java 如果它是空的，则切换 observable
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Switch observable if it is empty
提问by Ignacio Giagante
I implemented two repository in order to manage my data. So, if there are not data in database, it should ask to the API about it. I saw in other post that this could be resolved using switchIfEmpty, but it didn't work for me.
我实现了两个存储库来管理我的数据。因此，如果数据库中没有数据，则应向 API 询问。我在其他帖子中看到这可以使用switchIfEmpty解决，但它对我不起作用。
I tried the following code. The line restApiFlavorRepository.query(specification)is called, but the subscriber is never notified.
我尝试了以下代码。调用了restApiFlavorRepository.query(specification)行，但永远不会通知订阅者。
public Observable query(Specification specification) {
final Observable observable = flavorDaoRepository.query(specification);
return observable.map(new Func1() {
@Override
public Observable<List<Flavor>> call(Object o) {
if(((ArrayList<Flavor>)o).isEmpty()) {
return restApiFlavorRepository.query(specification);
}
return null;
}
});
}
and this
还有这个
public Observable query(Specification specification) {
final Observable observable = flavorDaoRepository.query(specification);
return observable.switchIfEmpty(restApiFlavorRepository.query(specification));
}
And I'm still getting empty list, when I should obtain two Flavors.
当我应该获得两种口味时，我仍然得到空清单。
UPDATED
更新
What I was looking for, was this...
我要找的，是这个……
public Observable query(Specification specification) {
Observable<List<Plant>> query = mRepositories.get(0).query(specification);
List<Plant> list = new ArrayList<>();
query.subscribe(plants -> list.addAll(plants));
Observable<List<Plant>> observable = Observable.just(list);
return observable.map(v -> !v.isEmpty()).firstOrDefault(false)
.flatMap(exists -> exists
? observable
: mRepositories.get(1).query(null));
}
and it works like charm!! :)
它的作用就像魅力一样！！:)
回答by akarnokd
The switchIfEmpty()requires the source to complete without any values in order to switch to the second source:
的switchIfEmpty()要求源，以便没有任何值，完成切换到第二源：
Observable.empty().switchIfEmpty(Observable.just(1))
.subscribe(System.out::println);
This one won't switch:
这个不会切换：
Observable.just(new ArrayList<Integer>())
.switchIfEmpty(Observable.just(Arrays.asList(2)))
.subscribe(System.out::println);
If you want to switch on a 'custom' notion of emptiness, you can use filter:
如果你想打开一个“自定义”的空性概念，你可以使用filter：
Observable.just(new ArrayList<Integer>())
.filter(v -> !v.isEmpty())
.switchIfEmpty(Observable.just(Arrays.asList(2)))
.subscribe(System.out::println);
