ViewModel 使用及原理解析

本文是基于 androidx.lifecycle:lifecycle-extensions:2.0.0 的源碼進(jìn)行分析

ViewModel旨在以生命周期意識的方式存儲和管理用戶界面相關(guān)的數(shù)據(jù),它可以用來管理Activity和Fragment中的數(shù)據(jù).還可以拿來處理Fragment與Fragment之間的通信等等.

當(dāng)Activity或者Fragment創(chuàng)建了關(guān)聯(lián)的ViewModel,那么該Activity或Fragment只要處于活動狀態(tài),那么該ViewModel就不會被銷毀,即使是該Activity屏幕旋轉(zhuǎn)時重建了.所以也可以拿來做數(shù)據(jù)的暫存.

ViewModel主要是拿來獲取或者保留Activity/Fragment所需要的數(shù)據(jù)的,開發(fā)者可以在Activity/Fragment中觀察ViewModel中的數(shù)據(jù)更改(這里需要配合LiveData食用).

ps: ViewModel只是用來管理UI的數(shù)據(jù)的,千萬不要讓它持有View、Activity或者Fragment的引用(小心內(nèi)存泄露)。

本文以由淺入深的方式學(xué)習(xí)ViewModel

一、ViewModel的使用

1. 引入ViewModel

//引入AndroidX吧,替換掉support包
implementation 'androidx.appcompat:appcompat:1.0.2'

def lifecycle_version = "2.0.0"
// ViewModel and LiveData
implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"

2. 簡單使用起來

  1. 定義一個User數(shù)據(jù)類
class User implements Serializable {

    public int age;
    public String name;

    public User(int age, String name) {
        this.age = age;
        this.name = name;
    }

    @Override
    public String toString() {
        return "User{" +
                "age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}
  1. 然后引出我們今天的主角ViewModel
public class UserModel extends ViewModel {

    public final MutableLiveData<User> mUserLiveData = new MutableLiveData<>();

    public UserModel() {
        //模擬從網(wǎng)絡(luò)加載用戶信息
        mUserLiveData.postValue(new User(1, "name1"));
    }
    
    //模擬 進(jìn)行一些數(shù)據(jù)騷操作
    public void doSomething() {
        User user = mUserLiveData.getValue();
        if (user != null) {
            user.age = 15;
            user.name = "name15";
            mUserLiveData.setValue(user);
        }
    }

}
  1. 這時候在Activity中就可以使用ViewModel了. 其實就是一句代碼簡單實例化,然后就可以使用ViewModel了.
//這些東西我是引入的androidx下面的
import androidx.fragment.app.FragmentActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;

public class MainActivity extends FragmentActivity {

    private TextView mContentTv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mContentTv = findViewById(R.id.tv_content);

        //構(gòu)建ViewModel實例
        final UserModel userModel = ViewModelProviders.of(this).get(UserModel.class);

        //讓TextView觀察ViewModel中數(shù)據(jù)的變化,并實時展示
        userModel.mUserLiveData.observe(this, new Observer<User>() {
            @Override
            public void onChanged(User user) {
                mContentTv.setText(user.toString());
            }
        });

        findViewById(R.id.btn_test).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //點擊按鈕  更新User數(shù)據(jù)  觀察TextView變化
                userModel.doSomething();
            }
        });
    }
}

這個時候,我們點擊一下按鈕(user中的age變?yōu)?5),我們可以旋轉(zhuǎn)手機(jī)屏幕(這個時候其實Activity是重新創(chuàng)建了,也就是onCreate()方法被再次調(diào)用,但是ViewModel其實是沒有重新創(chuàng)建的,還是之前那個ViewModel),但是當(dāng)我們旋轉(zhuǎn)之后,發(fā)現(xiàn)TextView上顯示的age居然還是15,,,,這就是ViewModel的魔性所在.這個就不得不提ViewModel的生命周期了,它只有在Activity銷毀之后,它才會自動銷毀(所以別讓ViewModel持有Activity引用啊,會內(nèi)存泄露的). 下面引用一下谷歌官方的圖片,將ViewModel的生命周期展示的淋漓盡致.

