一、LiveData介绍
LiveData顾名思义就是活着的数据，它是一种可以观察源数据变化的数据存储类。同时具有感知Activity/Fragment生命周期的能力，只在Activity/Fragment活跃状态下更新数据。
二、LiveData源码解析
MutableLiveData
public class MutableLiveData<T> extends LiveData<T> {
/**
* Creates a MutableLiveData initialized with the given {@code value}.
*
* @param value initial value
*/
public MutableLiveData(T value) {
super(value);
}
/**
* Creates a MutableLiveData with no value assigned to it.
*/
public MutableLiveData() {
super();
}
@Override
public void postValue(T value) {
super.postValue(value);
}
@Override
public void setValue(T value) {
super.setValue(value);
}
}
MutableLiveData是LiveData的子类，也是我们常用的数据源存储类，这个类代码很简单他重写了父类的postValue和setValue方法，我们看下父类中的实现。
LiveData
public abstract class LiveData<T> {
@SuppressWarnings("WeakerAccess") /* synthetic access */
final Object mDataLock = new Object();
static final int START_VERSION = -1;
@SuppressWarnings("WeakerAccess") /* synthetic access */
static final Object NOT_SET = new Object();
private SafeIterableMap<Observer<? super T>, ObserverWrapper> mObservers =
new SafeIterableMap<>();
// how many observers are in active state
@SuppressWarnings("WeakerAccess") /* synthetic access */
int mActiveCount = 0;
// to handle active/inactive reentry, we guard with this boolean
private boolean mChangingActiveState;
private volatile Object mData;
// when setData is called, we set the pending data and actual data swap happens on the main
// thread
@SuppressWarnings("WeakerAccess") /* synthetic access */
volatile Object mPendingData = NOT_SET;
private int mVersion;
private boolean mDispatchingValue;
@SuppressWarnings("FieldCanBeLocal")
private boolean mDispatchInvalidated;
private final Runnable mPostValueRunnable = new Runnable() {
@SuppressWarnings("unchecked")
@Override
public void run() {
Object newValue;
synchronized (mDataLock) {
newValue = mPendingData;
mPendingData = NOT_SET;
}
setValue((T) newValue);
}
};
/**
* Creates a LiveData initialized with the given {@code value}.
*
* @param value initial value
*/
public LiveData(T value) {
mData = value;
mVersion = START_VERSION + 1;
}
/**
* Creates a LiveData with no value assigned to it.
*/
public LiveData() {
mData = NOT_SET;
mVersion = START_VERSION;
}
@SuppressWarnings("unchecked")
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;
observer.mObserver.onChanged((T) mData);
}
@SuppressWarnings("WeakerAccess") /* synthetic access */
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;
}
/**
* Adds the given observer to the observers list within the lifespan of the given
* owner. The events are dispatched on the main thread. If LiveData already has data
* set, it will be delivered to the observer.
* <p>
* The observer will only receive events if the owner is in {@link Lifecycle.State#STARTED}
* or {@link Lifecycle.State#RESUMED} state (active).
* <p>
* If the owner moves to the {@link Lifecycle.State#DESTROYED} state, the observer will
* automatically be removed.
* <p>
* When data changes while the {@code owner} is not active, it will not receive any updates.
* If it becomes active again, it will receive the last available data automatically.
* <p>
* LiveData keeps a strong reference to the observer and the owner as long as the
* given LifecycleOwner is not destroyed. When it is destroyed, LiveData removes references to
* the observer & the owner.
* <p>
* If the given owner is already in {@link Lifecycle.State#DESTROYED} state, LiveData
* ignores the call.
* <p>
* If the given owner, observer tuple is already in the list, the call is ignored.
* If the observer is already in the list with another owner, LiveData throws an
* {@link IllegalArgumentException}.
*
* @param owner The LifecycleOwner which controls the observer
* @param observer The observer that will receive the events
*/
@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> 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);
}
/**
* Adds the given observer to the observers list. This call is similar to
* {@link LiveData#observe(LifecycleOwner, Observer)} with a LifecycleOwner, which
* is always active. This means that the given observer will receive all events and will never
* be automatically removed. You should manually call {@link #removeObserver(Observer)} to stop
* observing this LiveData.
* While LiveData has one of such observers, it will be considered
* as active.
* <p>
* If the observer was already added with an owner to this LiveData, LiveData throws an
* {@link IllegalArgumentException}.
