一箕昭、ViewModel的簡(jiǎn)介

2018年谷歌IO大會(huì)上正式發(fā)布了JetPack組件灵妨，其中包括Databing、Lifecycles落竹、LiveData泌霍、Navigation、Paging述召、Room朱转、ViewModel、WorkManager等一系列框架积暖。并且發(fā)布androidx包藤为，這些框架的最新版本組件將引入到androidx包下。
JetPack架構(gòu)組件的發(fā)布呀酸，意味著google終于拿出了官方推薦的Android開發(fā)架構(gòu)凉蜂，一直以來在Android開發(fā)上老生常談三種開發(fā)模式琼梆，MVC性誉、MVP、MVVM茎杂。整體的思想就是做一些代碼封裝和操作上的分離错览，具體三種架構(gòu)的區(qū)別不在這詳細(xì)描述。說一點(diǎn)兒個(gè)人對(duì)這三中開發(fā)模式的理解煌往。
MVC：Activity承擔(dān)C的任務(wù)倾哺，強(qiáng)行carry大量業(yè)務(wù)和數(shù)據(jù)代碼。
MVP：Presenter承擔(dān)主要代碼任務(wù)刽脖，連接數(shù)據(jù)層和視圖層的交互羞海。
MVVM：Model負(fù)責(zé)數(shù)據(jù)、View負(fù)責(zé)展示曲管、ViewModel負(fù)責(zé)Model和View的交互却邓，同時(shí)，最好的一點(diǎn)是View層自動(dòng)監(jiān)聽ViewModel的數(shù)據(jù)變化院水。
在JetPack組件出現(xiàn)之前腊徙，一直以來構(gòu)建MVVM架構(gòu)的方式都是代用Databing來充當(dāng)ViewModel简十，而Databing是采用標(biāo)簽方式寫入布局文件中的，這樣一來當(dāng)出現(xiàn)問題的時(shí)候不是太好進(jìn)行調(diào)試撬腾。所以ViewMode的出現(xiàn)對(duì)于MVVM模式開發(fā)具有重大的意義螟蝙。

二、ViewModel的用法和特點(diǎn)
ViewModel的使用是要結(jié)合LiveData框架進(jìn)行的民傻，LiveData框架也是JetPack組件的一部分胰默，這里先不詳細(xì)進(jìn)行介紹。ViewModel有兩個(gè)特點(diǎn)漓踢，一是更加方便的保存數(shù)據(jù)初坠，第二個(gè)特點(diǎn)，也是最重要的特點(diǎn)彭雾，就是保證數(shù)據(jù)不受Activity的銷毀重建所影響碟刺，當(dāng)Activity銷毀重建后仍然能收到之前的數(shù)據(jù)。

1）繼承ViewModel

 class MainViewModel : ViewModel() {
    private val repertory: MainRepository by lazy { MainRepository() }
    var data: MutableLiveData<JsonBean> = MutableLiveData()
    fun getDataFromServer(){
    repertory.getDataFromServer(data)
     }
    }

ViewModel中持有LiveData薯酝，LiveData是ViewModel持有數(shù)據(jù)的載體半沽。

2）Activity通過ViewModelProviders獲取ViewModel

class MainActivity : AppCompatActivity() {
private lateinit var mModel: MainViewModel

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    initData()
}

private fun initData() {
    mModel = ViewModelProviders.of(this)[MainViewModel::class.java]
    mModel.data?.observe(this, Observer {
        val mainAdapter = MainAdapter(this, it)
        val linearLayoutManager = LinearLayoutManager(this)
        rv.layoutManager = linearLayoutManager
        rv.adapter = mainAdapter
    })
    mModel.getDataFromServer()
 }  
}

3）通過ViewModel中的方法獲取數(shù)據(jù)然后改變LiveData狀態(tài)，通過響應(yīng)式的方式通知到Activity進(jìn)行視圖更新吴菠。用法上比較簡(jiǎn)單者填。

三、源碼分析

個(gè)人認(rèn)為對(duì)于一個(gè)框架的學(xué)習(xí)僅僅掌握框架的用法是不夠的做葵，因?yàn)檫@太簡(jiǎn)單了占哟，曾經(jīng)看到過一篇博客，里面有一句話說的很好酿矢，”如果你只會(huì)用這個(gè)框架榨乎，而不清楚其中的原理和設(shè)計(jì)，那么即便這個(gè)框架性能再好瘫筐，再牛逼蜜暑，那也是寫框架的那個(gè)人牛逼，而不是你牛逼“策肝。我認(rèn)為這句話說的很好肛捍，所以當(dāng)我們學(xué)習(xí)一中新框架的時(shí)候，不能單單只會(huì)用之众，一定要清楚其中的原理拙毫，盡量去理解作者的設(shè)計(jì)思想，就像品茶一樣棺禾，這樣才能品出里面的滋味缀蹄。不扯閑篇了，由于作者能力也有限，所以以上和以下分析袍患，如有不同觀點(diǎn)或作者描述有誤請(qǐng)給予批評(píng)指正坦康，歡迎來噴。

