介绍
LiveData 是一种持有可被观察数据的类（an observable data holder class）。和其他可被观察的类不同的是，LiveData是有生命周期感知能力的（lifecycle-aware,），这意味着它可以在 activities, fragments, 或者 services 生命周期是活跃状态时更新这些组件。
用法
1.继承ViewModel
2.将实例类封装成MutableLiveData类型
基础代码如下
public class UserViewModel extends ViewModel {
private MutableLiveData<User> mutableLiveData;
String name = "asdasdasdadadada";
public MutableLiveData<User> getData(){
if (null == mutableLiveData){
mutableLiveData = new MutableLiveData<>();
}
mutableLiveData.setValue(loadData());
return this.mutableLiveData;
}
private User loadData() {
User user = new User();
user.age = new Random().nextInt(50);
user.name = name.substring(0,new Random().nextInt(15));
Log.d("UserViewModel",user.toString());
return user;
}
public void change(){
if(mutableLiveData != null){
mutableLiveData.setValue(loadData());
}
}
}
界面上使用：
userViewModel = ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication()).create(UserViewModel.class);
userViewModel.getData().observe(this, new Observer<User>() {
@Override
public void onChanged(User user) {
tv.setText(user.toString());
}
});
当改变调用change改变MutableLiveData的值时，界面会同步改变。
简单看下关联的代码：
当我们点击的时候会调用我们自己写的change方法，触发MutableLiveData的setValue方法：
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
setValue方法中会触发分发的方法dispatchingValue：
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;
}
该方法最终会执行considerNotify方法：
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);
}
最后一句代码ObserverWrapper.mObserver就是我们在界面中设置的Observer对象，因此最终回调会返回到onChanged触发界面更新。
注意点
LiveData与MutableLiveData区别
LiveData与MutableLiveData的其实在概念上是一模一样的.唯一几个的区别如下:
1.MutableLiveData的父类是LiveData
2.LiveData在实体类里可以通知指定某个字段的数据更新.
3.MutableLiveData则是完全是整个实体类或者数据类型变化后才通知.不会细节到某个字段
