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operators: add compactScan() and tryCompactScan()
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Thibault Wittemberg
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Sep 4, 2021
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,92 @@ | ||
| // | ||
| // CompactScan.swift | ||
| // CombineExt | ||
| // | ||
| // Created by Thibault Wittemberg on 04/09/2021. | ||
| // Copyright © 2021 Combine Community. All rights reserved. | ||
| // | ||
|
|
||
| #if canImport(Combine) | ||
| import Combine | ||
|
|
||
| @available(OSX 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *) | ||
| public extension Publisher { | ||
| /// Transforms elements from the upstream publisher by providing the current | ||
| /// element to a closure along with the last value returned by the closure. | ||
| /// | ||
| /// The ``nextPartialResult`` closure might return nil values. In that case the accumulator won't change until the next non-nil upstream publisher value. | ||
| /// | ||
| /// Use ``Publisher/compactScan(_:_:)`` to accumulate all previously-published values into a single | ||
| /// value, which you then combine with each newly-published value. | ||
| /// | ||
| /// The following example logs a running total of all values received | ||
| /// from the sequence publisher. | ||
| /// | ||
| /// let range = (0...5) | ||
| /// let cancellable = range.publisher | ||
| /// .compactScan(0) { | ||
| /// guard $1.isMultiple(of: 2) else { return nil } | ||
| /// return $0 + $1 | ||
| /// } | ||
| /// .sink { print ("\($0)", terminator: " ") } | ||
| /// // Prints: "0 2 6 ". | ||
| /// | ||
| /// - Parameters: | ||
| /// - initialResult: The previous result returned by the `nextPartialResult` closure. | ||
| /// - nextPartialResult: A closure that takes as its arguments the previous value returned by the closure and the next element emitted from the upstream publisher. | ||
| /// - Returns: A publisher that transforms elements by applying a closure that receives its previous return value and the next element from the upstream publisher. | ||
| func compactScan<T>(_ initialResult: T, _ nextPartialResult: @escaping (T, Output) -> T?) -> AnyPublisher<T, Failure> { | ||
| self.scan((initialResult, initialResult)) { accumulator, value -> (T, T?) in | ||
| let lastNonNilAccumulator = accumulator.0 | ||
| let newAccumulator = nextPartialResult(lastNonNilAccumulator, value) | ||
| return (newAccumulator ?? lastNonNilAccumulator, newAccumulator) | ||
| } | ||
| .compactMap { $0.1 } | ||
| .eraseToAnyPublisher() | ||
| } | ||
|
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| /// Transforms elements from the upstream publisher by providing the current element to an error-throwing closure along with the last value returned by the closure. | ||
| /// | ||
| /// The ``nextPartialResult`` closure might return nil values. In that case the accumulator won't change until the next non-nil upstream publisher value. | ||
| /// | ||
| /// Use ``Publisher/tryCompactScan(_:_:)`` to accumulate all previously-published values into a single value, which you then combine with each newly-published value. | ||
| /// If your accumulator closure throws an error, the publisher terminates with the error. | ||
| /// | ||
| /// In the example below, ``Publisher/tryCompactScan(_:_:)`` calls a division function on elements of a collection publisher. The resulting publisher publishes each result until the function encounters a `DivisionByZeroError`, which terminates the publisher. | ||
| /// | ||
| /// struct DivisionByZeroError: Error {} | ||
| /// | ||
| /// /// A function that throws a DivisionByZeroError if `current` provided by the TryScan publisher is zero. | ||
| /// func myThrowingFunction(_ lastValue: Int, _ currentValue: Int) throws -> Int? { | ||
| /// guard currentValue.isMultiple(of: 2) else { return nil } | ||
| /// guard currentValue != 0 else { throw DivisionByZeroError() } | ||
| /// return lastValue / currentValue | ||
| /// } | ||
| /// | ||
| /// let numbers = [1, 2, 3, 4, 5, 0, 6, 7, 8, 9] | ||
| /// let cancellable = numbers.publisher | ||
| /// .tryCompactScan(10) { try myThrowingFunction($0, $1) } | ||
| /// .sink( | ||
| /// receiveCompletion: { print ("\($0)") }, | ||
| /// receiveValue: { print ("\($0)", terminator: " ") } | ||
| /// ) | ||
| /// | ||
| /// // Prints: "6 2 failure(DivisionByZeroError())". | ||
| /// | ||
| /// If the closure throws an error, the publisher fails with the error. | ||
| /// | ||
| /// - Parameters: | ||
| /// - initialResult: The previous result returned by the `nextPartialResult` closure. | ||
| /// - nextPartialResult: An error-throwing closure that takes as its arguments the previous value returned by the closure and the next element emitted from the upstream publisher. | ||
