ios - 如何忽略RxSwift上的behaviorRelayRelay元素？

Question

我正在观察BehaviorRelay，并且仅在项目数量增加时才想订阅。我尝试了distinct / dinstinctUntilChanged ，但它不符合我的需求，因为它将跳过太多或太少的次数。

behaviorRelay
  .compactMap { $0?.items }
  .subscribe(onNext: { elements in
      print("items has one more element.")
  }).disposed(by: bag)

var behaviorRelay = BehaviorRelay<[Car]?>(value: [])

class Car {
  var items: [Any] // whatever... just an example.
}

Answer 1

首先，使用map从数组映射到数字(元素):

.map { $0?.count ?? 0 } // return 0 if array is nil

比使用 scan检索当前元素和上一个元素，如下所示:

.scan((0, 0)) { previousPairOfValues, newValue in
    return (previousPairOfValues.1, newValue) // create new pair from newValue and second item of previous pair
}

然后使用 filter，仅传递递增的值:

.filter { $0.1 > $0.0 } // newer value greater than older value

而不是将其映射回最新值:

.map { $0.1 }

放在一起:

behaviorRelay
  .compactMap { $0?.items }
  .map { $0?.count ?? 0 } // return 0 if array is nil
  .scan((0, 0)) { previousPairOfValues, newValue in
    return (previousPairOfValues.1, newValue) // create new pair from newValue and second item of previous pair
  }
  .filter { $0.1 > $0.0 } // newer value greater than older value
  .map { $0.1 }
  .subscribe(onNext: { elementCount in
      print("items has one more element.")
      print("there are \(elementCount) items now")
  }).disposed(by: bag)