image

3. ViewModel妙用1: Activity與Fragment"通信"

有了ViewModel,Activity與Fragment可以共享一個ViewModel,因為Fragment是依附在Activity上的,在實例化ViewModel時將該Activity傳入ViewModelProviders,它會給你一個該Activity已創(chuàng)建好了的ViewModel,這個Fragment可以方便的訪問該ViewModel中的數(shù)據(jù).在Activity中修改userModel數(shù)據(jù)后,該Fragment就能拿到更新后的數(shù)據(jù).

public class MyFragment extends Fragment {
     public void onStart() {
        //這里拿到的ViewModel實例,其實是和Activity中創(chuàng)建的是一個實例
         UserModel userModel = ViewModelProviders.of(getActivity()).get(UserModel.class);
     }
 }

4. ViewModel妙用2: Fragment與Fragment"通信"

下面我們來看一個例子(Google官方例子)

public class SharedViewModel extends ViewModel {
    private final MutableLiveData<Item> selected = new MutableLiveData<Item>();

    public void select(Item item) {
        selected.setValue(item);
    }

    public LiveData<Item> getSelected() {
        return selected;
    }
}


public class MasterFragment extends Fragment {
    private SharedViewModel model;
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
        itemSelector.setOnClickListener(item -> {
            model.select(item);
        });
    }
}

public class DetailFragment extends Fragment {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        SharedViewModel model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
        model.getSelected().observe(this, { item ->
           // Update the UI.
        });
    }
}
  1. 首先定義一個ViewModel,在里面放點數(shù)據(jù)
  2. 然后在MasterFragment和DetailFragment都可以拿到該ViewModel,拿到了該ViewModel就可以拿到里面的數(shù)據(jù)了,相當(dāng)于間接通過ViewModel通信了. so easy....

二、ViewModel源碼解析

又到了我們熟悉的源碼解析環(huán)節(jié)

我們從下面這句代碼start.

final UserModel userModel = ViewModelProviders.of(this).get(UserModel.class);

我們跟著ViewModelProviders.of(this)打開新世界的大門

1. ViewModelProviders.of(this) 方法

/**
 * 用于構(gòu)建一個ViewModelProvider,當(dāng)Activity是alive時它會保留所有的該Activity對應(yīng)的ViewModels.
 */
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity) {
    return of(activity, null);
}

@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
        @Nullable Factory factory) {
    //檢查application是否為空,不為空則接收
    Application application = checkApplication(activity);
    if (factory == null) {
        //構(gòu)建一個ViewModelProvider.AndroidViewModelFactory
        factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
    }
    return new ViewModelProvider(activity.getViewModelStore(), factory);
}

ViewModelProviders里面的of()函數(shù)其實是為了方便我們構(gòu)建一個ViewModelProvider.而ViewModelProvider,一看名字就知道干啥的了,就是提供ViewModel的.

Factory是ViewModelProvider的一個內(nèi)部接口,它的實現(xiàn)類是拿來構(gòu)建ViewModel實例的.它里面只有一個方法,就是創(chuàng)建一個ViewModel.

/**
 * Implementations of {@code Factory} interface are responsible to instantiate ViewModels.
 */
public interface Factory {
    /**
     * Creates a new instance of the given {@code Class}.
     * <p>
     *
     * @param modelClass a {@code Class} whose instance is requested
     * @param <T>        The type parameter for the ViewModel.
     * @return a newly created ViewModel
     */
    @NonNull
    <T extends ViewModel> T create(@NonNull Class<T> modelClass);
}

Factory有2個實現(xiàn)類:一個是NewInstanceFactory, 一個是AndroidViewModelFactory .