| /// - Returns: A publisher that transforms elements by applying a closure that receives its previous return value and the next element from the upstream publisher. | ||
| func tryCompactScan<T>(_ initialResult: T, _ nextPartialResult: @escaping (T, Output) throws -> T?) -> AnyPublisher<T, Error> { | ||
| self.tryScan((initialResult, initialResult)) { accumulator, value -> (T, T?) in | ||
| let lastNonNilAccumulator = accumulator.0 | ||
| let newAccumulator = try nextPartialResult(lastNonNilAccumulator, value) | ||
| return (newAccumulator ?? lastNonNilAccumulator, newAccumulator) | ||
| } | ||
| .compactMap { $0.1 } | ||
| .eraseToAnyPublisher() | ||
| } | ||
| } | ||
| #endif |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,98 @@ | ||
| // | ||
| // CompactScanTests.swift | ||
| // CombineExtTests | ||
| // | ||
| // Created by Thibault Wittemberg on 04/09/2021. | ||
| // Copyright © 2021 Combine Community. All rights reserved. | ||
| // | ||
|
|
||
| #if !os(watchOS) | ||
| import XCTest | ||
| import Combine | ||
| import CombineExt | ||
|
|
||
| @available(OSX 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *) | ||
| final class CompactScanTests: XCTestCase { | ||
| func testCompactScan_drops_nil_values() { | ||
| let expectedValues = [0, 2, 6] | ||
| var receivedValues = [Int]() | ||
|
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| // Given: a stream of integers from 0 to 5 | ||
| let sut = (0...5).publisher | ||
|
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| // When: using a compactScan operator using a closure that returns nil when the value from the upstream publisher is odd | ||
| let cancellable = sut | ||
| .compactScan(0) { | ||
| guard $1.isMultiple(of: 2) else { return nil } | ||
| return $0 + $1 | ||
| } | ||
| .assertNoFailure() | ||
| .sink { receivedValues.append($0) } | ||
|
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| // Then: the nil results have been discarded | ||
| XCTAssertEqual(receivedValues, expectedValues) | ||
|
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| cancellable.cancel() | ||
| } | ||
|
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| func testTryCompactScan_drops_nil_values() { | ||
| let expectedValues = [0, 2, 6] | ||
| var receivedValues = [Int]() | ||
|
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| // Given: a stream of integers from 0 to 5 | ||
| let sut = (0...5).publisher | ||
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| // When: using a tryCompactScan operator using a closure that returns nil when the value from the upstream publisher is odd | ||
| let cancellable = sut | ||
| .tryCompactScan(0) { | ||
| guard $1.isMultiple(of: 2) else { return nil } | ||
| return $0 + $1 | ||
| } | ||
| .assertNoFailure() | ||
| .sink { receivedValues.append($0) } | ||
|
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| // Then: the nil results have been discarded | ||
| XCTAssertEqual(receivedValues, expectedValues) | ||
|
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| cancellable.cancel() | ||
| } | ||
|
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| func testTryCompactScan_drops_nil_values_and_throws_error() { | ||
| struct DivisionByZeroError: Error, Equatable {} | ||
|
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| let expectedValues = [6, 2] | ||
| var receivedValues = [Int]() | ||
|
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| let expectedError = DivisionByZeroError() | ||
| var receivedCompletion: Subscribers.Completion<Error>? | ||
|
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| // Given: a sequence a integers containing a 0 | ||
| let sut = [1, 2, 3, 4, 5, 0, 6, 7, 8, 9].publisher | ||
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| // When: using a tryCompactScan operator using a closure that returns nil when the value from the upstream publisher is odd | ||
| // and throws when the value is 0 | ||
| let cancellable = sut | ||
| .tryCompactScan(10) { | ||
| guard $1.isMultiple(of: 2) else { return nil } | ||
| guard $1 != 0 else { throw expectedError } | ||
| return ($0 + $1) / $1 | ||
| } | ||
| .sink { | ||
| receivedCompletion = $0 | ||
| } receiveValue: { | ||
| receivedValues.append($0) | ||
| } | ||
|
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| cancellable.cancel() | ||
|
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| // Then: the nil results have been discarded | ||
| XCTAssertEqual(receivedValues, expectedValues) | ||
|
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| // Then: the thrown error provoqued a failure | ||
| switch receivedCompletion { | ||
| case let .failure(receivedError): XCTAssertEqual(receivedError as? DivisionByZeroError, expectedError) | ||
| default: XCTFail() | ||
| } | ||
| } | ||
| } | ||
| #endif |