從API調用入手
舉個栗子:Activity中訂閱數據變更
viewModel.contentLiveData.observe(this, Observer<String> { content ->
tv.text = content
})
ViewModel中初始化LiveData
val contentLiveData: MutableLiveData<String> by lazy {
MutableLiveData<String>()
}
LiveData.observe()
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
//檢查是否爲ui線程
assertMainThread("observe");
//頁面DESTROYED狀態不執行訂閱流程
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
return;
}
//包裝
LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
//添加到集合mObservers
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);
}
owner爲上述this--->Activity,owner和用戶自定義的observer包裝到LifecycleBoundObserver
LifeCycle原理分析過Activity.getLifecycle()返回LifecycleRegistry--->LifecycleRegistry.addObserver(wrapper)
LifecycleBoundObserver實現LifecycleEventObserver接口
class LifecycleBoundObserver extends ObserverWrapper implements LifecycleEventObserver{
final LifecycleOwner mOwner;
LifecycleBoundObserver(@NonNull LifecycleOwner owner, Observer<? super T> observer) {
super(observer);
mOwner = owner;
}
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
removeObserver(mObserver);
return;
}
activeStateChanged(shouldBeActive());
}
}
owner生命週期變更時會觸發onStateChanged()方法,DESTROYED狀態移除訂閱。
接下來觸發數據變更
contentLiveData.value = ""
MutableLiveData.setValue()
@Override
public void setValue(T value) {
super.setValue(value);
}
調用父類LiveData.setValue()
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
LiveData.dispatchingValue()
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;
}
遍歷集合調用considerNotify(ObserverWrapper)
LiveData.considerNotify()
private void considerNotify(ObserverWrapper observer) {
if (!observer.mActive) {
return;
}
if (!observer.shouldBeActive()) {
observer.activeStateChanged(false);
return;
}
if (observer.mLastVersion >= mVersion) {
return;
}
observer.mLastVersion = mVersion;
observer.mObserver.onChanged((T) mData);
}
回調到Observer.onChanged()方法
LiveData還有異步線程更新數據的api
contentLiveData.postValue("")
MutableLiveData.postValue()
@Override
public void postValue(T value) {
super.postValue(value);
}
LiveData.postValue()
protected void postValue(T value) {
boolean postTask;
synchronized (mDataLock) {
postTask = mPendingData == NOT_SET;
mPendingData = value;
}
if (!postTask) {
return;
}
ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
}
mPostValueRunnable
private final Runnable mPostValueRunnable = new Runnable() {
@SuppressWarnings("unchecked")
@Override
public void run() {
Object newValue;
synchronized (mDataLock) {
newValue = mPendingData;
mPendingData = NOT_SET;
}
setValue((T) newValue);
}
};
在Runnable中調用LiveData.setValue()
ArchTaskExecutor
public class ArchTaskExecutor extends TaskExecutor {
public static ArchTaskExecutor getInstance() {
if (sInstance != null) {
return sInstance;
}
synchronized (ArchTaskExecutor.class) {
if (sInstance == null) {
sInstance = new ArchTaskExecutor();
}
}
return sInstance;
}
private ArchTaskExecutor() {
mDefaultTaskExecutor = new DefaultTaskExecutor();
mDelegate = mDefaultTaskExecutor;
}
@Override
public void postToMainThread(Runnable runnable) {
mDelegate.postToMainThread(runnable);
}
}
DefaultTaskExecutor.postToMainThread()
@Override
public void postToMainThread(Runnable runnable) {
if (mMainHandler == null) {
synchronized (mLock) {
if (mMainHandler == null) {
mMainHandler = createAsync(Looper.getMainLooper());
}
}
}
mMainHandler.post(runnable);
}
mMainHandler.post(runnable)
通過handler.post()切換到主線程執行
LiveData組件實現依賴於LifeCycle組件,分析完LifeCycle流程很容易理解其執行過程。