Add "Property" struct

Signals/Signal.swift

@@ -309,3 +309,38 @@ fileprivate func signalsAssert(_ condition: Bool, _ message: String) {
 #if DEBUG
 var assertionHandlerOverride:((_ condition: Bool, _ message: String) -> ())?
 #endif
+
+/**
+    A wrapper for a variable that allows observing its value changes in the `KVO` fashion
+    ### Usage Example: ###
+    ````
+     class MyClass {
+        // Custom Access Control for setter
+        private(set) var property = Property(value: 5)
+     }
+    ````
+ */
+public struct Property<T> {
+
+    /// The underlying signal.
+    public private(set) var signal: Signal<T>
+
+    /// The current value of the property.
+    public var value: T {
+        get { return signal.lastDataFired! }
+        set {
+            assert(Thread.isMainThread, "The property mutation must occur from the main thread because UI code might be observing changes")
+            signal.fire(newValue)
+        }
+    }
+
+    public init(value: T) {
+        signal = Signal<T>(retainLastData: true)
+        signal.fire(value)
+    }
+
+    /// Subscribes an observer to the Property and invokes its callback immediately with the current value
+    public func observe(with observer: AnyObject, callback: @escaping (T) -> Void) {
+        signal.subscribePast(with: observer, callback: callback)
+    }
+}

SignalsTests/SignalsTests.swift

@@ -407,3 +407,76 @@ class SignalsTests: XCTestCase {
         }
     }
 }
+
+class PropertyTests: XCTestCase {
+
+    func test_observe_shouldFireCallbackWithCurrentValue() {
+        let sut = Property(value: 10)
+        XCTAssertEqual(sut.value, 10)
+        let subscription = NSObject()
+        let expectation = XCTestExpectation(description: "Subscriber should be notified about the update")
+        sut.observe(with: subscription) { value in
+            XCTAssertEqual(value, 10)
+            expectation.fulfill()
+        }
+        wait(for: [expectation], timeout: 0.1)
+    }
+
+    func test_valueSetter_shouldChangeTheValueAndFireSubscriptionCallback() {
+        var sut = Property(value: 10)
+        XCTAssertEqual(sut.value, 10)
+        let subscription = NSObject()
+        let expectation = XCTestExpectation(description: "Subscriber should be notified about the update")
+        var dispatchCount = 0
+        sut.observe(with: subscription) { value in
+            dispatchCount += 1
+            if dispatchCount > 1 {
+                XCTAssertEqual(value, 5)
+                expectation.fulfill()
+            }
+        }
+        sut.value = 5
+        wait(for: [expectation], timeout: 0.1)
+    }
+
+    func test_automaticCancellation() {
+        var sut = Property(value: "abc")
+        var subscription: NSObject? = NSObject()
+        var dispatchCount = 0
+        sut.observe(with: subscription!) { value in
+            dispatchCount += 1
+            if dispatchCount > 1 {
+                XCTFail("Subscription should be cancelled")
+            }
+        }
+        subscription = nil
+        let expectation = XCTestExpectation(description: "Delay")
+        DispatchQueue.main.async {
+            sut.value = "def"
+            expectation.fulfill()
+        }
+        wait(for: [expectation], timeout: 0.1)
+    }
+
+    func test_manualCancellation() {
+        var sut = Property(value: false)
+        let subscription = NSObject()
+        var dispatchCount = 0
+        sut.observe(with: subscription) { value in
+            dispatchCount += 1
+            if dispatchCount > 1 {
+                XCTFail("Subscription should be cancelled")
+            }
+        }
+        sut.signal.cancelSubscription(for: subscription)
+        sut.value = true
+        sut.observe(with: subscription) { value in
+            dispatchCount += 1
+            if dispatchCount > 2 {
+                XCTFail("Subscription should be cancelled")
+            }
+        }
+        sut.signal.cancelAllSubscriptions()
+        sut.value = false
+    }
+}