  • NewInstanceFactory源碼
public static class NewInstanceFactory implements Factory {

        @SuppressWarnings("ClassNewInstance")
        @NonNull
        @Override
        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);
            }
        }
    }

NewInstanceFactory專門用來實例化那種構(gòu)造方法里面沒有參數(shù)的class,并且ViewModel里面是不帶Context的,然后它是通過newInstance()去實例化的.

  • AndroidViewModelFactory 源碼
public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {

    private static AndroidViewModelFactory sInstance;

    /**
     * Retrieve a singleton instance of AndroidViewModelFactory.
     *
     * @param application an application to pass in {@link AndroidViewModel}
     * @return A valid {@link AndroidViewModelFactory}
     */
    @NonNull
    public static AndroidViewModelFactory getInstance(@NonNull Application application) {
        if (sInstance == null) {
            sInstance = new AndroidViewModelFactory(application);
        }
        return sInstance;
    }

    private Application mApplication;

    /**
     * Creates a {@code AndroidViewModelFactory}
     *
     * @param application an application to pass in {@link AndroidViewModel}
     */
    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);
    }
}

AndroidViewModelFactory專門用來實例化那種構(gòu)造方法里面有參數(shù)的class,并且ViewModel里面可能是帶Context的.

  • 它是通過newInstance(application)去實例化的.如果有帶application參數(shù)則是這樣實例化
  • 如果沒有帶application參數(shù)的話,則還是會走newInstance()方法去構(gòu)建實例.

AndroidViewModelFactory通過構(gòu)造方法給ViewModel帶入Application,就可以在ViewModel里面拿到Context,因為Application是APP全局的,那么不存在內(nèi)存泄露的問題.完美解決了有些ViewModel里面需要Context引用,但是又擔(dān)心內(nèi)存泄露的問題.

下面我們繼續(xù)ViewModelProviders.of(this)方法繼續(xù)分析吧,注意最后一句new ViewModelProvider(activity.getViewModelStore(), factory);第一個參數(shù)會調(diào)用activity的getViewModelStore()方法(這個方法會返回ViewModelStore,這個類是拿來存儲ViewModel的,下面會說到),這里的activity是androidx.fragment.app.FragmentActivity,看一下這個getViewModelStore()方法

/**
 * 獲取這個Activity相關(guān)聯(lián)的ViewModelStore
 */
@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) {
        //獲取最近一次橫豎屏切換時保存下來的數(shù)據(jù)
        NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
            // Restore the ViewModelStore from NonConfigurationInstances
            mViewModelStore = nc.viewModelStore;
        }
        if (mViewModelStore == null) {
            mViewModelStore = new ViewModelStore();
        }
    }
    return mViewModelStore;
}

//沒想到吧,Activity在橫豎屏切換時悄悄保存了viewModelStore
//注意,這是FragmentActivity中的NonConfigurationInstances(其實Activity中還定義了一個NonConfigurationInstances,內(nèi)容要比這個多一些,但是由于沒有關(guān)系到它,這里就不提及了)
static final class NonConfigurationInstances {
    Object custom;
    ViewModelStore viewModelStore;
    FragmentManagerNonConfig fragments;
}

Android橫豎屏切換時會觸發(fā)onSaveInstanceState()，而還原時會調(diào)用onRestoreInstanceState()，但是Android的Activity類還有2個方法名為onRetainNonConfigurationInstance()和getLastNonConfigurationInstance()這兩個方法。

來具體看看這2個素未謀面的方法

/**
 保留所有fragment的狀態(tài)。你不能自己覆寫它！如果要保留自己的狀態(tài)，請使用onRetainCustomNonConfigurationInstance（）
 這個方法在FragmentActivity里面
 */
@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;
}


//這個方法在Activity里面,而mLastNonConfigurationInstances.activity實際就是就是上面方法中年的nci
public Object getLastNonConfigurationInstance() {
    return mLastNonConfigurationInstances != null
            ? mLastNonConfigurationInstances.activity : null;
}

