（*useful）标记：目前觉得有用的函数
//FIXME 标记：待补充

帮助捕获错误，并可以发送消息，平滑的处理error

catchErrorJustReturn

example("catchErrorJustReturn") {
    let disposeBag = DisposeBag()
    
    let sequenceThatFails = PublishSubject<String>()
    
    sequenceThatFails
        .catchErrorJustReturn("😊")
        .subscribe { print($0) }
        .disposed(by: disposeBag)
    
    sequenceThatFails.onNext("😬")
    sequenceThatFails.onNext("😨")
    sequenceThatFails.onNext("😡")
    sequenceThatFails.onNext("🔴")
    sequenceThatFails.onError(TestError.test)
}

打印：
--- catchErrorJustReturn example ---
next(😬)
next(😨)
next(😡)
next(🔴)
next(😊) //捕获的error
completed

catchError: 捕获错误，并return一个Observable sequence

example("catchError") {
    let disposeBag = DisposeBag()
    
    let sequenceThatFails = PublishSubject<String>()
    let recoverySequence = PublishSubject<String>()
    
    sequenceThatFails
        .catchError {
            print("Error:", $0)
            return recoverySequence
        }
        .subscribe { print($0) }
        .disposed(by: disposeBag)
    
    sequenceThatFails.onNext("😬")
    sequenceThatFails.onNext("😨")
    sequenceThatFails.onNext("😡")
    sequenceThatFails.onNext("🔴")
    sequenceThatFails.onError(TestError.test)
    sequenceThatFails.onNext("errored")  
    recoverySequence.onNext("😊")
}

打印：
next(😬)
next(😨)
next(😡)
next(🔴)
Error: test //切换到了recoverySequence
next(😊)

retry

重复订阅onError的sequence，indefinitely（可无限订阅

example("retry") {
    let disposeBag = DisposeBag()
    var count = 1
    
    let sequenceThatErrors = Observable<String>.create { observer in
        observer.onNext("🍎")
        observer.onNext("🍐")
        observer.onNext("🍊")
        
        if count == 1 {
            observer.onError(TestError.test)
            print("Error encountered")
            count += 1
        }
        
        observer.onNext("🐶")
        observer.onNext("🐱")
        observer.onNext("🐭")
        observer.onCompleted()
        
        return Disposables.create()
    }
    
    sequenceThatErrors
        .retry()
        .subscribe(onNext: { print($0) })
        .disposed(by: disposeBag)
}

打印：
--- retry example ---
🍎
🍐
🍊
Error encountered
🍎 //retry从这里开始 ，demo只进行了一个次error
🍐
🍊
🐶
🐱
🐭

retry(_:) 跟上面的一样，加入了参数（控制retry的次数）

example("retry maxAttemptCount") {
    let disposeBag = DisposeBag()
    var count = 1
    
    let sequenceThatErrors = Observable<String>.create { observer in
        observer.onNext("🍎")
        observer.onNext("🍐")
        observer.onNext("🍊")
        
        if count < 5 {
            observer.onError(TestError.test)
            print("Error encountered")
            count += 1
        }
        
        observer.onNext("🐶")
        observer.onNext("🐱")
        observer.onNext("🐭")
        observer.onCompleted()
        
        return Disposables.create()
    }
    
    sequenceThatErrors
        .retry(3)
        .subscribe(onNext: { print($0) })
        .disposed(by: disposeBag)
}

打印：
--- retry maxAttemptCount example ---
🍎
🍐
🍊
Error encountered
🍎
🍐
🍊
Error encountered
🍎
🍐
🍊
Error encountered
Unhandled error happened: test
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