Android Jetpack架构组件（五）一文带你了解LiveData(原理篇）

正文

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2.LiveData的observe方法回调

LifecycleBoundObservers是LiveData的内部类，代码如下所示。 frameworks/support/lifecycle/livedata-core/src/main/java/androidx/lifecycle/LiveData.java

 class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver {
        @NonNull final LifecycleOwner mOwner;

        LifecycleBoundObserver(@NonNull LifecycleOwner owner, Observer<? super T> observer) {
            super(observer);
            mOwner = owner;
        }

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

        @Override
        public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
            if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
                removeObserver(mObserver);//1
                return;
            }
            activeStateChanged(shouldBeActive());//2
        }

        @Override
        boolean isAttachedTo(LifecycleOwner owner) {
            return mOwner == owner;
        }

        @Override
        void detachObserver() {
            mOwner.getLifecycle().removeObserver(this);
        }
    }
        
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LifecycleBoundObserver继承了ObserverWrapper类，重写了shouldBeActive方法，用于判断当前传入的组件的状态是否是Active的，Active状态包括STARTED和RESUMED状态。

LifecycleBoundObserver实现了GenericLifecycleObserver接口，当组件状态发生变化时，会调用onStateChanged方法，当组件处于DESTROYED状态时，会调用注释1处的removeObserver方法，来移除observer。 这样在文章开头的疑问就解决了，为什么一个观察者（组件）处于DESTROYED状态时，它将不会收到通知。

接着会调用注释2处的activeStateChange方法，代码如下所示。

frameworks/support/lifecycle/livedata-core/src/main/java/androidx/lifecycle/LiveData.java

  private abstract class ObserverWrapper {
        final Observer<? super T> mObserver;
        boolean mActive;
        int mLastVersion = START_VERSION;

        ObserverWrapper(Observer<? super T> observer) {
            mObserver = observer;
        }

        abstract boolean shouldBeActive();

        boolean isAttachedTo(LifecycleOwner owner) {
            return false;
        }

        void detachObserver() {
        }

        void activeStateChanged(boolean newActive) {
            if (newActive == mActive) {
                return;
            }
            mActive = newActive;
            boolean wasInactive = LiveData.this.mActiveCount == 0;
            LiveData.this.mActiveCount += mActive ? 1

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