LiveData的使用和原理

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//创建一个LiveData对象
private val livedata = MutableLiveData<String>();

//为LiveData设置监听
livedata.observe(this, Observer {
    Log.e("znh", "监听到livedata的数据了：${it}")
})

//设置LiveData的值
livedata.value = "测试数据"

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@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
      assertMainThread("observe");

	  //注释1 判断owner的生命周期状态是不是被销毁了
      if (owner.getLifecycle().getCurrentState() == DESTROYED) {
          // ignore
          return;
      }

	  //注释2
      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;
      }

	  //注释3 为lifecycle添加观察者
      owner.getLifecycle().addObserver(wrapper);
 }

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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);
        }

		//注释4
        @Override
        public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
        	//注释5
            if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
                removeObserver(mObserver);
                return;
            }
            
            //注释6
            activeStateChanged(shouldBeActive());
        }

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

        @Override
        void detachObserver() {
        	//注释7
            mOwner.getLifecycle().removeObserver(this);
        }
}

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private void considerNotify(ObserverWrapper observer) {
       if (!observer.mActive) {
           return;
       }

       if (!observer.shouldBeActive()) {
           observer.activeStateChanged(false);
           return;
       }

	   //注释8
       if (observer.mLastVersion >= mVersion) {
           return;
       }
       observer.mLastVersion = mVersion;
       
       //注释9
       observer.mObserver.onChanged((T) mData);
}