為什么要使用ViewModel胧砰？

1. 在日常開發(fā)中，橫豎屏切換是非常常見的功能旦委，由于橫豎屏切換會使得Activity重建奇徒，導(dǎo)致界面相關(guān)數(shù)據(jù)都會丟失。為了避免這種情況缨硝，一般會有兩種做法：第一種是在AndroidManifest.xml文件中摩钙，將Activity的一個(gè)屬性設(shè)置為android:configChanges="orientation|keyboardHidden|screenSize"；第二種方案是使用onSaveInstanceState方法保存數(shù)據(jù)查辩，但此方法僅適合可以序列化和反序列化的少量數(shù)據(jù)胖笛，而不適合數(shù)量可能較大的數(shù)據(jù)，如用戶列表或位圖宜岛。

2. 在使用MVP架構(gòu)時(shí)长踊，當(dāng)View層使用Presenter層去異步加載一個(gè)耗時(shí)任務(wù)，若在任務(wù)結(jié)果返回回來之前萍倡，界面已經(jīng)被銷毀身弊，我們都需要手動判斷View層的狀態(tài)；若Presenter層還有持有了View層的Context列敲，若不清理掉的話可能會造成內(nèi)存泄露阱佛。

ViewModel的出現(xiàn)能夠很優(yōu)雅地解決上述兩個(gè)問題。

簡單用法

// 1. 創(chuàng)建一個(gè)繼承ViewModel的類
public class MyVideoModel extends ViewModel {}
// 2. 創(chuàng)建一個(gè)ViewModelProvider實(shí)例
ViewModelProvider provider = new ViewModelProvider(this);
// 3. 通過provider獲取MyVideoModel實(shí)例
MyVideoModel videoModel = provider.get(MyVideoModel.class);

源碼分析

先來看看第2步創(chuàng)建ViewModelProvider時(shí)做了些什么事

public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
    this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
         ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
         : NewInstanceFactory.getInstance());
}

第一個(gè)參數(shù)調(diào)用了owner.getViewModelStore()戴而，這個(gè)ViewModelStoreOwner是一個(gè)接口凑术，我們繼承的父類Activity--ComponentActivity實(shí)現(xiàn)了這個(gè)接口。

@NonNull
@Override
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) {
            // Restore the ViewModelStore from NonConfigurationInstances
            mViewModelStore = nc.viewModelStore;
        }
        if (mViewModelStore == null) {
            mViewModelStore = new ViewModelStore();
        }
    }
    return mViewModelStore;
}

當(dāng)我們在自己的Activity#onCreate方法內(nèi)new ViewModelProvider(this);時(shí)所意，這個(gè)
mViewModelStore就已經(jīng)不為空了淮逊，所以直接返回。ViewModelStore就是用來保存我們自己創(chuàng)建的
ViewModel對象扁眯。那么它是什么時(shí)候被創(chuàng)建賦值的呢？在FragmentActivity#onCreate方法內(nèi)調(diào)用了
mFragments.attachHost(null /*parent*/);

// FragmentActivity.java
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    mFragments.attachHost(null /*parent*/);
    ...
    super.onCreate(savedInstanceState);
    ...
}
// FragmentController.java
public void attachHost(@Nullable Fragment parent) {
    mHost.mFragmentManager.attachController(
        mHost, mHost /*container*/, parent);
}
// FragmentManagerImpl.java
public void attachController(@NonNull FragmentHostCallback host,
                             @NonNull FragmentContainer container, @Nullable final Fragment parent) {
    ...
    // Get the FragmentManagerViewModel
    if (parent != null) {
        mNonConfig = parent.mFragmentManager.getChildNonConfig(parent);
    } else if (host instanceof ViewModelStoreOwner) {
        ViewModelStore viewModelStore = ((ViewModelStoreOwner) host).getViewModelStore();
        mNonConfig = FragmentManagerViewModel.getInstance(viewModelStore);
    } else {
        mNonConfig = new FragmentManagerViewModel(false);
    }
}

attachController方法中的parent為空翅帜，所以不會走第一個(gè)if條件姻檀；第一個(gè)參數(shù)host，它的實(shí)現(xiàn)類是
HostCallbacks涝滴。

class HostCallbacks extends FragmentHostCallback<FragmentActivity> implements
    ViewModelStoreOwner,
OnBackPressedDispatcherOwner {
    ...
    @NonNull
    @Override
    public ViewModelStore getViewModelStore() {
        return FragmentActivity.this.getViewModelStore();
    }
    ...
}

