Adds Publisher.weaklyAssign(to:on:).

CombineExt.xcodeproj/project.pbxproj

@@ -41,6 +41,8 @@
 		78C193E0241D4D8D0001B7FD /* MaterializeTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 78C193DF241D4D8D0001B7FD /* MaterializeTests.swift */; };
 		78C193E2241D596F0001B7FD /* Dematerialize.swift in Sources */ = {isa = PBXBuildFile; fileRef = 78C193E1241D596F0001B7FD /* Dematerialize.swift */; };
 		78C193E4241D63620001B7FD /* DematerializeTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 78C193E3241D63620001B7FD /* DematerializeTests.swift */; };
+		AAEAF0E32436C8A8007C35E0 /* WeakAssign.swift in Sources */ = {isa = PBXBuildFile; fileRef = AAEAF0E22436C8A8007C35E0 /* WeakAssign.swift */; };
+		AAEAF0E52436C9C2007C35E0 /* WeakAssignTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = AAEAF0E42436C9C2007C35E0 /* WeakAssignTests.swift */; };
 		BF50924B241FFE8E00600DF4 /* ZipManyTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = BF50924A241FFE8E00600DF4 /* ZipManyTests.swift */; };
 		BF8121BC241FF42C006A93B8 /* ZipMany.swift in Sources */ = {isa = PBXBuildFile; fileRef = BF8121BB241FF42C006A93B8 /* ZipMany.swift */; };
 		BFB4EA132428256B0096E9E9 /* CombineLatestMany.swift in Sources */ = {isa = PBXBuildFile; fileRef = BFB4EA122428256B0096E9E9 /* CombineLatestMany.swift */; };
@@ -83,6 +85,8 @@
 		78C193DF241D4D8D0001B7FD /* MaterializeTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = MaterializeTests.swift; sourceTree = "<group>"; };
 		78C193E1241D596F0001B7FD /* Dematerialize.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Dematerialize.swift; sourceTree = "<group>"; };
 		78C193E3241D63620001B7FD /* DematerializeTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = DematerializeTests.swift; sourceTree = "<group>"; };
+		AAEAF0E22436C8A8007C35E0 /* WeakAssign.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = WeakAssign.swift; sourceTree = "<group>"; };
+		AAEAF0E42436C9C2007C35E0 /* WeakAssignTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = WeakAssignTests.swift; sourceTree = "<group>"; };
 		BF50924A241FFE8E00600DF4 /* ZipManyTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ZipManyTests.swift; sourceTree = "<group>"; };
 		BF8121BB241FF42C006A93B8 /* ZipMany.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ZipMany.swift; sourceTree = "<group>"; };
 		BFB4EA122428256B0096E9E9 /* CombineLatestMany.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CombineLatestMany.swift; sourceTree = "<group>"; };
@@ -166,6 +170,7 @@
 				BF50924A241FFE8E00600DF4 /* ZipManyTests.swift */,
 				BFB4EA1424283ECF0096E9E9 /* CombineLatestManyTests.swift */,
 				DC1691112428228200B234C4 /* MapManyTests.swift */,
+				AAEAF0E42436C9C2007C35E0 /* WeakAssignTests.swift */,
 			);
 			path = Tests;
 			sourceTree = SOURCE_ROOT;
@@ -215,6 +220,7 @@
 				BF8121BB241FF42C006A93B8 /* ZipMany.swift */,
 				BFB4EA122428256B0096E9E9 /* CombineLatestMany.swift */,
 				DC16910E24281A1800B234C4 /* MapMany.swift */,
+				AAEAF0E22436C8A8007C35E0 /* WeakAssign.swift */,
 			);
 			path = Operators;
 			sourceTree = "<group>";
@@ -331,6 +337,7 @@
 				78C193E2241D596F0001B7FD /* Dematerialize.swift in Sources */,
 				78002BB7241E915E0018AA28 /* CurrentValueRelay.swift in Sources */,
 				78C193DE241D46F40001B7FD /* Materialize.swift in Sources */,
+				AAEAF0E32436C8A8007C35E0 /* WeakAssign.swift in Sources */,
 				78988A23241FFE2400F3A4AF /* Partition.swift in Sources */,
 				BF8121BC241FF42C006A93B8 /* ZipMany.swift in Sources */,
 				78C193D4241C2DE00001B7FD /* Create.swift in Sources */,
@@ -362,6 +369,7 @@
 				78C193E0241D4D8D0001B7FD /* MaterializeTests.swift in Sources */,
 				78002BBB241E97350018AA28 /* CurrentValueRelayTests.swift in Sources */,
 				78C193E4241D63620001B7FD /* DematerializeTests.swift in Sources */,
+				AAEAF0E52436C9C2007C35E0 /* WeakAssignTests.swift in Sources */,
 				BF50924B241FFE8E00600DF4 /* ZipManyTests.swift in Sources */,
 				78988A1E241EAFDD00F3A4AF /* PassthroughRelayTests.swift in Sources */,
 				DC1691122428228200B234C4 /* MapManyTests.swift in Sources */,

