原理
基于Android 10.0
LiveData是一个可观察的数据持有者，是具有组件生命周期感知的，LiveData和RxJava不同，LiveData并不会通知所有的观察者，只会通知处于Active状态的观察者，如果一个观察者处于DESTROYED状态，他将不会收到通知
源码
LiveData.java的observe
private SafeIterableMap, ObserverWrapper> mObservers = new SafeIterableMap<>();
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer observer) {
assertMainThread("observe");
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
// ignore
return;
}
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;
}
owner.getLifecycle().addObserver(wrapper);
}
第1个if处，如果观察者当前状态是DESTROYED，就直接返回，此状态不允许注册LiveData
紧接着创建了一个LifecycleBoundObserver包装类，并将owner和observer传入包装类
下面又调用putIfAbsent方法将包装类和observer传入SafeIterableMap。putIfAbsent方法如果传入的key对应的value已经存在，则返回存在的value，不进行替换。如果传入的key对应的value不存在，则添加key和value，返回null
如果existing为null，就会将wrapper添加到Lifecycle中完成注册，这样当我们调用LiveData的observe方法时，实际上是LiveData内部完成了Lifecycle观察者的注册，这样LiveData就具有了观察组件生命周期的能力
LiveData.java的LifecycleBoundObserver
class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver {
@NonNull final LifecycleOwner mOwner;
LifecycleBoundObserver(@NonNull LifecycleOwner owner, Observer 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);
return;
}
activeStateChanged(shouldBeActive());
}
@Override
boolean isAttachedTo(LifecycleOwner owner) {
return mOwner == owner;
}
@Override
void detachObserver() {
mOwner.getLifecycle().removeObserver(this);
}
}
这是一个内部类，继承了ObserverWrapper，其中shouldBeActive方法用于判断当前传入的组件的状态是否是Active状态。Active状态包括STARTED和RESUMED状态
同时这个内部类也实现了GenericLifecycleObserver接口，当组件状态发生变化时，会调用onStateChanged方法。这个方法里会判断组件处于DESTROYED状态时，会removeObserver移除观察者，此时，这个观察者不会收到通知
否则，会调用activeStateChanged方法
LiveData.java的activeStateChanged
private abstract class ObserverWrapper {
final Observer mObserver;
boolean mActive;
int mLastVersion = START_VERSION;
ObserverWrapper(Observer observer) {
mObserver = observer;
}
abstract boolean shouldBeActive();
boolean isAttachedTo(LifecycleOwner owner) {
return false;
}
void detachObserver() {
}
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);
}
}
}
activeStateChanged方法在抽象类ObserverWrapper中，会根据Active状态和处于Active状态的组件的数量，对onActive方法和onInactive方法进行回调，这两个方法用于扩展LiveData对象。在最后一个if处，如果处于Active状态，则会调用dispatchingValue方法，并将自身传入其中
LiveData.java的dispatchingValue
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, ObserverWrapper>> iterator =
mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
considerNotify(iterator.next().getValue());
if (mDispatchInvalidated) {
break;
}
}
}
} while (mDispatchInvalidated);
mDispatchingValue = false;
}
mDispatchingValue用于标记当前是否处于分发状态，如果处于分发状态，则在第1个if处标记当前分发无效，直接返回
若分发有效，则会进入do-while，不论ObserverWrapper参数initiator是否为null，都会调用considerNotify方法，要么传入当前initiator，要么传入下一个
LiveData.java的considerNotify
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);
}
方法中进行了多次判断，第1个if处，如果ObserverWrapper的mActive值不为true，就直接return
第2个if处，如果当前observer对应组件的状态不是Active，就会再次调用activeStateChanged方法，传入false，其方法内部会再次判断是否执行onActive方法和onInactive方法的回调
如果判断条件都满足则会调用Observer的onChanged方法，这个方法正是LiveData的observer方法的回调
总结
LiveData的核心原理就是通过observe方法获取并注册观察对象的生命周期，当生命周期状态发生变化时会回调onStateChanged方法判断是否已经把最新的值通知给观察者。如果已经通知就直接返回，如果没有通知，就调用dispatchingValue将最新的值通知给Active状态的观察者进行数据更新
postValue和setValue除了运行的线程不同，最后都会调用dispatchingValue方法
postValue & setValue
调用MutableLiveData的observe方法后，还需要通过这两个方法进行数据更新
源码
LiveData.java的postValue和setValue
private final Runnable mPostValueRunnable = new Runnable() {
@Override
public void run() {
Object newValue;
synchronized (mDataLock) {
newValue = mPendingData;
mPendingData = NOT_SET;
}
//noinspection unchecked
setValue((T) newValue);
}
};
protected void postValue(T value) {
boolean postTask;
synchronized (mDataLock) {
postTask = mPendingData == NOT_SET;
mPendingData = value;
}
if (!postTask) {
return;
}
ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
}
@MainThread
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
我们可以观察到postValue内部加了锁，最后就是将setValue切换到主线程调用
setValue是运行在主线程的，内部调用了dispatchingValue，上面介绍过，也就是其参数ObserverWrapper为null的情况
无论是LiveData的observe方法还是LiveData的postValue和setValue方法，都会调用dispatchingValue方法
Transformations.map
这个方法用于在LiveData对象分发给观察者之前对其中存储的值进行更改
源码
Transformations.java
@MainThread
public static LiveData map(
@NonNull LiveData source,
@NonNull final Function mapFunction) {
final MediatorLiveData result = new MediatorLiveData<>();
result.addSource(source, new Observer() {
@Override
public void onChanged(@Nullable X x) {
result.setValue(mapFunction.apply(x));
}
});
return result;
}
这个方法运行在主线程，先创建一个final的MediatorLiveData，然后调用addSource方法
