Repository: ReactiveX/RxSwift
File: RxSwift/Observables/Sample.swift (Swift)

Code before:
//
//  Sample.swift
//  RxSwift
//
//  Created by Krunoslav Zaher on 5/1/15.
//  Copyright © 2015 Krunoslav Zaher. All rights reserved.
//

extension ObservableType {

    /**
     Samples the source observable sequence using a sampler observable sequence producing sampling ticks.

     Upon each sampling tick, the latest element (if any) in the source sequence during the last sampling interval is sent to the resulting sequence.

     **In case there were no new elements between sampler ticks, if a default value has been set then it is sent
       to the resulting sequence otherwise no element is sent.**

     - seealso: [sample operator on reactivex.io](http://reactivex.io/documentation/operators/sample.html)

     - parameter sampler: Sampling tick sequence.
                 defaultValue: a value to return if there are new new elements between sampler ticks
     - returns: Sampled observable sequence.
     */
    public func sample<Source: ObservableType>(_ sampler: Source, defaultValue: Element? = nil)
        -> Observable<Element> {
            return Sample(source: self.asObservable(), sampler: sampler.asObservable(), defaultValue: defaultValue)
    }
}

final private class SamplerSink<Observer: ObserverType, SampleType>
    : ObserverType
    , LockOwnerType
    , SynchronizedOnType {
    typealias Element = SampleType
    
    typealias Parent = SampleSequenceSink<Observer, SampleType>
    
    private let parent: Parent

    var lock: RecursiveLock {

Review comment (suggestion):
```suggestion
     **In case there were no new elements between sampler ticks, you may provide a default value to be emitted, instead
```

Code after:
//
//  Sample.swift
//  RxSwift
//
//  Created by Krunoslav Zaher on 5/1/15.
//  Copyright © 2015 Krunoslav Zaher. All rights reserved.
//

extension ObservableType {

    /**
     Samples the source observable sequence using a sampler observable sequence producing sampling ticks.

     Upon each sampling tick, the latest element (if any) in the source sequence during the last sampling interval is sent to the resulting sequence.

     **In case there were no new elements between sampler ticks, you may provide a default value to be emitted, instead
       to the resulting sequence otherwise no element is sent.**

     - seealso: [sample operator on reactivex.io](http://reactivex.io/documentation/operators/sample.html)

     - parameter sampler: Sampling tick sequence.
     - parameter defaultValue: a value to return if there are no new elements between sampler ticks
     - returns: Sampled observable sequence.
     */
    public func sample<Source: ObservableType>(_ sampler: Source, defaultValue: Element? = nil)
        -> Observable<Element> {
            return Sample(source: self.asObservable(), sampler: sampler.asObservable(), defaultValue: defaultValue)
    }
}

final private class SamplerSink<Observer: ObserverType, SampleType>
    : ObserverType
    , LockOwnerType
    , SynchronizedOnType {
    typealias Element = SampleType
    
    typealias Parent = SampleSequenceSink<Observer, SampleType>
    
    private let parent: Parent

    var lock: RecursiveLock {