add configurable exponential backoff

Source/SocketIO/Client/SocketIOClientOption.swift

@@ -75,8 +75,14 @@ public enum SocketIOClientOption : ClientOption {
     /// The number of times to try and reconnect before giving up. Pass `-1` to [never give up](https://www.youtube.com/watch?v=dQw4w9WgXcQ).
     case reconnectAttempts(Int)
 
-    /// The number of seconds to wait before reconnect attempts.
+    /// The minimum number of seconds to wait before reconnect attempts.
     case reconnectWait(Int)
+
+    /// The maximum number of seconds to wait before reconnect attempts.
+    case reconnectWaitMax(Int)
+
+    /// The randomization factor for calculating reconnect jitter.
+    case randomizationFactor(Double)
 
     /// Set `true` if your server is using secure transports.
     case secure(Bool)
@@ -125,6 +131,10 @@ public enum SocketIOClientOption : ClientOption {
             description = "reconnectAttempts"
         case .reconnectWait:
             description = "reconnectWait"
+        case .reconnectWaitMax:
+            description = "reconnectWaitMax"
+        case .randomizationFactor:
+            description = "randomizationFactor"
         case .secure:
             description = "secure"
         case .selfSigned:
@@ -170,6 +180,10 @@ public enum SocketIOClientOption : ClientOption {
             value = attempts
         case let .reconnectWait(wait):
             value = wait
+        case let .reconnectWaitMax(wait):
+            value = wait
+        case let .randomizationFactor(factor):
+            value = factor
         case let .secure(secure):
             value = secure
         case let .security(security):

Source/SocketIO/Manager/SocketManager.swift

@@ -97,9 +97,15 @@ open class SocketManager : NSObject, SocketManagerSpec, SocketParsable, SocketDa
     /// If `true`, this client will try and reconnect on any disconnects.
     public var reconnects = true
 
-    /// The number of seconds to wait before attempting to reconnect.
+    /// The minimum number of seconds to wait before attempting to reconnect.
     public var reconnectWait = 10
 
+    /// The maximum number of seconds to wait before attempting to reconnect.
+    public var reconnectWaitMax = 30
+
+    /// The randomization factor for calculating reconnect jitter.
+    public var randomizationFactor = 0.5
+
     /// The status of this manager.
     public private(set) var status: SocketIOStatus = .notConnected {
         didSet {
@@ -474,7 +480,21 @@ open class SocketManager : NSObject, SocketManagerSpec, SocketParsable, SocketDa
         currentReconnectAttempt += 1
         connect()
 
-        handleQueue.asyncAfter(deadline: DispatchTime.now() + Double(reconnectWait), execute: _tryReconnect)
+        let interval = reconnectInterval(attempts: currentReconnectAttempt)
+        DefaultSocketLogger.Logger.log("Scheduling reconnect in \(interval)s", type: SocketManager.logType)
+        handleQueue.asyncAfter(deadline: DispatchTime.now() + interval, execute: _tryReconnect)
+    }
+
+    func reconnectInterval(attempts: Int) -> Double {
+        // apply exponential factor
+        let backoffFactor = pow(1.5, attempts)
+        let interval = Double(reconnectWait) * Double(truncating: backoffFactor as NSNumber)
+        // add in a random factor smooth thundering herds
+        let rand = Double.random(in: 0 ..< 1)
+        let randomFactor = rand * randomizationFactor * Double(truncating: interval as NSNumber)
+        // add in random factor, and clamp to min and max values
+        let combined = interval + randomFactor
+        return Double(fmax(Double(reconnectWait), fmin(combined, Double(reconnectWaitMax))))
     }
 
     /// Sets manager specific configs.

Source/SocketIO/Manager/SocketManagerSpec.swift

@@ -69,8 +69,14 @@ public protocol SocketManagerSpec : AnyObject, SocketEngineClient {
     /// If `true`, this manager will try and reconnect on any disconnects.
     var reconnects: Bool { get set }
 
-    /// The number of seconds to wait before attempting to reconnect.
+    /// The minimum number of seconds to wait before attempting to reconnect.
     var reconnectWait: Int { get set }
+
+    /// The maximum number of seconds to wait before attempting to reconnect.
+    var reconnectWaitMax: Int { get set }
+
+    /// The randomization factor for calculating reconnect jitter.
+    var randomizationFactor: Double { get set }
 
     /// The URL of the socket.io server.
     var socketURL: URL { get }

Tests/TestSocketIO/SocketMangerTest.swift

@@ -15,6 +15,8 @@ class SocketMangerTest : XCTestCase {
         XCTAssertEqual(manager.handleQueue, DispatchQueue.main)
         XCTAssertTrue(manager.reconnects)
         XCTAssertEqual(manager.reconnectWait, 10)
+        XCTAssertEqual(manager.reconnectWaitMax, 30)
+        XCTAssertEqual(manager.randomizationFactor, 0.5)
         XCTAssertEqual(manager.status, .notConnected)
     }
 
@@ -27,6 +29,21 @@ class SocketMangerTest : XCTestCase {
 
         XCTAssertEqual(manager.config.first!, .secure(true))
     }
+
+    func testBackoffIntervalCalulation() {
+        XCTAssertLessThanOrEqual(manager.reconnectInterval(attempts: -1), Double(manager.reconnectWaitMax))
+        XCTAssertLessThanOrEqual(manager.reconnectInterval(attempts: 0), 15)
+        XCTAssertLessThanOrEqual(manager.reconnectInterval(attempts: 1), 22.5)
+        XCTAssertLessThanOrEqual(manager.reconnectInterval(attempts: 2), 33.75)
+        XCTAssertLessThanOrEqual(manager.reconnectInterval(attempts: 50), Double(manager.reconnectWaitMax))
+        XCTAssertLessThanOrEqual(manager.reconnectInterval(attempts: 10000), Double(manager.reconnectWaitMax))
+
+        XCTAssertGreaterThanOrEqual(manager.reconnectInterval(attempts: -1), Double(manager.reconnectWait))
+        XCTAssertGreaterThanOrEqual(manager.reconnectInterval(attempts: 0), Double(manager.reconnectWait))
+        XCTAssertGreaterThanOrEqual(manager.reconnectInterval(attempts: 1), 15)
+        XCTAssertGreaterThanOrEqual(manager.reconnectInterval(attempts: 2), 22.5)
+        XCTAssertGreaterThanOrEqual(manager.reconnectInterval(attempts: 10000), Double(manager.reconnectWait))
+    }
 
     func testManagerCallsConnect() {
         setUpSockets()