*
* @param observer The observer that will receive the events
*/
@MainThread
public void observeForever(@NonNull Observer<? super T> observer) {
assertMainThread("observeForever");
AlwaysActiveObserver wrapper = new AlwaysActiveObserver(observer);
ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
if (existing instanceof LiveData.LifecycleBoundObserver) {
throw new IllegalArgumentException("Cannot add the same observer"
+ " with different lifecycles");
}
if (existing != null) {
return;
}
wrapper.activeStateChanged(true);
}
/**
* Removes the given observer from the observers list.
*
* @param observer The Observer to receive events.
*/
@MainThread
public void removeObserver(@NonNull final Observer<? super T> observer) {
assertMainThread("removeObserver");
ObserverWrapper removed = mObservers.remove(observer);
if (removed == null) {
return;
}
removed.detachObserver();
removed.activeStateChanged(false);
}
/**
* Removes all observers that are tied to the given {@link LifecycleOwner}.
*
* @param owner The {@code LifecycleOwner} scope for the observers to be removed.
*/
@SuppressWarnings("WeakerAccess")
@MainThread
public void removeObservers(@NonNull final LifecycleOwner owner) {
assertMainThread("removeObservers");
for (Map.Entry<Observer<? super T>, ObserverWrapper> entry : mObservers) {
if (entry.getValue().isAttachedTo(owner)) {
removeObserver(entry.getKey());
}
}
}
/**
* Posts a task to a main thread to set the given value. So if you have a following code
* executed in the main thread:
* <pre class="prettyprint">
* liveData.postValue("a");
* liveData.setValue("b");
* </pre>
* The value "b" would be set at first and later the main thread would override it with
* the value "a".
* <p>
* If you called this method multiple times before a main thread executed a posted task, only
* the last value would be dispatched.
*
* @param value The new value
*/
protected void postValue(T value) {
boolean postTask;
synchronized (mDataLock) {
postTask = mPendingData == NOT_SET;
mPendingData = value;
}
if (!postTask) {
return;
}
ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
}
/**
* Sets the value. If there are active observers, the value will be dispatched to them.
* <p>
* This method must be called from the main thread. If you need set a value from a background
* thread, you can use {@link #postValue(Object)}
*
* @param value The new value
*/
@MainThread
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
/**
* Returns the current value.
* Note that calling this method on a background thread does not guarantee that the latest
* value set will be received.
*
* @return the current value
*/
@SuppressWarnings("unchecked")
@Nullable
public T getValue() {
Object data = mData;
if (data != NOT_SET) {
return (T) data;
}
return null;
}
int getVersion() {
return mVersion;
}
/**
* Called when the number of active observers change from 0 to 1.
* <p>
* This callback can be used to know that this LiveData is being used thus should be kept
* up to date.
*/
protected void onActive() {
}
/**
* Called when the number of active observers change from 1 to 0.
* <p>
* This does not mean that there are no observers left, there may still be observers but their
* lifecycle states aren't {@link Lifecycle.State#STARTED} or {@link Lifecycle.State#RESUMED}
* (like an Activity in the back stack).
* <p>
* You can check if there are observers via {@link #hasObservers()}.
*/
protected void onInactive() {
}
/**
* Returns true if this LiveData has observers.
*
* @return true if this LiveData has observers
*/
@SuppressWarnings("WeakerAccess")
public boolean hasObservers() {
return mObservers.size() > 0;
}
/**
* Returns true if this LiveData has active observers.
