Jetpack的ViewModel与LiveData

本文基于SDK 29

一、ViewModel与LiveData的作用：

1、viewModel：

数据共享，屏幕旋转不丢失数据，并且在Activity与Fragment之间共享数据。

2、LiveData：

感知生命周期并且通知观察者刷新，防止内存泄漏。

二、用法

三、原理：

1、ViewModel：

ViewModelProviders.of(this).get(MyViewModel::class.java)

我们通过这个方法来构造ViewModel。

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

/**
 * Creates a {@link ViewModelProvider}, which retains ViewModels while a scope of given Activity
 * is alive. More detailed explanation is in {@link ViewModel}.
 * <p>
 * It uses the given {@link Factory} to instantiate new ViewModels.
 *
 * @param activity an activity, in whose scope ViewModels should be retained
 * @param factory  a {@code Factory} to instantiate new ViewModels
 * @return a ViewModelProvider instance
 */
@NonNull
@MainThread
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);
}

从源码中可以看出，ViewModelProviders.of(this)获取了一个ViewModelProvider 对象，而该对象中持有一个ViewModelProvider.AndroidViewModelFactory（因为我们传进入的是null）

和activity.getViewModelStore()。

private final Factory mFactory;
private final ViewModelStore mViewModelStore;

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

我们再来看看ViewModelStore这个类，从名字中已经可以看出它的用途，那便是存储ViewModel。

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();
    }
}

我们的ViewModel便是存储在上面的HashMap中。

接下来我们再来看ViewModelProviders.of(this).get(MyViewModel::class.java)的get方法：

 @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);
 }

@NonNull
@MainThread
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;
}

可以看出，所以会去存储ViewModel的ViewModelStore中拿，发现已经有了便直接返回，如果没有的话，那边使用mFactory工厂进行构建，然后再放进ViewModelStore中。

从之前的分析可以看出，这里的mFactory便是AndroidViewModelFactory。

@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);
}

其实该工厂也只是直接实例出该类而已。

此时我们便已经拿到了ViewModel。

可是它是怎么做到数据共享的呢，想做到数据共享，按理说它应该只有一个实例对象，我们且看。

@NonNull
@MainThread
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);
}

在获取ViewModelProvider的时候传进去了activity.getViewModelStore()，那我们看一下activity.getViewModelStore()是怎么获取ViewModelStore的。

@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;
}

关键的代码在于这一句：NonConfigurationInstances nc =  (NonConfigurationInstances) getLastNonConfigurationInstance();

static final class NonConfigurationInstances {
        Object activity;
        HashMap<String, Object> children;
        FragmentManagerNonConfig fragments;
        ArrayMap<String, LoaderManager> loaders;
        VoiceInteractor voiceInteractor;
    }
    /* package */ NonConfigurationInstances mLastNonConfigurationInstances;

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

将mLastNonConfigurationInstances.activity强转成FragmentActivity中的一个类：NonConfigurationInstances，然后获取ViewModelStore

static final class NonConfigurationInstances {
    Object custom;
    ViewModelStore viewModelStore;
    FragmentManagerNonConfig fragments;
}

NonConfigurationInstances是个静态类，所以里面的ViewModelStore 也是唯一的，因此ViewModelStore 能做到数据共享。

2、LivaData

我们先看这个语句：

viewModel?.livaData?.observe(this, Observer<Int> { integer -> Log.d("MainActivity", integer!!.toString()) })

从这个语句往源码里面探究：

@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
    assertMainThread("observe");
    if (owner.getLifecycle().getCurrentState() == DESTROYED) {
        // ignore
        return;
    }
    LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
    ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
    if (existing != null && !existing.isAttachedTo(owner)) {
        throw new IllegalArgumentException("Cannot add the same observer"
                + " with different lifecycles");
    }
    if (existing != null) {
        return;
    }
    owner.getLifecycle().addObserver(wrapper);
}

如果这个activity处于销毁状态，那么便不会添加该观察者，否则，构造一个LifecycleBoundObserver对象，放进mObservers里面，mObservers即为：

private SafeIterableMap<Observer<? super T>, ObserverWrapper> mObservers =
            new SafeIterableMap<>();

然后将LifecycleBoundObserver对象放进LifecycleRegistry里面。

LifecycleBoundObserver里面持有的对象如下：

当我们给LiveData设置值的时候：livaData.value = i

public class MutableLiveData<T> extends LiveData<T> {
    @Override
    public void postValue(T value) {
        super.postValue(value);
    }

    @Override
    public void setValue(T value) {
        super.setValue(value);
    }
}

