lorentey · July 6, 2018 09:52 · Oct 21, 2016 · Oct 21, 2016
diff --git a/Zero-allocation observers with Generalized Existentials.swift b/Zero-allocation observers with Generalized Existentials.swift
@@ -40,9 +40,8 @@ extension Source {
     }
 }
 
-struct ClosureSink<S: Source>: Sink {
-    let source: S
-    let sink: (S.Value) -> Void
+struct ClosureSink<Value>: Sink {
+    let sink: (Value) -> Void
 
     func receive(_ value: Value) {
         sink(value)
@@ -55,8 +54,7 @@ class Foo {
     }
 }
 
-struct FooSink<S: Source where S.Value == String>: Sink {
-    let source: S
+struct FooSink: Sink {
     let target: Foo
     let context: Int
     func receive(_ value: String) { target.receive(value, with: context) }

diff --git a/Zero-allocation observers with Generalized Existentials.swift b/Zero-allocation observers with Generalized Existentials.swift
@@ -0,0 +1,79 @@
+protocol Sink {
+    associatedtype Value
+    func receive(_ value: Value)
+}
+
+// This assumes Generalized Existentials are a thing
+// https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#generalized-existentials
+typealias AnySink<Value> = Any<Sink where .Value == Value>
+
+protocol Source {
+    associatedtype Value
+
+    func subscribe(_ sink: Sink) -> Int
+    func unsubscribe(_ token: Int)
+}
+
+class Signal<Value>: Source {
+    private var _sinks: [Int: AnySink<Value>] = [:]
+    private var _nextKey = 0
+
+    func subscribe(_ sink: Sink) -> Int {
+        let key = _nextKey
+        _sinks[key] = sink
+        _nextKey += 1
+        return key
+    }
+
+    func unsubscribe(_ key: Int) {
+        _sinks[key] = nil
+    }
+
+    func send(_ value: Value) {
+        _sinks.forEach { _, sink in sink.receive(value) }
+    }
+}
+
+extension Source {
+    func subscribe(_ sink: @escaping (Value) -> Void) -> Int {
+        return subscribe(ClosureSink(source: self, sink: sink))
+    }
+}
+
+struct ClosureSink<S: Source>: Sink {
+    let source: S
+    let sink: (S.Value) -> Void
+
+    func receive(_ value: Value) {
+        sink(value)
+    }
+}
+
+class Foo {
+    func receive(_ value: String, with context: Int) {
+        print("\(value) from Foo with context \(context)")
+    }
+}
+
+struct FooSink<S: Source where S.Value == String>: Sink {
+    let source: S
+    let target: Foo
+    let context: Int
+    func receive(_ value: String) { target.receive(value, with: context) }
+}
+
+let foo = Foo()
+
+let signal = Signal<String>()
+
+let k1 = signal.subscribe { print("\($0) from closure observer") }
+
+// Note that FooSink fits in 3 words, so AnySink probably won't allocate a box for it.
+let k2 = signal.subscribe(FooSink(source: signal, target: foo, context: 42))
+
+signal.send("Hello")
+// => Hello from closure observer
+// => Hello from Foo with context 42
+
+signal.unsubscribe(k1)
+signal.unsubscribe(k2)