再回到ComponentActivity#getViewModelStore方法

@NonNull
@Override
public ViewModelStore getViewModelStore() {
    ...
    if (mViewModelStore == null) {
        NonConfigurationInstances nc =
            (NonConfigurationInstances) getLastNonConfigurationInstance();
        // Activity第一次啟動時(shí)绣版，得到nc為null胶台。但是在橫豎屏切換時(shí)，這個(gè)nc不為空了
        if (nc != null) {
            // Restore the ViewModelStore from NonConfigurationInstances
            // 所以這里就拿到了之前保存的ViewModelStore杂抽，也就意味著拿到之前我們的ViewModel實(shí)例诈唬。
            // 后面會繼續(xù)分析到
            mViewModelStore = nc.viewModelStore;
        }
        // 執(zhí)行這里
        if (mViewModelStore == null) {
            mViewModelStore = new ViewModelStore();
        }
    }
    return mViewModelStore;
}

此時(shí)再回到上面創(chuàng)建ViewModelProvider時(shí)的第二個(gè)參數(shù)，因?yàn)?code>ComponentActivity并沒有實(shí)現(xiàn)
HasDefaultViewModelProviderFactory缩麸，所以第二個(gè)參數(shù)的類型是NewInstanceFactory铸磅。第2步源碼分析完成。

接下來看第3步源碼實(shí)現(xiàn)杭朱。

@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);
}

繼續(xù)追蹤get的重載函數(shù)

@NonNull
@MainThread
public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
    // 注釋1
    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.
        }
    }
    // 注釋2
    if (mFactory instanceof KeyedFactory) {
        viewModel = ((KeyedFactory) mFactory).create(key, modelClass);
    } else {
    
        // 注釋3
        viewModel = mFactory.create(modelClass);
    }
    mViewModelStore.put(key, viewModel);
    return (T) viewModel;
}

方法中的key為androidx.lifecycle.ViewModelProvider.DefaultKey:xxx.xxx.MyViewModel阅仔，
modelClass就是我們自己傳的MyViewModel.class。注釋1處代碼第一次執(zhí)行時(shí)肯定為空弧械，從第2步源碼分析可知mFactory的類型為NewInstanceFactory八酒，所以會執(zhí)行到第注釋3。通過反射獲取到我們自己的
ViewModel對象刃唐，然后將其保存到ViewModelStore中羞迷。

當(dāng)因配置更新導(dǎo)致界面重建時(shí)如何做到保存數(shù)據(jù)的？

當(dāng)界面發(fā)生橫豎屏切換時(shí)画饥，Activity會被銷毀再重建衔瓮，在調(diào)用onDestroy方法之前系統(tǒng)會調(diào)用一個(gè)方法

// Activity.java
/**
  * Called by the system, as part of destroying an
  * activity due to a configuration change, when it is known that a new
  * instance will immediately be created for the new configuration.  You
  * can return any object you like here, including the activity instance
  * itself, which can later be retrieved by calling
  * {@link #getLastNonConfigurationInstance()} in the new activity
  * instance.
  */
public Object onRetainNonConfigurationInstance() {
    return null;
}

來看看ComponentActivity中是如何將其重寫的

@Override
@Nullable
public final Object onRetainNonConfigurationInstance() {
    // 這里直接返回的是null
    Object custom = onRetainCustomNonConfigurationInstance();