里面还有个postValue方法：

protected void postValue(T value) {
    boolean postTask;
    synchronized (mDataLock) {
        postTask = mPendingData == NOT_SET;
        mPendingData = value;
    }
    if (!postTask) {
        return;
    }
    ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
}

postValue最终也会调用到主线程。postValue可以在子线程调用，而setValue必须在主线程调用，否则会抛出异常。

我们看setValue方法：

@MainThread
protected void setValue(T value) {
    assertMainThread("setValue");
    mVersion++;
    mData = value;
    dispatchingValue(null);
}

void dispatchingValue(@Nullable ObserverWrapper initiator) {
    if (mDispatchingValue) {
        mDispatchInvalidated = true;
        return;
    }
    mDispatchingValue = true;
    do {
        mDispatchInvalidated = false;
        if (initiator != null) {
            considerNotify(initiator);
            initiator = null;
        } else {
            for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =
                    mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
                considerNotify(iterator.next().getValue());
                if (mDispatchInvalidated) {
                    break;
                }
            }
        }
    } while (mDispatchInvalidated);
    mDispatchingValue = false;
}

这里我们传进来的initiator为null，所以我们主要看：

for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =
        mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
    considerNotify(iterator.next().getValue());
    if (mDispatchInvalidated) {
        break;
    }
}

这里的mObservers即为：

private SafeIterableMap<Observer<? super T>, ObserverWrapper> mObservers =
        new SafeIterableMap<>();

里面存放着我们之前放进去的LifecycleBoundObserver对象。

iterator.next().getValue()获取的便是LifecycleBoundObserver对象。

private void considerNotify(ObserverWrapper observer) {
    if (!observer.mActive) {
        return;
    }
    // Check latest state b4 dispatch. Maybe it changed state but we didn't get the event yet.
    //
    // we still first check observer.active to keep it as the entrance for events. So even if
    // the observer moved to an active state, if we've not received that event, we better not
    // notify for a more predictable notification order.
    if (!observer.shouldBeActive()) {
        observer.activeStateChanged(false);
        return;
    }
    if (observer.mLastVersion >= mVersion) {
        return;
    }
    observer.mLastVersion = mVersion;
    //noinspection unchecked
    observer.mObserver.onChanged((T) mData);
}

检测当前生命周期，至少是处于start。

@Override
boolean shouldBeActive() {
    return mOwner.getLifecycle().getCurrentState().isAtLeast(STARTED);
}

然后执行observer.mObserver.onChanged((T) mData);回调出去。

observer.mObserver便是我们传进去的观察者：

Observer<Int> { integer -> Log.d("MainActivity", integer!!.toString()) }

由以上也可以看出：我们是可以注册多个观察者的，所以要注意在一个Activity中只能够注册一次，否则会发生多个回调。

那么有个疑问，我们这样已经实现了，那问什么在liveData?.observe方法里面，不但将LifecycleBoundObserver放进LiveData的SafeIterableMap里面，还要将其放入LifecycleRegistry

里面。owner.getLifecycle()获取到的便是LifecycleRegistry

这是为了在相关的生命周期内做相关的操作，根据上一篇文章，我们可以知道，当activity的生命周期发生改变的时候，会获取添加进LifecycleRegistry的观察者，然后对每个观察者进行回调处理。

而在这里便会回调LifecycleBoundObserver的onStateChanged方法。

@Override
public void onStateChanged(@NonNull LifecycleOwner source,
        @NonNull Lifecycle.Event event) {
    if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
        removeObserver(mObserver);
        return;
    }
    activeStateChanged(shouldBeActive());
}

判断如果当前处于DESTROYED状态，那么便将我们添加进入的观察者移除。

否则会调用activeStateChanged(shouldBeActive())方法。

如果当前的活跃状态与上一次一样，那么就直接返回。

否则如果变为活跃的状态，那么会调用dispatchingValue(this);

 这里要注意，我们之前调用LiveData的setValue的时候，走的的2，但是现在走的是1，因为这次传进来的参数不为空。

private void considerNotify(ObserverWrapper observer) {
    if (!observer.mActive) {
        return;
    }
    // Check latest state b4 dispatch. Maybe it changed state but we didn't get the event yet.
    //
    // we still first check observer.active to keep it as the entrance for events. So even if
    // the observer moved to an active state, if we've not received that event, we better not
    // notify for a more predictable notification order.
    if (!observer.shouldBeActive()) {
        observer.activeStateChanged(false);
        return;
    }
    if (observer.mLastVersion >= mVersion) {
        return;
    }
    observer.mLastVersion = mVersion;
    observer.mObserver.onChanged((T) mData);
}

然后进入considerNotify这个方法，里面有一个判断十分重要：

if (observer.mLastVersion >= mVersion) {
    return;
}

这个判断是做什么用的呢？mVersion是什么时候被赋值的，这时候就要我们回过去头去看LiveData的setValue方法：

每调用一次，那么这个mVersion就会自加一。

所以这个判断便保证了，必须是刷新了LiveData里面的data值，才能够回调观察者事件：observer.mObserver.onChanged((T) mData);

如果生命周期变化的时候，LiveData里面的data值没有刷新，就不能回调出去。所以如果刷新LiveData里面的值的时候不处于活跃状态导致没有回调，当生命周期来到onStart的时候就会去回调。