一 入口
閱讀源碼需要從源碼的入口處著手，先看先官方的例子(https://developer.android.com/topic/libraries/architecture/livedata)：
ViewModel
public class NameViewModel extends ViewModel {
// Create a LiveData with a String
private MutableLiveData<String> mCurrentName;
public MutableLiveData<String> getCurrentName() {
if (mCurrentName == null) {
mCurrentName = new MutableLiveData<String>();
}
return mCurrentName;
}
...
}
里面有一個(gè)LiveData數(shù)據(jù)和getLivedata方法
二 使用：
public class NameActivity extends AppCompatActivity {
private NameViewModel mModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Other code to setup the activity...
// Get the ViewModel.
mModel = ViewModelProviders.of(this).get(NameViewModel.class);
// Create the observer which updates the UI.
final Observer<String> nameObserver = new Observer<String>() {
@Override
public void onChanged(@Nullable final String newName) {
// Update the UI, in this case, a TextView.
mNameTextView.setText(newName);
}
};
// Observe the LiveData, passing in this activity as the LifecycleOwner and the observer.
mModel.getCurrentName().observe(this, nameObserver);
}
}
更新LiveData：
mButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
String anotherName = "John Doe";
mModel.getCurrentName().setValue(anotherName);
}
});
mModel.getCurrentName().observe(this, nameObserver)就是注冊(cè)了觀察者，接上一篇。
三 LiveData是如何工作的？
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<T> observer) {
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
// ignore
return;
}
LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
...
owner.getLifecycle().addObserver(wrapper);
}
DESTROYED狀態(tài)就不進(jìn)行注冊(cè)操作。 否則addObserver(wrapper)。
在什么時(shí)候回調(diào)Observer的void onChanged(@Nullable T t)呢？
有兩個(gè)地方會(huì)回調(diào)：
- LifecycleBoundObserver的onStateChanged處
- LiveData void setValue(T value)和 void postValue(T value)內(nèi)部調(diào)用setValue。
下面分開看。
在上一篇中，我們知道， lifeCircle感知到生命周期時(shí)，進(jìn)行對(duì)應(yīng)的回調(diào)給Observer， 即回調(diào)這里的LifecycleBoundObserver的onStateChanged方法。
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
removeObserver(mObserver);
return;
}
activeStateChanged(shouldBeActive());
}
void activeStateChanged(boolean newActive) {
if (newActive == mActive) {
return;
}
// immediately set active state, so we'd never dispatch anything to inactive
// owner
mActive = newActive;
boolean wasInactive = LiveData.this.mActiveCount == 0;
LiveData.this.mActiveCount += mActive ? 1 : -1;
if (wasInactive && mActive) {
onActive();
}
if (LiveData.this.mActiveCount == 0 && !mActive) {
onInactive();
}
if (mActive) {
dispatchingValue(this);
}
}
在wasInactive && mActive調(diào)用了LiveData的onActive()， 在這里我們可以釋放資源。onInactive()相反。 最后，如果是在mActive為true情況下， dispatchingValue(this)，分發(fā)數(shù)據(jù)。
private 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<T>, ObserverWrapper>> iterator =
mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
considerNotify(iterator.next().getValue());
if (mDispatchInvalidated) {
break;
}
}
}
} while (mDispatchInvalidated);
mDispatchingValue = false;
}
分兩種情況：
1 initiator為null， 就considerNotify所有的Observer。
- 否則，就considerNotify(initiator)；
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);
}
調(diào)用onChanged方法。即回調(diào)到google 示例的方法：
final Observer<String> nameObserver = new Observer<String>() {
@Override
public void onChanged(@Nullable final String newName) {
// Update the UI, in this case, a TextView.
mNameTextView.setText(newName);
}
};
然后，我們setValue處的回調(diào)。
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
dispatchingValue(null)由上面可見，會(huì)調(diào)用所有Observer的onChanged方法。post方法省略。google的例子就是通過setValue方法更新LiveData的數(shù)據(jù)的。LiveData的源碼就閱讀完了。
四 ViewModel 的創(chuàng)建
mModel = ViewModelProviders.of(this).get(NameViewModel.class)
代碼比較少，我們來看看。
of 的參數(shù)主要是兩種 activity， fragment。 最終走如下兩個(gè)分支
public static ViewModelProvider of(@NonNull Fragment fragment, @Nullable Factory factory) {
Application application = checkApplication(checkActivity(fragment));
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
return new ViewModelProvider(ViewModelStores.of(fragment), factory);
}
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(ViewModelStores.of(activity), factory);
}
可以看出都是新建ViewModelProvider， 參數(shù)為@NonNull ViewModelStore store, @NonNull Factory factory
分別看， 第二個(gè)參數(shù)兩個(gè)方法是類似的，不同的是第一個(gè)參數(shù)。ViewModelStore的of重載方法。
public static ViewModelStore of(@NonNull FragmentActivity activity) {
if (activity instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) activity).getViewModelStore();
}
return holderFragmentFor(activity).getViewModelStore();
}
public static ViewModelStore of(@NonNull Fragment fragment) {
if (fragment instanceof ViewModelStoreOwner) {
return ((ViewModelStoreOwner) fragment).getViewModelStore();
}
return holderFragmentFor(fragment).getViewModelStore();
}
看到這里，其實(shí)可以猜到，也是采用的透明的fragment存儲(chǔ)viewModel。繼續(xù)跟進(jìn)去：
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(FragmentActivity activity) {
return sHolderFragmentManager.holderFragmentFor(activity);
}
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(Fragment fragment) {
return sHolderFragmentManager.holderFragmentFor(fragment);
}
HolderFragment holderFragmentFor(FragmentActivity activity) {
FragmentManager fm = activity.getSupportFragmentManager();
HolderFragment holder = findHolderFragment(fm);
if (holder != null) {
return holder;
}
holder = mNotCommittedActivityHolders.get(activity);
if (holder != null) {
return holder;
}
if (!mActivityCallbacksIsAdded) {
mActivityCallbacksIsAdded = true;
activity.getApplication().registerActivityLifecycleCallbacks(mActivityCallbacks);
}
holder = createHolderFragment(fm);
mNotCommittedActivityHolders.put(activity, holder);
return holder;
}
HolderFragment holderFragmentFor(Fragment parentFragment) {
FragmentManager fm = parentFragment.getChildFragmentManager();
HolderFragment holder = findHolderFragment(fm);
if (holder != null) {
return holder;
}
holder = mNotCommittedFragmentHolders.get(parentFragment);
if (holder != null) {
return holder;
}
parentFragment.getFragmentManager()
.registerFragmentLifecycleCallbacks(mParentDestroyedCallback, false);
holder = createHolderFragment(fm);
mNotCommittedFragmentHolders.put(parentFragment, holder);
return holder;
}
即不存在時(shí)，都通過 createHolderFragment(fm)創(chuàng)建對(duì)應(yīng)holder，不過生命周期不同而已。
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;
}
作了一份內(nèi)存級(jí)緩存。
image.png
整個(gè)架構(gòu)如圖， 這里看出，ViewModel持有activity或fragment，需要注意的就是內(nèi)存泄漏， 記得在適當(dāng)時(shí)機(jī)調(diào)用onCleared()方法。
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