Add multicast latest (#27)

Author: vdudchak

Sources/CombineExtensions/MulticastLatest.swift

@@ -0,0 +1,10 @@
+import Combine
+
+public extension Publisher {
+    func multicastLatest() -> some Publisher<Output, Failure> {
+        map(Optional.some)
+            .multicast({ CurrentValueSubject<Output?, Failure>(nil) })
+            .autoconnect()
+            .compactMap { $0 }
+    }
+}

Tests/CombineExtensionsTests/MulticastLatestTests.swift

@@ -0,0 +1,43 @@
+import Combine
+import CombineExtensions
+import CombineTestExtensions
+import XCTest
+
+class MulticastLatestSubjectTests: XCTestCase {
+    func testMulticastLatest() {
+        let voidSubject = PassthroughSubject<Void, Never>()
+        let intSubject = PassthroughSubject<Int, Never>()
+        var mapCalledCount = 0
+        
+        let multicastedPublisher = voidSubject
+            .map {
+                mapCalledCount += 1
+                return intSubject
+            }
+            .switchToLatest()
+            .multicastLatest()
+        
+        let outputExpectation = multicastedPublisher.expectOutput([1, 2])
+
+        voidSubject.send(())
+        intSubject.send(1)
+        intSubject.send(2)
+        
+        let expectation1 = expectation(description: "first subscriber")
+        let subscription1 = multicastedPublisher.sink { value in
+            XCTAssertEqual(value, 2)
+            expectation1.fulfill()
+        }
+        defer { subscription1.cancel() }
+        
+        let expectation2 = expectation(description: "second subscriber")
+        let subscription2 = multicastedPublisher.sink { value in
+            XCTAssertEqual(value, 2)
+            expectation2.fulfill()
+        }
+        defer { subscription2.cancel() }
+        
+        wait(for: [outputExpectation, expectation1, expectation2], timeout: 0.5)
+        XCTAssertEqual(mapCalledCount, 1)
+    }
+}