README.md

@@ -30,6 +30,7 @@ All operators, utilities and helpers respect Combine's publisher contract, inclu
 * [zip(with:) and Collection.zip](#ZipMany)
 * [combineLatest(with:) and Collection.combineLatest](#CombineLatestMany)
 * [mapMany(_:)](#MapMany)
+* [weaklyAssign(to:on:)](#weaklyAssign)
 
 ### Publishers
 * [AnyPublisher.create](#anypublisher.create)
@@ -362,6 +363,10 @@ intArrayPublisher.send([10, 2, 2, 4, 3, 8])
 ["10", "2", "2", "4", "3", "8"]
 ```
 
+### weaklyAssign
+
+Like Apple’s [`Publisher.assign(to:on)`](https://developer.apple.com/documentation/combine/publisher/3235801-assign), except it only `weak`ly captures `on` instead of `strong`ly.
+
 ## Publishers
 
 This section outlines some of the custom Combine publishers CombineExt provides

Sources/Operators/WeakAssign.swift

@@ -0,0 +1,25 @@
+//
+//  WeakAssign.swift
+//  CombineExt
+//
+//  Created by Jasdev Singh on 2/04/2020.
+//  Copyright © 2020 Combine Community. All rights reserved.
+//
+
+import Combine
+
+public extension Publisher where Failure == Never {
+    /// `weak`ly assigns onto an object at a specified key path.
+    ///
+    /// - Parameters:
+    ///   - keyPath: The key path to assign onto on the referenced object.
+    ///   - object: The object reference to assign onto.
+    ///
+    /// - Returns: A subscription cancellation token.
+    func weaklyAssign<Root: AnyObject>(to keyPath: ReferenceWritableKeyPath<Root, Output>,
+                                       on object: Root) -> AnyCancellable {
+        sink { [weak object] value in
+            object?[keyPath: keyPath] = value
+        }
+    }
+}

Tests/WeakAssignTests.swift

@@ -0,0 +1,46 @@
+//
+//  WeakAssignTests.swift
+//  CombineExtTests
+//
+//  Created by Jasdev Singh on 2/04/2020.
+//  Copyright © 2020 Combine Community. All rights reserved.
+//
+
+import XCTest
+import Combine
+import CombineExt
+
+final class WeakAssignTests: XCTestCase {
+    private var subscription: AnyCancellable!
+
+    func testWeakAssignment() {
+        let source = PassthroughSubject<Int, Never>()
+        var destination = Fake(property: 1)
+
+        XCTAssertEqual(destination.property, 1)
+
+        subscription = source
+            .assign(to: \.property, on: destination)
+
+        XCTAssertFalse(isKnownUniquelyReferenced(&destination))
+
+        subscription = source
+            .weaklyAssign(to: \.property, on: destination)
+
+        XCTAssertTrue(isKnownUniquelyReferenced(&destination))
+
+        source.send(2)
+        XCTAssertEqual(destination.property, 2)
+    }
+}
+
+// MARK: - Private Helpers
+
+private final class Fake<PropertyType> {
+    var property: PropertyType
+
+    init(property: PropertyType) {
+        self.property = property
+    }
+}
+