我們來看看getLastNonConfigurationInstance()的調(diào)用時機(jī),

protected void onCreate(@Nullable Bundle savedInstanceState) {
    ......
    super.onCreate(savedInstanceState);

    NonConfigurationInstances nc =
            (NonConfigurationInstances) getLastNonConfigurationInstance();
    if (nc != null && nc.viewModelStore != null && mViewModelStore == null) {
        mViewModelStore = nc.viewModelStore;
    }
    ......
}

沒想到吧,Activity在橫豎屏切換時悄悄保存了viewModelStore,放到了NonConfigurationInstances實例里面,橫豎屏切換時保存了又恢復(fù)了回來,相當(dāng)于ViewModel實例就還在啊,也就避免了橫豎屏切換時的數(shù)據(jù)丟失.

2. viewModelProvider.get(UserModel.class)

下面我們來到那句構(gòu)建ViewModel代碼的后半段,它是ViewModelProvider的get()方法,看看實現(xiàn),其實很簡單

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) {
    //先取緩存  有緩存則用緩存
    ViewModel viewModel = mViewModelStore.get(key);

    if (modelClass.isInstance(viewModel)) {
        //noinspection unchecked
        return (T) viewModel;
    } else {
        //noinspection StatementWithEmptyBody
        if (viewModel != null) {
            // TODO: log a warning.
        }
    }
    
    //無緩存  則重新通過mFactory構(gòu)建
    viewModel = mFactory.create(modelClass);
    //緩存起來
    mViewModelStore.put(key, viewModel);
    //noinspection unchecked
    return (T) viewModel;
}

大體思路是利用一個key來緩存ViewModel,有緩存則用緩存的,沒有則重新構(gòu)建.構(gòu)建時使用的factory是上面of()方法的那個factory.

3. ViewModelStore

上面多個地方用到了ViewModelStore,它其實就是一個普普通通的保存ViewModel的類.

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();
    }
}

ViewModelStore有一個HashMap專門用于存儲,普通吧.

下面看看何時調(diào)用的clear()

4. ViewModel.onCleared() 資源回收

既然ViewModel是生命周期感知的,那么何時應(yīng)該清理ViewModel呢?

我們來到FragmentActivity的onDestroy()方法,發(fā)現(xiàn)它是在這里清理的.

/**
 * Destroy all fragments.
 */
@Override
protected void onDestroy() {
    super.onDestroy();

    if (mViewModelStore != null && !isChangingConfigurations()) {
        mViewModelStore.clear();
    }

    mFragments.dispatchDestroy();
}

5. 再看 ViewModel

很多朋友可能就要問了,ViewModel到底是什么?

public abstract class ViewModel {
    /**
     * 這個方法會在ViewModel即將被銷毀時調(diào)用,可以在這里清理垃圾
     */
    @SuppressWarnings("WeakerAccess")
    protected void onCleared() {
    }
}

其實很簡單,就一個抽象類,里面就一個空方法??? 我擦,搞了半天,原來ViewModel不是主角....

6. AndroidViewModel

ViewModel有一個子類,是AndroidViewModel.它里面有一個Application的屬性,僅此而已,為了方便在ViewModel里面使用Context.

public class AndroidViewModel extends ViewModel {
    @SuppressLint("StaticFieldLeak")
    private Application mApplication;

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

    /**
     * Return the application.
     */
    @SuppressWarnings("TypeParameterUnusedInFormals")
    @NonNull
    public <T extends Application> T getApplication() {
        //noinspection unchecked
        return (T) mApplication;
    }
}

三、小結(jié)

ViewModel 的源碼其實不多，理解起來比較容易，主要是官方FragmentActivity提供了技術(shù)實現(xiàn)，onRetainNonConfigurationInstance（）保存狀態(tài)，getLastNonConfigurationInstance()恢復(fù)。

原來Activity還有這么2個玩意兒，之前我還只是知道onSaveInstanceState()和onRestoreInstanceState()，漲姿勢了。

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