*
* @return true if this LiveData has active observers
*/
@SuppressWarnings("WeakerAccess")
public boolean hasActiveObservers() {
return mActiveCount > 0;
}
@MainThread
void changeActiveCounter(int change) {
int previousActiveCount = mActiveCount;
mActiveCount += change;
if (mChangingActiveState) {
return;
}
mChangingActiveState = true;
try {
while (previousActiveCount != mActiveCount) {
boolean needToCallActive = previousActiveCount == 0 && mActiveCount > 0;
boolean needToCallInactive = previousActiveCount > 0 && mActiveCount == 0;
previousActiveCount = mActiveCount;
if (needToCallActive) {
onActive();
} else if (needToCallInactive) {
onInactive();
}
}
} finally {
mChangingActiveState = false;
}
}
class LifecycleBoundObserver extends ObserverWrapper implements LifecycleEventObserver {
@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(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
Lifecycle.State currentState = mOwner.getLifecycle().getCurrentState();
if (currentState == DESTROYED) {
removeObserver(mObserver);
return;
}
Lifecycle.State prevState = null;
while (prevState != currentState) {
prevState = currentState;
activeStateChanged(shouldBeActive());
currentState = mOwner.getLifecycle().getCurrentState();
}
}
@Override
boolean isAttachedTo(LifecycleOwner owner) {
return mOwner == owner;
}
@Override
void detachObserver() {
mOwner.getLifecycle().removeObserver(this);
}
}
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;
}
// immediately set active state, so we'd never dispatch anything to inactive
// owner
mActive = newActive;
changeActiveCounter(mActive ? 1 : -1);
if (mActive) {
dispatchingValue(this);
}
}
}
private class AlwaysActiveObserver extends ObserverWrapper {
AlwaysActiveObserver(Observer<? super T> observer) {
super(observer);
}
@Override
boolean shouldBeActive() {
return true;
}
}
static void assertMainThread(String methodName) {
if (!ArchTaskExecutor.getInstance().isMainThread()) {
throw new IllegalStateException("Cannot invoke " + methodName + " on a background"
+ " thread");
}
}
}
LiveData是一个抽象类，它有一个有参构造函数和一个无参构造函数，看下构造函数中做了哪些处理:
/**
* Creates a LiveData initialized with the given {@code value}.
*
* @param value initial value
*/
public LiveData(T value) {
mData = value;
mVersion = START_VERSION + 1;
}
/**
* Creates a LiveData with no value assigned to it.
*/
public LiveData() {
mData = NOT_SET;
mVersion = START_VERSION;
}
在有参的构造函数中将value赋值给mData并将mVersion+1记录版本号，而无参的构造函数中将Object类型静态全局常量NOT_SET赋值给mData。
下面看下LiveData注册数据变化监听的方法实现：
@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
assertMainThread("observe");
//如果当前组件的状态等于DESTROYED则不注册返回。
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
// ignore
return;
}
//如果observer不存在则存储在map中
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;
}
//注册观察者到被观察者中，所以具有生命周期感知能力
owner.getLifecycle().addObserver(wrapper);
}
Observe方法的参数有两个一个是LifecycleOwner 用来获取LifeCycle，一个是Observer对象，用来通知value变化。LifecycleBoundObserver 对参数进行包装，看下LifecycleBoundObserver的代码可以看到它实现了LifecycleEventObserver，所以具有组件生命周期变化感知的能力。当currentState == DESTROYED时自动移除LiveData中map保存的Observer。
class LifecycleBoundObserver extends ObserverWrapper implements LifecycleEventObserver {
@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(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
Lifecycle.State currentState = mOwner.getLifecycle().getCurrentState();
//移除Observer
if (currentState == DESTROYED) {
removeObserver(mObserver);
return;
}
Lifecycle.State prevState = null;
while (prevState != currentState) {
prevState = currentState;
activeStateChanged(shouldBeActive());
currentState = mOwner.getLifecycle().getCurrentState();
}
}
@Override
boolean isAttachedTo(LifecycleOwner owner) {
return mOwner == owner;
}
@Override
void detachObserver() {
mOwner.getLifecycle().removeObserver(this);
}
}
这里注意下LifecycleBoundObserver父类中的成员变量mLastVersion的值默认就是START_VERSION
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;
}
// immediately set active state, so we'd never dispatch anything to inactive
// owner
mActive = newActive;
changeActiveCounter(mActive ? 1 : -1);
if (mActive) {
dispatchingValue(this);
}
}
}
下面看下更新value的两个方法setValue和postValue方法，setValue必须在主进程中使用，如果需要在子线程中更新数据则使用postValue方法。
/**
* Sets the value. If there are active observers, the value will be dispatched to them.