    ViewModelStore viewModelStore = mViewModelStore;
    // 正常情況下，此時(shí)的mViewModelStore不可能為空
    if (viewModelStore == null) {
        // No one called getViewModelStore(), so see if there was an existing
        // ViewModelStore from our last NonConfigurationInstance
        NonConfigurationInstances nc =
            (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
            viewModelStore = nc.viewModelStore;
        }
    }

    if (viewModelStore == null && custom == null) {
        return null;
    }

    NonConfigurationInstances nci = new NonConfigurationInstances();
    nci.custom = custom;
    nci.viewModelStore = viewModelStore;
    return nci;
}

根據(jù)代碼中的注釋荒澡，最終ViewModelStore對象被保存在ComponentActivity的內(nèi)部類
NonConfigurationInstances對象中报辱。這個(gè)方法是在Activity中被調(diào)用的。

// Activity.java
NonConfigurationInstances retainNonConfigurationInstances() {
    Object activity = onRetainNonConfigurationInstance();
    ...
    NonConfigurationInstances nci = new NonConfigurationInstances();
    nci.activity = activity;
    ...
    return nci;
}

將ComponentActivity#NonConfigurationInstances對象賦值給了
Activity#NonConfigurationInstances對象的activity屬性单山。而retainNonConfigurationInstances方法是在Activity執(zhí)行onDestroy生命周期方法之前執(zhí)行的碍现。

// ActivityThread.java
/** Core implementation of activity destroy call. */
ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
                                            int configChanges, boolean getNonConfigInstance, String reason) {
    ActivityClientRecord r = mActivities.get(token);
        ... 
        if (getNonConfigInstance) {
            try {
                // 將其返回給了ActivityClientRecord屬性
                r.lastNonConfigurationInstances
                    = r.activity.retainNonConfigurationInstances();
            } catch (Exception e) {
                ...
            }
        }
        try {
            r.activity.mCalled = false;
            // 執(zhí)行onDestroy方法
            mInstrumentation.callActivityOnDestroy(r.activity);
            ...
        } catch (Exception e) {
            ...
        }
    return r;
}

最終mViewModelStore對象被保存在ActivityClientRecord對象的
lastNonConfigurationInstances屬性上。很顯然米奸，mActivities屬性的生命周期與ActivityThread保持一致昼接，那么lastNonConfigurationInstances也一樣，因此不會受Activity的銷毀再重建的影響悴晰。

當(dāng)Activity重建時(shí)慢睡，又會執(zhí)行onCreate方法，此時(shí)铡溪，我們再回到getViewModelStore方法

@NonNull
@Override
public ViewModelStore getViewModelStore() {
    ...
        if (mViewModelStore == null) {
            // 會執(zhí)行到這里
            NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
    return mViewModelStore;
}


@Nullable
public Object getLastNonConfigurationInstance() {
    return mLastNonConfigurationInstances != null
        ? mLastNonConfigurationInstances.activity : null;
}

mLastNonConfigurationInstances不為空漂辐，所以直接返回它的屬性activity的值。還記得這個(gè)屬性保存的是什么值嗎棕硫？上文已經(jīng)提到過了髓涯，它就是ComponentActivity的NonConfigurationInstances對象，而它的viewModelStore就保存著橫豎屏銷毀前的那個(gè)ViewModelStore對象哈扮，那也就意味著拿到了之前的ViewModel對象纬纪。

那mLastNonConfigurationInstances這個(gè)屬性是在哪里被賦值的呢蚓再？它是Activity對象在ActivityThread中被創(chuàng)建后，調(diào)用其attach方法時(shí)進(jìn)行賦值的包各。上文也提到過摘仅，我們的ViewModel被保存在ViewModelStore，而ViewModelStore又被保存在CompponentActivity#NonConfigurationInstances對象的viewModelStore屬性上问畅，而這個(gè)對象又被保存到了Activity#NonConfigurationInstances對象的activity屬性上娃属，它又被保存到了ActivityThread#ActivityClientRecord對象的lastNonConfigurationInstances屬性上。