1）先從獲取ViewModel的類入手诡延，”ViewModelProviders“從類名中也能分析出這個(gè)類是用于提供ViewModel的滞欠。源碼如下：

public class ViewModelProviders {
public ViewModelProviders() {
}

private static Application checkApplication(Activity activity) {
    Application application = activity.getApplication();
    if (application == null) {
        throw new IllegalStateException("Your activity/fragment is not yet attached to "
                + "Application. You can't request ViewModel before onCreate call.");
    }
    return application;
}

private static Activity checkActivity(Fragment fragment) {
    Activity activity = fragment.getActivity();
    if (activity == null) {
        throw new IllegalStateException("Can't create ViewModelProvider for detached fragment");
    }
    return activity;
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull Fragment fragment) {
    ViewModelProvider.AndroidViewModelFactory factory =
            ViewModelProvider.AndroidViewModelFactory.getInstance(
                    checkApplication(checkActivity(fragment)));
    return new ViewModelProvider(ViewModelStores.of(fragment), factory);
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity) {
    ViewModelProvider.AndroidViewModelFactory factory =
            ViewModelProvider.AndroidViewModelFactory.getInstance(
                    checkApplication(activity));
    return new ViewModelProvider(ViewModelStores.of(activity), factory);
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull Fragment fragment, @NonNull Factory factory) {
    checkApplication(checkActivity(fragment));
    return new ViewModelProvider(ViewModelStores.of(fragment), factory);
}
@NonNull
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
        @NonNull Factory factory) {
    checkApplication(activity);
    return new ViewModelProvider(ViewModelStores.of(activity), factory);
}
@SuppressWarnings("WeakerAccess")
@Deprecated
public static class DefaultFactory extends ViewModelProvider.AndroidViewModelFactory {
    /**
     * Creates a {@code AndroidViewModelFactory}
     *
     * @param application an application to pass in {@link AndroidViewModel}
     * @deprecated Use {@link ViewModelProvider.AndroidViewModelFactory} or
     * {@link ViewModelProvider.AndroidViewModelFactory#getInstance(Application)}.
     */
    @Deprecated
    public DefaultFactory(@NonNull Application application) {
        super(application);
    }
}
}

我們從中可以看到，這個(gè)類中的方法全是靜態(tài)的肆良，說明這個(gè)類相當(dāng)于一個(gè)工具類筛璧，從of方法中我們可以看到真正實(shí)現(xiàn)ViewModel創(chuàng)建的類是ViewModelProvider，而這個(gè)類的構(gòu)建又需要通過ViewModelStores的of方法獲取惹恃，ViewModelStores的代碼如下：

public class ViewModelStores {
private ViewModelStores() {
}

@NonNull
@MainThread
public static ViewModelStore of(@NonNull FragmentActivity activity) {
    if (activity instanceof ViewModelStoreOwner) {
        return ((ViewModelStoreOwner) activity).getViewModelStore();
    }
    return holderFragmentFor(activity).getViewModelStore();
}

@NonNull
@MainThread
public static ViewModelStore of(@NonNull Fragment fragment) {
    if (fragment instanceof ViewModelStoreOwner) {
        return ((ViewModelStoreOwner) fragment).getViewModelStore();
    }
    return holderFragmentFor(fragment).getViewModelStore();
}
}

從這個(gè)類中可以發(fā)現(xiàn)獲取ViewModelStore是通過構(gòu)建一個(gè)HoldFragment夭谤，而這個(gè)HoldFragment的作用就是持有ViewModelStore的引用，然后將這個(gè)Fragment添加到Activity中巫糙，而且這個(gè)Fragmet有一個(gè)重要的特點(diǎn)朗儒，在實(shí)例化的時(shí)候會(huì)調(diào)用setRetainInstance(true)方法，這個(gè)方法的作用是讓Fragment不受Activity銷毀重建影響参淹，這樣一來就能保證ViewModel不會(huì)由于Activity的銷毀重建導(dǎo)致數(shù)據(jù)丟失醉锄，這是ViewModel的一個(gè)重要特性。這種設(shè)計(jì)確實(shí)很巧妙浙值，HolderFragment的代碼如下：

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class HolderFragment extends Fragment implements ViewModelStoreOwner {
.......省略部分代碼

public HolderFragment() {
    setRetainInstance(true);
}

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    sHolderFragmentManager.holderFragmentCreated(this);
}
@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
}
@Override
public void onDestroy() {
    super.onDestroy();
    mViewModelStore.clear();
}
@NonNull
@Override
public ViewModelStore getViewModelStore() {
    return mViewModelStore;
}
/**
 * @hide
 */
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(FragmentActivity activity) {
    return sHolderFragmentManager.holderFragmentFor(activity);
}

