Add sample code for Swift Package Index playground

.arena-sample.swift

@@ -0,0 +1,111 @@
+/*:
+ # WriteFreely Swift Package: Playground Sample
+ 
+ * Note:
+    To view this Playground as documentation, go to **Editor** > **Show Rendered Markup**.
+ */
+import Foundation
+import WriteFreely
+
+/*:
+ * Important:
+    To test the WriteFreely package in this Playground, you'll need to set up an account on an open WriteFreely instance
+    first. You can find **Open** instances at [writefreely.org/instances](https://writefreely.org/instances), or create
+    a free account on [Write.as](https://write.as). Once that's done, supply your `username`, `password`, and `instance`
+    to the constants below.
+ */
+let username: String = "hello"// "username"
+let password: String = "hi"// "password"
+let instance: String = "https://pencil.writefree.ly" //"https://write.as"
+
+/*:
+ * callout(The Plan):
+    Next, let's define what we want the app to do:
+    1. Log in to your account on the WriteFreely instance,
+    2. Publish an anonymous/draft post, and
+    3. Log out.
+ */
+/*:
+ * callout(The WFClient):
+    Most of what you'll use to interact with the WriteFreely instance are functions vended by the **WFClient**. Let's
+    start by initializing it, then log in. Most WFClient functions have a signature like this:
+  
+    `func methodName(parameter: Type, completion: @escaping (Result<WFType, Error>) -> Void)`
+  
+    For example, check out the `login` method:
+  
+    `func login(username: String, password: String, completion: @escaping (Result<WFUser, Error>) -> Void)`
+ */
+guard let instanceURL = URL(string: instance) else { fatalError() }
+let client = WFClient(for: instanceURL)
+client.login(username: username, password: password, completion: loginHandler)
+
+/*:
+ * callout(The Completion Handlers):
+    You might have noticed above that when logging in, we're passing a `loginHandler` to the `completion:` parameter.
+    As the function signature shows, this is a closure that takes a `Result<WFUser, Error>` parameter and returns
+    nothing. That means that once the WFClient has finished with the login attempt, it'll call this closure, which you
+    write to handle both the case of a successful login attempt, or any error, like so:
+ */
+func loginHandler(result: Result<WFUser, Error>) {
+    switch result {
+    
+    /// If the login attempt was successful, get the returned `WFUser` from the `Result`, print a greeting to the
+    /// console, and then call the `createAndPublishSamplePost()` function.
+    case .success(let user):
+        print("Hi there, \(user.username ?? "anonymous user")!")
+        createAndPublishSamplePost()
+
+    /// Otherwise, if the login attempt failed, get the returned `Error`, and print it to the console.
+    case .failure(let error):
+        print("Oops! \(error)")
+    }
+}
+
+/*:
+ * callout(Publishing A Post):
+    When the `loginHander(result:)` function above is called for a successful login attempt, it calls the function below
+    to create and publish an draft post to your WriteFreely instance. We start by creating a very simple `WFPost` object
+    with a `title` and a `body`, and then pass it in to the `createPost(post:completion:)` function. In this case, we
+    write the completion block inline with the function.
+ */
+func createAndPublishSamplePost() {
+    // Prepare a draft post for publishing.
+    let draft = WFPost(
+        body: "This is a sample post created from the Swift Package playground!",
+        title: "My First Draft!"
+    )
+
+    // Then, we attempt to publish the post!
+    client.createPost(post: draft, completion: { result in
+        switch result {
+
+        /// If the post is published successfully, get the returned `WFPost`, print a success message to the console
+        /// with a link to the draft, and then call the `logout()` function.
+        case .success(let post):
+            print("See your post live, here: \(instance)/\(post.postId ?? "")")
+            logout()
+
+        /// Otherwise, if the publishing attempt failed, get the returned `Error`, and print it to the console.
+        case .failure(let error): print("Oops! \(error)")
+        }
+    })
+}
+
+/*:
+ * callout(Logging Out)
+    This is very important! When you're done with the session, make sure to call the `logout(completion:)` function to
+    make sure that the user's access token is invalidated by the server.
+ */
+func logout() {
+    client.logout(completion: { result in
+        switch result {
+
+        /// If the logout attempt is successful, print a success message to the console.
+        case .success(_): print("Logged out!")
+
+        /// Otherwise, if the logout attempt failed, get the returned `Error`, and print it to the console.
+        case .failure(let error): print("Oops! \(error)")
+        }
+    })
+}