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Add count operator (#190)
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‎CHANGELOG.md

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- rolled `map(to:)` back to `mapTo(_:)` for `SharedSequenceConvertibleType`
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- added `unwrap()` operator for SharedSequence
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- added `apply(_:)` for `Single`
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- added `count` operator
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3.3.0
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-----

‎Playground/RxSwiftExtPlayground.playground/Pages/Index.xcplaygroundpage/Contents.swift

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- [Observable.fromAsync()](fromAsync) constructor, translates an async function that returns data through a completionHandler in a function that returns data through an Observable
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- [ofType()](ofType) operator, filters the elements of an observable sequence, if that is an instance of the supplied type.
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- [toSortedArray()](mapMany) operator, converts an Observable into another Observable that emits the whole sequence as a single array sorted using the provided closure and then terminates.
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- [count(predicate)](count) operator, counts the number of items emitted by an Observable. If predicate exists, then counts the number of items satisfying it.
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- **UIViewPropertyAnimator** [animate()](UIViewPropertyAnimator.animate) operator, returns a Completable that completes as soon as the animation ends.
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- **UIViewPropertyAnimator** [fractionComplete](UIViewPropertyAnimator.fractionComplete) binder, provides a reactive way to bind to `UIViewPropertyAnimator.fractionComplete`.
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*/

‎Playground/RxSwiftExtPlayground.playground/Pages/count.xcplaygroundpage/Contents.swift

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/*:
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> # IMPORTANT: To use `RxSwiftExtPlayground.playground`, please:
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1. Make sure you have [Carthage](https://github.com/Carthage/Carthage) installed
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1. Fetch Carthage dependencies from shell: `carthage bootstrap --platform ios`
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1. Build scheme `RxSwiftExtPlayground` scheme for a simulator target
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1. Choose `View > Show Debug Area`
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*/
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//: [Previous](@previous)
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import RxSwift
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import RxSwiftExt
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/*:
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## count
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Count the number of items emitted by an Observable
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- seealso: [count operator on reactivex.io](http://reactivex.io/documentation/operators/count.html)
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- parameter predicate: predicate determines what elements to be counted.
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- returns: An Observable sequence containing a value that represents how many elements in the specified observable sequence satisfy a condition if provided, else the count of items.
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*/
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example("count") {
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// count even number in the sequence
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let _ = Observable.from([1...10])
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.count { $0 % 2 == 0 }
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.subscribe(onNext: {
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print ($0)
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})
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}
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//: [Next](@next)

‎Playground/RxSwiftExtPlayground.playground/contents.xcplayground

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<page name='apply'/>
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<page name='cascade'/>
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<page name='catchErrorJustComplete'/>
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<page name='count'/>
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<page name='distinct'/>
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<page name='ignore'/>
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<page name='not'/>
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<page name='nwise'/>
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<page name='zipWith'/>
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<page name='ofType'/>
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<page name='mapMany'/>
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<page name='toSortedArray'/>
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<page name='UIViewPropertyAnimator.animate'/>
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<page name='UIViewPropertyAnimator.fractionComplete'/>
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<page name='withUnretained'/>
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</pages>
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</playground>
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</playground>

‎Readme.md

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* [Observable.fromAsync](#fromasync)
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* [Observable.zip(with:)](#zipwith)
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* [withUnretained](#withunretained)
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* [count](#count)
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There are two more available operators for `materialize()`'d sequences:
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```
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next(One)
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next(Three)
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completed
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completed
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```
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#### ignoreWhen
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Convenience version of `Observable.zip(_:)`. Merges the specified observable sequences into one observable sequence by using the selector function whenever all
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of the observable sequences have produced an element at a corresponding index.
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```
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```swift
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let first = Observable.from(numbers)
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let second = Observable.from(strings)
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completed
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```
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#### [count](http://reactivex.io/documentation/operators/count.html)
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Emits the number of items emitted by an Observable once it terminates with no errors. If a predicate is given, only elements matching the predicate will be counted.
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```swift
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Observable.from([1, 2, 3, 4, 5, 6])
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.count { $0 % 2 == 0 }
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.subscribe()
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```
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```
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next(3)
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completed
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```
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Reactive Extensions details
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===========
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‎RxSwiftExt.xcodeproj/project.pbxproj

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‎Source/RxSwift/count.swift

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//
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// count.swift
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// RxSwiftExt-iOS
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//
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// Created by Fred on 06/11/2018.
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// Copyright © 2018 RxSwiftCommunity. All rights reserved.
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//
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import Foundation
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import RxSwift
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extension Observable {
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/**
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Count the number of items emitted by an Observable
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- seealso: [count operator on reactivex.io](http://reactivex.io/documentation/operators/count.html)
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- returns: An Observable sequence containing a value that represents how many elements in the specified observable sequence satisfy a condition if provided, else the count of items.
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*/
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public func count() -> Observable<Int> {
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return reduce(0) { count, _ in count + 1 }
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}
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/**
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Count the number of items emitted by an Observable
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- seealso: [count operator on reactivex.io](http://reactivex.io/documentation/operators/count.html)
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- parameter predicate: predicate determines what elements to be counted.
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- returns: An Observable sequence containing a value that represents how many elements in the specified observable sequence satisfy a condition if provided, else the count of items.
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*/
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public func count(_ predicate: @escaping (Element) throws -> Bool) -> Observable<Int> {
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return filter(predicate).count()
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}
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}

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