MediatorLiveData.java的addSource
@MainThread
public void addSource(@NonNull LiveData source, @NonNull Observer onChanged) {
Source e = new Source<>(source, onChanged);
Source existing = mSources.putIfAbsent(source, e);
if (existing != null && existing.mObserver != onChanged) {
throw new IllegalArgumentException(
"This source was already added with the different observer");
}
if (existing != null) {
return;
}
if (hasActiveObservers()) {
e.plug();
}
}
将传进来的LiveData和onChanged封装到Source类中，调用了plug方法
Source.java的plug
private static class Source implements Observer {
final LiveData mLiveData;
final Observer mObserver;
int mVersion = START_VERSION;
Source(LiveData liveData, final Observer observer) {
mLiveData = liveData;
mObserver = observer;
}
void plug() {
mLiveData.observeForever(this);
}
void unplug() {
mLiveData.removeObserver(this);
}
@Override
public void onChanged(@Nullable V v) {
if (mVersion != mLiveData.getVersion()) {
mVersion = mLiveData.getVersion();
mObserver.onChanged(v);
}
}
}
可以看到，传入的Observer回调onChanged在这里调用。plug方法里调用了observeForever方法
LiveData.java的observeForever
@MainThread
public void observeForever(@NonNull Observer observer) {
assertMainThread("observeForever");
AlwaysActiveObserver wrapper = new AlwaysActiveObserver(observer);
ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
if (existing != null && existing instanceof LiveData.LifecycleBoundObserver) {
throw new IllegalArgumentException("Cannot add the same observer"
+ " with different lifecycles");
}
if (existing != null) {
return;
}
wrapper.activeStateChanged(true);
}
开头调创建AlwaysActive Observer对Observer进行包装，然后调用AlwaysActiveObserver的activeStateChanged方法，内部实际调用的是ObserverWrapper的activeStateChanged方法，上面已经说过，这里不再赘述，看一下这个类是如何实现的
LiveData.java的AlwaysActiveObserver
private class AlwaysActiveObserver extends ObserverWrapper {
AlwaysActiveObserver(Observer observer) {
super(observer);
}
@Override
boolean shouldBeActive() {
return true;
}
}
是LiveData的内部类，继承自ObserverWrapper，和ObserverWrapper唯一的区别就是，他永远处于Active状态
应用举例
Transformations.map
在数据发送给观察者之前进行修改
public class TransformActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_transform);
MutableLiveData mutableLiveData = new MutableLiveData<>();
mutableLiveData.observe(this, new Observer() {
@Override
public void onChanged(String s) {
Log.d("Main", "onChanged1:" + s);
}
});
LiveData transformedLiveData = Transformations.map(mutableLiveData, new Function() {
@Override
public Object apply(String input) {
return input + "+Android进阶揭秘";
}
});
transformedLiveData.observe(this, new Observer() {
@Override
public void onChanged(Object o) {
Log.d("Main", "onChanged2:" + o.toString());
}
});
mutableLiveData.postValue("Android进阶之光");
}
}
Transformations.switchMap
手动控制监听某一个数据源，并根据需要随时进行切换
public class SwitchActivity extends AppCompatActivity {
private static final String TAG = "MainAc";
MutableLiveData mutableLiveData1;
MutableLiveData mutableLiveData2;
MutableLiveData liveDataSwitch;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_switch);
mutableLiveData1 = new MutableLiveData<>();
mutableLiveData2 = new MutableLiveData<>();
liveDataSwitch = new MutableLiveData<>();
//switchmap必定返回一个LiveData对象
LiveData transformedLiveData = Transformations.switchMap(liveDataSwitch, new Function>() {
@Override
public LiveData apply(Boolean input) {
if(input){
//切换监听
return mutableLiveData1;
}else{
return mutableLiveData2;
}
}
});
transformedLiveData.observe(this, new Observer() {
@Override
public void onChanged(String s) {
Log.d(TAG, "onChanged:" + s);
}
});
liveDataSwitch.postValue(false);
mutableLiveData1.postValue("Android进阶之光");
mutableLiveData2.postValue("Android进阶揭秘");
}
}
MediatorLiveData
合并多个数据源一起监听
public class MediatorActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_mediator);
MutableLiveData mutableLiveData1 = new MutableLiveData<>();
MutableLiveData mutableLiveData2 = new MutableLiveData<>();
MediatorLiveData liveDataMerger = new MediatorLiveData();
//合并两个数据源
liveDataMerger.addSource(mutableLiveData1, new Observer() {
@Override
public void onChanged(Object o) {
Log.d("Main", "onChanged1:" + o.toString());
}
});
//合并两个数据源
liveDataMerger.addSource(mutableLiveData2, new Observer() {
@Override
public void onChanged(Object o) {
Log.d("Main", "onChanged2:" + o.toString());
}
});
liveDataMerger.observe(this, new Observer() {
@Override
public void onChanged(Object o) {
Log.d("Main", "onChanged:" + o.toString());
}
});
//这样任意一个数据源有消息变动都可以观察到
mutableLiveData1.postValue("Android进阶之光");
}
}