LiveData常用方法源码分析

本文主要是介绍LiveData常用方法源码分析，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

1、postValue

protected void postValue(T value) {boolean postTask;// 同步锁synchronized (mDataLock) {// 判断mPendingData是否为NOT_SETpostTask = mPendingData == NOT_SET;// 对mPendingData赋值mPendingData = value;}// 防止数据重复设置if (!postTask) {return;}// 将数据发送到主线程处理ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);}

步骤分解：

• 1、LiveData通过postValue方法在子线程同步方法中设置数据
• 2、数据NOT_SET校验
• 3、将数据发送到主线程进行处理

2、setValue

 @MainThreadprotected void setValue(T value) {assertMainThread("setValue"); // 1mVersion++; // 2mData = value; // 3dispatchingValue(null); // 4}

步骤分解：

• 1、主线程校验
• 2、计数器自加
• 3、为mData赋值
• 4、分发数据（详见dispatchingValue）

3、assertMainThread 主线程校验

 static void assertMainThread(String methodName) {if (!ArchTaskExecutor.getInstance().isMainThread()) {throw new IllegalStateException("Cannot invoke " + methodName + " on a background"+ " thread");}}

4、dispatchingValue

   void dispatchingValue(@Nullable ObserverWrapper initiator) {// 1if (mDispatchingValue) {mDispatchInvalidated = true;return;}// 2mDispatchingValue = true;do {mDispatchInvalidated = false;// 3if (initiator != null) {considerNotify(initiator);initiator = null;} else {// 4for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {considerNotify(iterator.next().getValue());if (mDispatchInvalidated) {break;}}}} while (mDispatchInvalidated);mDispatchingValue = false;}

步骤分解：

• 1、分发状态标记，防止重复分发
• 2、修改分发状态
• 3、分发通过参数传递进来的迭代器中数据
• 4、将数据分发给所有观察者

5、considerNotify

 private void considerNotify(ObserverWrapper observer) {// 1if (!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.// 2if (!observer.shouldBeActive()) {observer.activeStateChanged(false);return;}// 3if (observer.mLastVersion >= mVersion) {return;}// 4observer.mLastVersion = mVersion;// 5observer.mObserver.onChanged((T) mData);}

步骤分解

• 1、观察者非active状态，拦截
• 2、通过shouldBeActive修改观察者active状态
• 3、version计数器比对
• 4、调用onChange方法，分发数据

6、observe

  @MainThreadpublic void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {assertMainThread("observe");// 1if (owner.getLifecycle().getCurrentState() == DESTROYED) {// ignorereturn;}// 2LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);// 3ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);// 4if (existing != null && !existing.isAttachedTo(owner)) {throw new IllegalArgumentException("Cannot add the same observer"+ " with different lifecycles");}if (existing != null) {return;}// 5owner.getLifecycle().addObserver(wrapper);}

步骤分解

• 1、如果owner为DESTROYED状态，直接return
• 2、构造LifecycleBoundObserver
• 3、构造ObserverWrapper（详见putIfAbsent）
• 4、对已经存在观察者抛出异常，防止重复添加监听
• 5、添加合法观察者

7、putIfAbsent

  public V putIfAbsent(@NonNull K key, @NonNull V v) {Entry<K, V> entry = get(key);if (entry != null) {return entry.mValue;}put(key, v);return null;}

• Entry形式存储K、V值，K不允许重复

8、map

 public static <X, Y> LiveData<Y> map(@NonNull LiveData<X> source,@NonNull final Function<X, Y> mapFunction) {// 1final MediatorLiveData<Y> result = new MediatorLiveData<>();// 2result.addSource(source, new Observer<X>() {@Overridepublic void onChanged(@Nullable X x) {// 3result.setValue(mapFunction.apply(x));}});// 4return result;}

步骤分解

• 1、创建MediatorLiveData
• 2、通过addSource将源数据添加到result中
• 3、为result添加源数据监听，并在接收到监听后setValue
• 4、返回MediatorLiveData类型数据作为result

9、switchMap

 @MainThread@NonNullpublic static <X, Y> LiveData<Y> switchMap(@NonNull LiveData<X> source,@NonNull final Function<X, LiveData<Y>> switchMapFunction) {// 1final MediatorLiveData<Y> result = new MediatorLiveData<>();// 2result.addSource(source, new Observer<X>() {LiveData<Y> mSource;@Overridepublic void onChanged(@Nullable X x) {// 3LiveData<Y> newLiveData = switchMapFunction.apply(x);// 4if (mSource == newLiveData) {return;}// 5if (mSource != null) {result.removeSource(mSource);}// 6mSource = newLiveData;// 7if (mSource != null) {result.addSource(mSource, new Observer<Y>() {@Overridepublic void onChanged(@Nullable Y y) {// 8result.setValue(y);}});}}});// 9return result;}

步骤分解

• 1、创建MediatorLiveData类型result
• 2、将源数据添加到result中
• 3、通过switchMapFunction的apply函数创建LiveData类型数据newLiveData
• 4、与前一次的数据相同，直接返回
• 5、数据不为空，通过removeSource方法将数据从result中移除
• 6、缓存newLiveData至mSource
• 7、mSource不为空，将mSource作为源数据添加到result中
• 8、为result添加源数据mSource监听，并在接收到监听后setValue
• 9、返回MediatorLiveData类型数据result

10、addSource

 @MainThreadpublic <S> void addSource(@NonNull LiveData<S> source, @NonNull Observer<? super S> onChanged) {// 1Source<S> e = new Source<>(source, onChanged);// 2Source<?> existing = mSources.putIfAbsent(source, e);// 3if (existing != null && existing.mObserver != onChanged) {throw new IllegalArgumentException("This source was already added with the different observer");}// 4if (existing != null) {return;}// 5if (hasActiveObservers()) {e.plug(); // 6}}

步骤分解

• 1、以source和onChanged构造Source类型数据e
• 2、以源数据source和e构造新的Source类型数据existing
• 3、如果existing数据存在，也就是说之前已经为source添加过观察者情况，抛异常
• 4、重复判断existing，进行返回
• 5、如果有active的Observers，添加监听
• 6、plug方法（详见plug）

11、plug() MediatorLiveData中

 void plug() {mLiveData.observeForever(this);}

• observeForever

  /*** Adds the given observer to the observers list. This call is similar to* {@link LiveData#observe(LifecycleOwner, Observer)} with a LifecycleOwner, which* is always active. This means that the given observer will receive all events and will never* be automatically removed. You should manually call {@link #removeObserver(Observer)} to stop* observing this LiveData.* While LiveData has one of such observers, it will be considered* as active.* <p>* If the observer was already added with an owner to this LiveData, LiveData throws an* {@link IllegalArgumentException}.** @param observer The observer that will receive the events*/@MainThreadpublic void observeForever(@NonNull Observer<? super T> observer) {assertMainThread("observeForever");// 1AlwaysActiveObserver wrapper = new AlwaysActiveObserver(observer);// 2ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);// 3if (existing instanceof LiveData.LifecycleBoundObserver) {throw new IllegalArgumentException("Cannot add the same observer"+ " with different lifecycles");}if (existing != null) {return;}// 4wrapper.activeStateChanged(true);}

• 1、 通过observer构造AlwaysActiveObserver
• 2、 通过observer和wrapper构造ObserverWrapper类型数据existing
• 3、 如果existing数据已经存在，也就是说LiveData已经添加过该observer了，抛出异常
• 4、修改wrapper的active状态为true

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