* <p>
* This method must be called from the main thread. If you need set a value from a background
* thread, you can use {@link #postValue(Object)}
*
* @param value The new value
*/
@MainThread
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
setValue方法中将mVersion++，并将mData更新，然后调用dispatchingValue(null)分发数据。
@SuppressWarnings("WeakerAccess") /* synthetic access */
void dispatchingValue(@Nullable ObserverWrapper initiator) {
//如果正在下发则返回
if (mDispatchingValue) {
mDispatchInvalidated = true;
return;
}
mDispatchingValue = true;
do {
mDispatchInvalidated = false;
if (initiator != null) {
considerNotify(initiator);
initiator = null;
} else {
//遍历map
for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =
mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
considerNotify(iterator.next().getValue());
if (mDispatchInvalidated) {
break;
}
}
}
} while (mDispatchInvalidated);
mDispatchingValue = false;
}
先进行状态检查：观察者是非活跃就return；若当前observer对应的owner非活跃，就会再调用activeStateChanged方法，并传入false，其内部会再次判断。最后回调真正的mObserver的onChanged方法，值是LivaData的变量mData。注意：这里面会判断if (observer.mLastVersion >= mVersion)如果observer的mLastversion大于等于mVersion也返回不处理
@SuppressWarnings("unchecked")
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;
observer.mObserver.onChanged((T) mData);
}
postValue实际上是将一个runnable放入主线程中执行
protected void postValue(T value) {
boolean postTask;
synchronized (mDataLock) {
postTask = mPendingData == NOT_SET;
mPendingData = value;
}
if (!postTask) {
return;
}
ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
}
Runnable中实际调用的还是setValue方法。
private final Runnable mPostValueRunnable = new Runnable() {
@SuppressWarnings("unchecked")
@Override
public void run() {
Object newValue;
synchronized (mDataLock) {
newValue = mPendingData;
mPendingData = NOT_SET;
}
setValue((T) newValue);
}
};
三、LiveData高级用法
（1）数据转换 Transformations.map
MutableLiveData<Integer> liveData = new MutableLiveData<>();
LiveData<String> stringLiveData = Transformations.map(liveData, new Function<Integer, String>() {
@Override
public String apply(Integer input) {
return Integer.toString(input);
}
});
（2） 数据切换 Transformations.switchMap
//两个liveData，由liveDataSwitch决定 返回哪个livaData数据
MutableLiveData<String> liveData3 = new MutableLiveData<>();
MutableLiveData<String> liveData4 = new MutableLiveData<>();
//切换条件LiveData，liveDataSwitch的value 是切换条件
MutableLiveData<Boolean> liveDataSwitch = new MutableLiveData<>();
//liveDataSwitchMap由switchMap()方法生成，用于添加观察者
LiveData<String> liveDataSwitchMap = Transformations.switchMap(liveDataSwitch, new Function<Boolean, LiveData<String>>() {
@Override
public LiveData<String> apply(Boolean input) {
//这里是具体切换逻辑：根据liveDataSwitch的value返回哪个liveData
if (input) {
return liveData3;
}
return liveData4;
}
});
liveDataSwitchMap.observe(this, new Observer<String>() {
@Override
public void onChanged(String s) {
Log.i(TAG, "onChanged2: " + s);
}
});
boolean switchValue = true;
liveDataSwitch.setValue(switchValue);//设置切换条件值
liveData3.setValue("liveData3");
liveData4.setValue("liveData4");
liveData3、liveData4是两个数据源，有一个判断条件来决定 取哪一个数据 ，这个条件就是liveDataSwitch，如果值为true则取liveData3，false则取liveData4。Transformations.switchMap()就用于实现这一逻辑，返回值liveDataSwitchMap添加观察者就可以了。
（3）观察多个数据 MediatorLiveData
MediatorLiveData 是 LiveData 的子类，允许合并多个 LiveData 源。只要任何原始的 LiveData 源对象发生更改，就会触发 MediatorLiveData 对象的观察者
MediatorLiveData<String> mediatorLiveData = new MediatorLiveData<>();
MutableLiveData<String> liveData5 = new MutableLiveData<>();
MutableLiveData<String> liveData6 = new MutableLiveData<>();
//添加 源 LiveData
mediatorLiveData.addSource(liveData5, new Observer<String>() {
@Override
public void onChanged(String s) {
Log.i(TAG, "onChanged3: " + s);
mediatorLiveData.setValue(s);
}
});
//添加 源 LiveData
mediatorLiveData.addSource(liveData6, new Observer<String>() {
@Override
public void onChanged(String s) {
Log.i(TAG, "onChanged4: " + s);
mediatorLiveData.setValue(s);
}
});
//添加观察
mediatorLiveData.observe(this, new Observer<String>() {
@Override
public void onChanged(String s) {
Log.i(TAG, "onChanged5: "+s);
//无论liveData5、liveData6更新，都可以接收到
}
});
liveData5.setValue("liveData5");
//liveData6.setValue("liveData6");
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