LiveData是一个与Activity/Fragment生命周期相关(lifecycle-aware)的Observer类。而这种相关性(awareness )会导致LiveData中的数据只有在Active状态下才会被回调。
我们可以通过实现了LifeCycleOwner
接口的对象来提供生命周期的感知。而LiveData这种方式会使得不用再考虑Activity或者Fragment的泄露,当生命周期结束时,会自动销毁Observer对象。
inactive
状态时,例如一个Activity回退到Activity栈中,就不会受到任何LiveData的事件了inactive
状态时,它接收最新的数据,当LifeCycle再次变成active
状态,会将最新的数据回调。比如在后台一个Activity收到了最新的数据,而当它回到前台时候,则会将数据回调。LiveData
对象,并且使用单例模式来封装一个系统的Service,以至于它可以在App内共享。LiveData
对象一旦连接系统Service,当有Observer需要资源的话,就可以监听LiveData对象了build.gradle
中添加配置dependencies {
def lifecycle_version = "1.1.1"
// ViewModel and LiveData
implementation "android.arch.lifecycle:extensions:$lifecycle_version"
// alternatively - just ViewModel
implementation "android.arch.lifecycle:viewmodel:$lifecycle_version" // use -ktx for Kotlin
// alternatively - just LiveData
implementation "android.arch.lifecycle:livedata:$lifecycle_version"
// alternatively - Lifecycles only (no ViewModel or LiveData).
// Support library depends on this lightweight import
implementation "android.arch.lifecycle:runtime:$lifecycle_version"
annotationProcessor "android.arch.lifecycle:compiler:$lifecycle_version"
// alternately - if using Java8, use the following instead of compiler
implementation "android.arch.lifecycle:common-java8:$lifecycle_version"
// optional - ReactiveStreams support for LiveData
implementation "android.arch.lifecycle:reactivestreams:$lifecycle_version"
// optional - Test helpers for LiveData
testImplementation "android.arch.core:core-testing:$lifecycle_version"
}
LifeCycleOwner
相关联的LiveData
对象public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ActivityMainBinding mBinding = DataBindingUtil.setContentView(this, R.layout.activity_main);
MutableLiveData<UserData> liveData = new MutableLiveData<>();
liveData.observe(this, new Observer<UserData>() {
@Override public void onChanged(@Nullable UserData userData) {
// TODO:
}
});
}
}
LiveData
中的数据有变化时候,则可以调用setValue
或者在非主线程中调用postValue
改变,而修改后的值会通过onChanged
方法回调。liveData.setValue(new UserData());
liveData.postValue(new UserData());
setValue
以及postValue
的作用于定义成protected
,除非继承LiveData否则访问这两个函数。而MutableLiveData只是Override这两个函数,将protected作用域扩大成了public,以至于外部可以访问。addSource
函数添加LiveData
类型的数据来源,而后回调给MediatorLiveData
达到多路数据源的效果。比如同一种数据,来自网络与文件缓存。MutableLiveData<UserData> localCache = new MutableLiveData<>();
localCache.observe(this, userData -> {
// TODO:
});
MutableLiveData<UserData> networkData = new MutableLiveData<>();
networkData.observe(this, userData -> {
// TODO:
});
MediatorLiveData<String> live1 = new MediatorLiveData<>();
live1.addSource(localCache, userData -> live1.setValue(userData.getUserName()));
live1.addSource(networkData, userData -> live1.setValue(userData.getUserName()));
通过继承LiveData类重写onActive
以及onInactive
函数来完成对数据请求/前台耗时操作的回调等等
public class StockLiveData extends LiveData<BigDecimal> {
private StockManager mStockManager;
private SimplePriceListener mListener = new SimplePriceListener() {
@Override
public void onPriceChanged(BigDecimal price) {
setValue(price);
}
};
public StockLiveData(String symbol) {
mStockManager = new StockManager(symbol);
}
@Override
protected void onActive() {
mStockManager.requestPriceUpdates(mListener);
}
@Override
protected void onInactive() {
mStockManager.removeUpdates(mListener);
}
}
以下为当使用单例LiveData时的情况
public class StockLiveData extends LiveData<BigDecimal> {
private static StockLiveData sInstance;
private StockManager mStockManager;
private SimplePriceListener mListener = new SimplePriceListener() {
@Override
public void onPriceChanged(BigDecimal price) {
setValue(price);
}
};
@MainThread
public static StockLiveData get(String symbol) {
if (sInstance == null) {
sInstance = new StockLiveData(symbol);
}
return sInstance;
}
private StockLiveData(String symbol) {
mStockManager = new StockManager(symbol);
}
@Override
protected void onActive() {
mStockManager.requestPriceUpdates(mListener);
}
@Override
protected void onInactive() {
mStockManager.removeUpdates(mListener);
}
}
主要为了在数据(Value)分发前,修改分发的值。
LiveData<User> userLiveData = ...;
LiveData<String> userName = Transformations.map(userLiveData, user -> {
user.name + " " + user.lastName
});
private LiveData<User> getUser(String id) {
...;
}
LiveData<String> userId = ...;
LiveData<User> user = Transformations.switchMap(userId, id -> getUser(id) );