Try to add initial macOS setup

Author: fwal

src/main.ts

@@ -1,9 +1,8 @@
 import { EOL } from "os";
 import { equalVersions, getOS } from "./core";
-import { installSwift, setupLinux } from "./swiftly";
+import { installSwift, setupLinux, setupMacOS } from "./swiftly";
 import { currentVersion } from "./swift";
 import { error, getInput, info, setFailed, setOutput } from "@actions/core";
-import { coerce, eq } from "semver";
 
 /**
  * Main entry point for the action
@@ -24,7 +23,8 @@ async function run() {
     // Setup Swiftly on the runner
     switch (os) {
       case "darwin":
-        throw Error("Not implemented yet on macOS");
+        await setupMacOS();
+        break;
       case "linux":
         await setupLinux({ skipVerifySignature: true });
         break;

src/swiftly/installers/index.ts

@@ -1 +1,2 @@
 export * from "./linux";
+export * from "./macos";

src/swiftly/installers/macos.ts

@@ -0,0 +1,25 @@
+import { downloadTool, find } from "@actions/tool-cache";
+import { cmd } from "../../core";
+import { addPath, debug } from "@actions/core";
+
+export async function setupMacOS() {
+  let path = find("swiftly", "1.0.0");
+
+  if (!path) {
+    path = await download();
+  } else {
+    debug("Found cached Swiftly");
+  }
+
+  addPath(path);
+
+  debug(`Added Swiftly to PATH: ${path}`);
+}
+
+async function download() {
+  const pkg = await downloadTool(
+    "https://download.swift.org/swiftly/darwin/swiftly.pkg",
+  );
+  await cmd("installer", "-pkg", pkg, "-target", "CurrentUserHomeDirectory");
+  return "~/.swiftly/bin/swiftly";
+}