Detect if swiftly is already installed

Author: roulpriya

src/toolchain/swiftly.ts

@@ -158,7 +158,7 @@ export class Swiftly {
         }
     }
 
-    private static isSupported() {
+    public static isSupported() {
         return process.platform === "linux" || process.platform === "darwin";
     }
 
@@ -245,4 +245,22 @@ export class Swiftly {
         );
         return JSON.parse(swiftlyConfigRaw);
     }
+
+    public static async isInstalled() {
+
+        if(!Swiftly.isSupported()) {
+            return false;
+        }
+
+        try {
+            await Swiftly.version();
+            return true;
+        } catch (error) {
+            if (error instanceof ExecFileError && 'code' in error && error.code === "ENOENT") {
+                return false;
+            }
+            throw error;
+        }
+
+    }
 }

src/ui/ToolchainSelection.ts

@@ -257,7 +257,7 @@ async function getQuickPickItems(
     }
     // Various actions that the user can perform (e.g. to install new toolchains)
     const actionItems: ActionItem[] = [];
-    if (process.platform === "linux" || process.platform === "darwin") {
+    if (Swiftly.isSupported() && !(await Swiftly.isInstalled())) {
         const platformName = process.platform === "linux" ? "Linux" : "macOS";
         actionItems.push({
             type: "action",