Add yarn install state

app/classes/state/node/YarnInstallState.js

@@ -0,0 +1,43 @@
+const State = require('../State')
+
+class YarnInstallState extends State {
+    #ran_once = false
+
+    static get services() {
+        return [...super.services, 'output']
+    }
+
+    async apply() {
+        const cmd = `cd "${this._config.path}" && yarn install`
+        await this._host.run(cmd)
+        this.#ran_once = true
+    }
+
+    async check() {
+        if ( this._config.force ) return this.#ran_once
+        else {
+            const node_modules = this._host.get_path(`${this._config.path}/node_modules`)
+            await node_modules.classify()
+            return node_modules.is_directory()
+        }
+    }
+
+    async reverse() {
+        const node_modules = this._host.get_path(`${this._config.path}/node_modules`)
+        await node_modules.unlink()
+    }
+
+    failure_message() {
+        return `The Yarn packages at ${this._config.path} need to be installed.`
+    }
+
+    check_message() {
+        return this.failure_message()
+    }
+
+    display() {
+        return `Ensure that the Yarn packages at ${this._config.path} are installed...`
+    }
+}
+
+module.exports = exports = YarnInstallState

app/services/states.service.js

@@ -41,6 +41,8 @@ class StatesService extends Service {
         'service.daemon.reloaded': require('../classes/state/os/ServiceDaemonReloadState'),
 
         'web.download': require('../classes/state/fs/DownloadState'),
+
+        'node.yarn.installed': require('../classes/state/node/YarnInstallState'),
     }
 
     static get services() {