1.定义
livedata是一个可以被观察的数据持有化类。livedata是生命周期感知的，它只通知处于活动生命周期状态的观察者组件更新
2. 作用
livedata用来持有数据，当数据发生变化时livedata通知处于活动生命周期状态的观察者。与此同时，livedata也会监控观察者的生命周期状态，当它处于DESTROYED状态时livedata会自动移除观察者，从而避免内存溢出的情况
3.实现原理
livedata类的实现基于观察者模式。观察者组件需要实现Observer接口，然后通过observe方法在livedata类中进行注册，observe方法需要传入一个LifecycleOwner对象参数，livedata会监控这个LifecycleOwner对象的生命周期状态，当LifecycleOwner对象的生命周期状态变成DESTROYED时livedata自动移除该观察者
@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer observer) {
assertMainThread("observe");
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
return;
}
//LifecycleBoundObserver类实现了LifecycleObserver接口，用来观察LifecycleOwner对象的生命周期
LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
//mObservers集合中持有wrapper对象，当livedata中数据发生变化时会遍历mObservers集合并且执行observer.onChange方法
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
owner.getLifecycle().addObserver(wrapper);
}
LifecycleBoundObserver类实现了LifecycleObserver接口，用来观察LifecycleOwner对象的生命周期，当LifecycleOwner对象的生命周期状态变成DESTROYED时自动移除该观察者
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
removeObserver(mObserver);
return;
}
activeStateChanged(shouldBeActive());
}
当更新livedata中的值时遍历mObservers集合，并且执行observer.onChange方法
@MainThread
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
4.实例
-
使用Activity作为LifecycleOwner
class LiveDataActivity : AppCompatActivity() {
private var mCustomLiveData: CustomLiveData = CustomLiveData()
private lateinit var mNameTextView: TextView
private lateinit var mButton : Button
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_livedata_main)
mNameTextView = findViewById(R.id.text_view)
mButton = findViewById(R.id.button)
mButton.setOnClickListener {
//setValue方法必须在主线程中执行
mCustomLiveData.currentNameLiveData.value = "new value"
}
val nameObserver = Observer { newName ->
mNameTextView.text = newName
}
// Observe the LiveData, passing in this activity as the LifecycleOwner and the observer.
mCustomLiveData.currentNameLiveData.observe(this, nameObserver)
}
}
livedata中的setVaule方法必须在主线程中执行。如果需要在工作线程中更新值应该使用postValue方法
class CustomLiveData {
val currentNameLiveData: MutableLiveData by lazy {
MutableLiveData().also {
it.value = "init"
}
}
}
通过懒加载方式初始化livedata对象，在使用时初始化
-
扩展livedata
class StockLiveData(symbol: String) : LiveData() {
private val stockManager: StockManager =
StockManager(symbol)
private val listener = { price: BigDecimal ->
value = price
}
override fun onActive() {
stockManager.requestPriceUpdates(listener)
}
override fun onInactive() {
stockManager.removeUpdates(listener)
}
companion object {
private lateinit var sInstance: StockLiveData
//创建一个StockLiveData单例对象
@MainThread
fun get(symbol: String): StockLiveData {
sInstance = if (Companion::sInstance.isInitialized) sInstance else StockLiveData(
symbol
)
return sInstance
}
}
}
当livedata有一个处于活动状态的观察者对象时调用onActive方法，可以在该方法中实现初始化操作
当livedata没有任何处于活动状态的观察者对象时调用onInactive方法，可以在该方法中实现清理操作
class MyFragment : Fragment() {
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
//Multiple fragments and activities can observe the livedata instance
StockLiveData.get("symbol")
.observe(this, Observer { price: BigDecimal? ->
// Update the UI.
})
}
}
livedata是生命周期感知组件，可以在多个activity或者fragment之间共享
-
livedata变换
Transformations.map()
val userLiveData: MutableLiveData = MutableLiveData()
val userName: LiveData = Transformations.map(userLiveData) {
user -> "${user.name} ${user.lastName}"
}
userName.observe(this, Observer {
println("userName............: $it")
})
mapTextView.setOnClickListener {
userLiveData.value = User("name", "last name")
}
map变换用来将原始的livedata泛型对象转换成新的livedata泛型，当原始livedata对象数据发生变化时新的livedata对象也能够收到通知
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;
}
map变换的实现基于MediatorLiveData。MediatorLiveData继承于LiveData，它会在原始livedata对象上注册新的观察者，当原始livedata对象数据发生变化时MediatorLiveData会回调mapFunction方法，然后将返回结果作为新的MediatorLiveData数据值更新，最后通知MediatorLiveData的观察者
MediatorLiveData可以用来实现同时观察多个livedata数据源，当任何一个livedata数据发生变换时MediatorLiveData都可以触发通知
Transformations.switchMap()
val userId: MutableLiveData = MutableLiveData()
val user: LiveData = Transformations.switchMap(userId) {
id ->
println("userId changed")
val result = getUser(id)
result.value = User("name", "lastName")
result
}
user.observe(this, Observer {
println("user............: $it")
})
switchMapTextView.setOnClickListener {
userId.value = "new userId"
}
private fun getUser(id: String): MutableLiveData {
return MutableLiveData()
}
switchMap变换的实现基于MediatorLiveData，用来观察getUser函数返回的livedata值的变化。当userId变化时switchMap会回调并且执行getUser函数，如果getUser函数返回新的livedata对象，MediatorLiveData会重新注册观察新的livedata对象
public static LiveData switchMap(
@NonNull LiveData source,
@NonNull final Function> switchMapFunction) {
final MediatorLiveData result = new MediatorLiveData<>();
result.addSource(source, new Observer() {
LiveData mSource;
@Override
public void onChanged(@Nullable X x) {
LiveData newLiveData = switchMapFunction.apply(x);
if (mSource == newLiveData) {
return;
}
if (mSource != null) {
result.removeSource(mSource);
}
mSource = newLiveData;
if (mSource != null) {
result.addSource(mSource, new Observer() {
@Override
public void onChanged(@Nullable Y y) {
result.setValue(y);
}
});
}
}
});
return result;
}
原文链接：https://github.com/rczh/JetpackGuide