Added withPrevious

Author: RuiAAPeres

Receiver/Sources/Receiver+Operators.swift

@@ -19,6 +19,23 @@ extension Receiver {
 
         return receiver
     }
+
+    func withPrevious() -> Receiver<(Wave?, Wave)> {
+        let (transmitter, receiver) = Receiver<(Wave?, Wave)>.make()
+        let values = Atomic<[Wave]>([])
+
+        self.listen { newValue in
+            values.apply { _values in
+
+                let previous = _values.last
+                _values.append(newValue)
+
+                transmitter.broadcast((previous, newValue))
+            }
+        }
+
+        return receiver
+    }
 }
 
 extension Receiver where Wave: Equatable {

ReceiverTests/ReceiverTests+Operators.swift

@@ -53,4 +53,26 @@ class ReceiverTests_Operators: XCTestCase {
 
         XCTAssertTrue(called == 5)
     }
+
+    func test_withPrevious_nil() {
+        let (transmitter, receiver) = Receiver<Int>.make()
+        let newReceiver = receiver.withPrevious()
+        var called = 0
+        var expected: (Int?, Int) = (0, 0)
+
+        newReceiver.listen { wave in
+            expected = wave
+            called = called + 1
+        }
+
+        transmitter.broadcast(1)
+        XCTAssertTrue(expected.0 == nil)
+        XCTAssertTrue(expected.1 == 1)
+
+        transmitter.broadcast(2)
+        XCTAssertTrue(expected.0 == 1)
+        XCTAssertTrue(expected.1 == 2)
+
+        XCTAssertTrue(called == 2)
+    }
 }