/**
 * @hide
 */
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(Fragment fragment) {
    return sHolderFragmentManager.holderFragmentFor(fragment);
}
.....省略部分代碼
    private static HolderFragment createHolderFragment(FragmentManager fragmentManager) {
        HolderFragment holder = new HolderFragment();
        fragmentManager.beginTransaction().add(holder, HOLDER_TAG).commitAllowingStateLoss();
        return holder;
    }

    HolderFragment holderFragmentFor(FragmentActivity activity) {
        FragmentManager fm = activity.getSupportFragmentManager();
        HolderFragment holder = findHolderFragment(fm);
        if (holder != null) {
            return holder;
        }
        holder = mNotCommittedActivityHolders.get(activity);
        if (holder != null) {
            return holder;
        }

        if (!mActivityCallbacksIsAdded) {
            mActivityCallbacksIsAdded = true;
            activity.getApplication().registerActivityLifecycleCallbacks(mActivityCallbacks);
        }
        holder = createHolderFragment(fm);
        mNotCommittedActivityHolders.put(activity, holder);
        return holder;
    }

    HolderFragment holderFragmentFor(Fragment parentFragment) {
        FragmentManager fm = parentFragment.getChildFragmentManager();
        HolderFragment holder = findHolderFragment(fm);
        if (holder != null) {
            return holder;
        }
        holder = mNotCommittedFragmentHolders.get(parentFragment);
        if (holder != null) {
            return holder;
        }

        parentFragment.getFragmentManager()
                .registerFragmentLifecycleCallbacks(mParentDestroyedCallback, false);
        holder = createHolderFragment(fm);
        mNotCommittedFragmentHolders.put(parentFragment, holder);
        return holder;
    }
}

之后來到ViewModelProvider恳不，代碼量不是很多，但是里面的設(shè)計(jì)確實(shí)很有意思开呐。首先來分析它的成員變量烟勋，成員變量只有兩個(gè)，F(xiàn)actory從字面的意思就能理解筐付，它是生產(chǎn)ViewModel的工廠卵惦，所以這里用到了工廠模式。ViewModelStore它用來存儲(chǔ)ViewModel家妆，相當(dāng)于ViewModel的緩存鸵荠。內(nèi)部是一個(gè)HashMap，key是ViewModel的類名稱伤极。ViewModelProvider代碼如下：

public class ViewModelProvider {
....省略部分代碼
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;
}

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) {
    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;
    .......省略部分代碼
}

2）Factory是ViewModelProvider中的一個(gè)接口，定義如下：

public interface Factory {
    @NonNull
    <T extends ViewModel> T create(@NonNull Class<T> modelClass);
}

通過create方法創(chuàng)建ViewModel姨伤。Factory有兩個(gè)實(shí)現(xiàn)類AndroidViewModelFactory哨坪、NewInstanceFactory都是ViewModelProvider中的靜態(tài)內(nèi)部類。同時(shí)AndroidViewModelFactory又繼承于NewInstanceFactory乍楚。有一種依賴注入的思想在里面当编。接下來我們來分析兩個(gè)Factory的實(shí)現(xiàn)類。NewInstanceFactory最終通過newinstance方法創(chuàng)建ViewModel實(shí)例徒溪。

public static class NewInstanceFactory implements Factory {
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        //noinspection TryWithIdenticalCatches
        try {
            return modelClass.newInstance();
        } catch (InstantiationException e) {
            throw new RuntimeException("Cannot create an instance of " + modelClass, e);
        } catch (IllegalAccessException e) {
            throw new RuntimeException("Cannot create an instance of " + modelClass, e);
        }
    }
}
public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {

    private static AndroidViewModelFactory sInstance;
    @NonNull
    public static AndroidViewModelFactory getInstance(@NonNull Application application) {
        if (sInstance == null) {
            sInstance = new AndroidViewModelFactory(application);
        }
        return sInstance;
    }
    private Application mApplication;

    public AndroidViewModelFactory(@NonNull Application application) {
        mApplication = application;
    }

    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
            //noinspection TryWithIdenticalCatches
            try {
                return modelClass.getConstructor(Application.class).newInstance(mApplication);
            } catch (NoSuchMethodException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (IllegalAccessException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (InstantiationException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            } catch (InvocationTargetException e) {
                throw new RuntimeException("Cannot create an instance of " + modelClass, e);
            }
        }
        return super.create(modelClass);
    }
}

最后忿偷，真正存儲(chǔ)ViewModel的容器是ViewModelStore金顿，ViewModelStore中維護(hù)了一個(gè)HashMap，代碼如下：

 public class ViewModelStore {

private final HashMap<String, ViewModel> mMap = new HashMap<>();

final void put(String key, ViewModel viewModel) {
    ViewModel oldViewModel = mMap.get(key);
    if (oldViewModel != null) {
        oldViewModel.onCleared();
    }
    mMap.put(key, viewModel);
}

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();
}

}

文章代碼庫：https://github.com/24KWYL/MVVM.git