)了ViewModel的共享嗜历。

即 傳入的Activity是同一個(gè)的前提下宣渗，多次調(diào)用下面的方法抖所，獲取到的ViewModel是同一個(gè)對象。

 mViewModel = ViewModelProviders.of(this).get(TranslateViewModel::class.java)

2.2 如何感知Activity的生命周期的

首先明確一點(diǎn)痕囱，ViewModel只能感知activity生命周期中的onDestory()田轧。在activity銷毀前，執(zhí)行viewModel的clear(回調(diào)onCleared()鞍恢，進(jìn)行資源的釋放)傻粘。

那么我們來看activity中是在什么時(shí)候觸發(fā)viewModel的clear()操作的

public class FragmentActivity extends ComponentActivity implements XXX{
    
}

public class ComponentActivity extends androidx.core.app.ComponentActivity implements
        LifecycleOwner,
        ViewModelStoreOwner,
        SavedStateRegistryOwner,
        OnBackPressedDispatcherOwner {
        
        public ComponentActivity() {
        Lifecycle lifecycle = getLifecycle();
        //noinspection ConstantConditions
      
      
        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();
                    }
                }
            }
        });

        }
}

可以看到，在基類ComponentActivity的構(gòu)造方法中,注冊了LifeCycleObserver帮掉，檢測到Lifecycle.Event.ON_DESTROY事件之后弦悉，會自動調(diào)用getViewModelStore().clear()

public class ViewModelStore {
     public final void clear() {
            for (ViewModel vm : mMap.values()) {
                vm.onCleared();
            }
            mMap.clear();
        }
}