我正在寻找一种更好的方法来实现使用 RxJava 时针对空结果的简单 Observable 后备系统。这个想法是,如果对一组数据的本地查询导致零项,则应该进行回退查询(可能是网络调用或其他查询)。目前,我的代码包含以下内容:
Observable.create(new Observable.OnSubscribe<Object>() {
@Override
public void call(Subscriber<? super Object> subscriber) {
List<Object> results = queryLocalDatabase();
if (results.isEmpty()) {
subscriber.onError(new Throwable("Empty results"));
} else {
// Process results...
}
}
}).onErrorResumeNext(/* Fallback Observable goes here */);
尽管这可行,但为空结果集引发异常并没有多大意义。我注意到有可用的条件运算符,例如isEmpty
,但它似乎并没有让我到达我想去的地方。例如,使用isEmpty
...
localObservable = Observable.create(new Observable.OnSubscribe<Object>() {
@Override
public void call(Subscriber<? super Object> subscriber) {
List<Object> results = queryLocalDatabase();
for (Object obj : results) {
// Process object
subscriber.onNext(object);
}
subscriber.onCompleted();
}
});
localObservable.isEmpty()
.switchMap(new Func1<Boolean, Observable<? extends Object>>() {
@Override
public Observable<? extends Object> call(Boolean isEmpty) {
if (isEmpty) {
// Return fallback Observable.
return fallbackObservable;
}
// Return original Observable, which would mean running the local query again... Not desired.
return localObservable;
}
});
这几乎让我到达了我想去的地方,除了localObservable
如果存在非空项目,则似乎会运行两次,这是一个破坏交易的情况。
Use switchIfEmpty
操作员。
有使用示例:
Observable.create(new Observable.OnSubscribe<String>() {
@Override
public void call(Subscriber<? super String> subscriber) {
// return no item
//subscriber.onNext(...);
System.out.println("Generating nothing :)");
subscriber.onCompleted();
}
}).switchIfEmpty(Observable.create(new Observable.OnSubscribe<String>() {
@Override
public void call(Subscriber<? super String> subscriber) {
System.out.println("Generating item");
subscriber.onNext("item");
subscriber.onCompleted();
}
})).subscribe(new Observer<String>() {
@Override
public void onCompleted() {
System.out.println("onCompleted");
}
@Override
public void onError(Throwable e) {
System.out.println("onError");
}
@Override
public void onNext(String s) {
System.out.println("onNext: " + s);
}
});
用 lambda 简化:
Observable.create(subscriber -> {
// return no item
//subscriber.onNext(...);
System.out.println("Generating nothing :)");
subscriber.onCompleted();
}).switchIfEmpty(Observable.create(subscriber -> {
System.out.println("Generating item");
subscriber.onNext("item");
subscriber.onCompleted();
})).subscribe(
s -> System.out.println("onNext: " + s),
throwable -> System.out.println("onError"),
() -> System.out.println("onCompleted")
);
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