feat: Swift error strategy (#721)

* Release flipt-client-swift-v0.1.0

Signed-off-by: Mark Phelps <209477+markphelps@users.noreply.github.com>

* feat(swift): add error strategy support to swift sdk

Signed-off-by: Mark Phelps <209477+markphelps@users.noreply.github.com>

---------

Signed-off-by: Mark Phelps <209477+markphelps@users.noreply.github.com>

Author: markphelps

README.md

@@ -12,7 +12,7 @@ Add the following to your `Package.swift` file:
 
 ```swift
 dependencies: [
-    .package(url: "https://github.com/flipt-io/flipt-client-swift.git", from: "0.0.x")
+    .package(url: "https://github.com/flipt-io/flipt-client-swift.git", from: "1.x.x")
 ]
 ```
 
@@ -86,6 +86,25 @@ defer {
 }
 ```
 
+### Client Options
+
+The `FliptClient` initializer accepts several options that can be used to configure the client. The available options are:
+
+- `namespace`: The namespace to fetch flag state from. If not provided, the client will default to the `default` namespace.
+- `url`: The URL of the upstream Flipt instance. If not provided, the client will default to `http://localhost:8080`.
+- `updateInterval`: The interval (in seconds) in which to fetch new flag state. If not provided, the client will default to 120 seconds.
+- `authentication`: The authentication strategy to use when communicating with the upstream Flipt instance. If not provided, the client will default to no authentication. See the [Authentication](#authentication) section for more information.
+- `reference`: The [reference](https://docs.flipt.io/guides/user/using-references) to use when fetching flag state. If not provided, reference will not be used.
+- `fetchMode`: The fetch mode to use when fetching flag state. If not provided, the client will default to polling.
+- `errorStrategy`: The error strategy to use when fetching flag state. If not provided, the client will default to `fail`. See the [Error Strategies](#error-strategies) section for more information.
+
+### Error Strategies
+
+The `FliptClient` supports the following error strategies:
+
+- `fail`: The client will throw an error if the flag state cannot be fetched. This is the default behavior.
+- `fallback`: The client will maintain the last known good state and use that state for evaluation in case of an error.
+
 ### Authentication
 
 The `FliptClient` supports the following authentication strategies:

Sources/FliptClient/FliptClient.swift

@@ -9,26 +9,30 @@ public class FliptClient {
     private var ref: String = ""
     private var updateInterval: Int = 0
     private var fetchMode: FetchMode = .polling
+    private var errorStrategy: ErrorStrategy = .fail
 
     public init(namespace: String = "default",
          url: String = "",
          authentication: Authentication? = nil,
          ref: String = "",
          updateInterval: Int = 120,
-         fetchMode: FetchMode = .polling) throws {
+         fetchMode: FetchMode = .polling,
+         errorStrategy: ErrorStrategy = .fail) throws {
         self.namespace = namespace
         self.url = url
         self.authentication = authentication
         self.ref = ref
         self.updateInterval = updateInterval
         self.fetchMode = fetchMode
+        self.errorStrategy = errorStrategy
 
         let clientOptions = ClientOptions(
             url: url,
             authentication: authentication,
             updateInterval: updateInterval,
             reference: ref,
-            fetchMode: fetchMode
+            fetchMode: fetchMode,
+            errorStrategy: errorStrategy
         )
 
         guard let jsonData = try? JSONEncoder().encode(clientOptions) else {
@@ -240,12 +244,18 @@ public enum FetchMode: String, Codable {
     case polling
 }
 
+public enum ErrorStrategy: String, Codable {
+    case fail
+    case fallback
+}
+
 public struct ClientOptions<T: Encodable>: Encodable {
     public let url: String
     public let authentication: T?
     public let updateInterval: Int
     public let reference: String
     public let fetchMode: FetchMode
+    public let errorStrategy: ErrorStrategy
 }
 
 public struct Flag: Codable {