Add once for client events

SocketIO-MacTests/SocketSideEffectTest.swift

@@ -83,6 +83,22 @@ class SocketSideEffectTest: XCTestCase {
         waitForExpectations(timeout: 3, handler: nil)
     }
 
+    func testHandleOnceClientEvent() {
+        let expect = expectation(description: "handled event")
+
+        socket.once(clientEvent: .connect) {data, ack in
+            XCTAssertEqual(self.socket.testHandlers.count, 0)
+            expect.fulfill()
+        }
+
+        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 0.1) {
+            // Fake connecting
+            self.socket.parseEngineMessage("0/")
+        }
+
+        waitForExpectations(timeout: 3, handler: nil)
+    }
+
     func testOffWithEvent() {
         socket.on("test") {data, ack in }
         socket.on("test") {data, ack in }

Source/SocketIOClient.swift

@@ -508,6 +508,16 @@ open class SocketIOClient : NSObject, SocketIOClientSpec, SocketEngineClient, So
         return handler.id
     }
 
+    /// Adds a single-use handler for a client event.
+    ///
+    /// - parameter clientEvent: The event for this handler.
+    /// - parameter callback: The callback that will execute when this event is received.
+    /// - returns: A unique id for the handler that can be used to remove it.
+    @discardableResult
+    open func once(clientEvent event: SocketClientEvent, callback: @escaping NormalCallback) -> UUID {
+        return once(event.rawValue, callback: callback)
+    }
+
     /// Adds a single-use handler for an event.
     ///
     /// - parameter event: The event name for this handler.