[GH-314] Persist user settings in native Linux app

This commit adds persistence for the user settings in the native Linux
app. The settings are written to $XDG_CONFIG_HOME/focalboard/settings.
If XDG_CONFIG_HOME is unset, the app falls back to $HOME/.config.

The change hooks into the existing settings export already used in the
nativve macOS app which means the settings are persisted immediately on
change.

Unfortunately, the golang webview package uses a custom native binding
technology and doesn't allow to define WebKit message handlers. As a
result, a dedicated message handler function was added to the existing
NativeApp object. In the native macOS app, said handler is
short-circuited to window.webkit.messagehandlers.[NAME].postMessage.
This has the benefit that the web app remains agnostic of the particular
native binding mechanism.

Relates to: #314

linux/main.go

@@ -1,9 +1,12 @@
 package main
 
 import (
+	"encoding/base64"
 	"fmt"
+	"io/ioutil"
 	"log"
 	"net"
+	"os"
 	"os/exec"
 	"runtime"
 
@@ -16,6 +19,7 @@ import (
 )
 
 var sessionToken string = "su-" + uuid.New().String()
+var settingsDir, settingsFile string = getSettingsPaths()
 
 func getFreePort() (int, error) {
 	addr, err := net.ResolveTCPAddr("tcp", "localhost:0")
@@ -101,6 +105,86 @@ func openBrowser(url string) {
 	}
 }
 
+func getSettingsPaths() (dir string, file string) {	
+	dir = fmt.Sprintf("%s/focalboard", getSettingsRoot())
+	file = fmt.Sprintf("%s/settings", dir)
+	log.Printf("Using settings file %v", file)
+	return
+}
+
+func getSettingsRoot() string {
+	xdgConfigHome := os.Getenv("XDG_CONFIG_HOME")
+	if len(xdgConfigHome) != 0 {
+		return xdgConfigHome
+	}
+
+	log.Println("XDG_CONFIG_HOME is not set, falling back to $HOME/.config")
+
+	home := os.Getenv("HOME")
+	if len(home) != 0 {
+		return fmt.Sprintf("%s/.config", home)
+	}
+
+	log.Fatal("HOME is not set, cannot store settings")
+	return ""
+}
+
+func loadSettings() string {
+	bytes, err := ioutil.ReadFile(settingsFile)
+	if err != nil {
+		log.Printf("Could not load user settings: %v", err)
+		return ""
+	}
+	return string(bytes)
+}
+
+func saveSettings(blob string) (err error) {
+	if _, statErr := os.Stat(settingsDir); os.IsNotExist(statErr) { 
+		err = os.MkdirAll(settingsDir, 0700)
+	}
+	if err == nil {
+		err = ioutil.WriteFile(settingsFile, []byte(blob), 0600)
+	}
+	return
+}
+
+func receiveMessage(msg map[string]interface{}) {
+	msgType, ok := msg["type"].(string)
+	if !ok {
+		log.Printf("Received unexpected script message, no value for key 'type': %v ", msg)
+		return
+	}
+
+	blob, ok := msg["settingsBlob"].(string)
+	if !ok {
+		log.Println("Received unexpected script message, no value for key 'settingsBlob': %v", msg)
+		return
+	}
+
+	log.Printf("Received message %v", msgType)
+
+	switch msgType {
+	case "didImportUserSettings":
+		log.Printf("Imported user settings keys %v", msg["keys"])
+	case "didNotImportUserSettings":
+		break
+	case "didChangeUserSettings":
+		err := saveSettings(blob)
+		if err == nil {
+			log.Printf("Persisted user settings after change for key %v", msg["key"])
+		} else {
+			log.Printf("Could not persist user settings: %v", err)
+		}
+	default:
+		log.Printf("Received script message of unknown type %v", msgType)
+	}
+
+	data, err := base64.StdEncoding.DecodeString(blob)
+	if err == nil {
+		log.Printf("Current user settings: %v", string(data))
+	}
+}
+
 func main() {
 	debug := true
 	w := webview.New(debug)
@@ -119,8 +203,13 @@ func main() {
 	w.SetTitle("Focalboard")
 	w.SetSize(1024, 768, webview.HintNone)
 
-	script := fmt.Sprintf("localStorage.setItem('focalboardSessionId', '%s');", sessionToken)
-	w.Init(script)
+	sessionTokenScript := fmt.Sprintf("localStorage.setItem('focalboardSessionId', '%s');", sessionToken)
+	w.Init(sessionTokenScript)
+
+	w.Bind("receiveMessage", receiveMessage)
+
+	userSettingsScript := fmt.Sprintf("const NativeApp = { settingsBlob: \"%s\", receiveMessage: receiveMessage };", loadSettings())
+	w.Init(userSettingsScript)
 
 	w.Navigate(fmt.Sprintf("http://localhost:%d", port))
 	w.Bind("openInNewBrowser", openBrowser)

mac/Focalboard/ViewController.swift

@@ -4,6 +4,8 @@
 import Cocoa
 import WebKit
 
+private let messageHandlerName = "nativeApp"
+
 class ViewController:
 	NSViewController,
 	WKUIDelegate,
@@ -21,7 +23,7 @@ class ViewController:
 		webView.navigationDelegate = self
 		webView.uiDelegate = self
 		webView.isHidden = true
-		webView.configuration.userContentController.add(self, name: "nativeApp")
+		webView.configuration.userContentController.add(self, name: messageHandlerName)
 
 		clearWebViewCache()
 
@@ -81,7 +83,12 @@ class ViewController:
 		)
 		let blob = UserDefaults.standard.string(forKey: "localStorage") ?? ""
 		let userSettingsScript = WKUserScript(
-			source: "const NativeApp = { settingsBlob: \"\(blob)\" };",
+			source: """
+				const NativeApp = {
+					settingsBlob: \"\(blob)\",
+					receiveMessage: msg => window.webkit.messageHandlers.\(messageHandlerName).postMessage(msg)
+				};
+				""",
 			injectionTime: .atDocumentStart,
 			forMainFrameOnly: true
 		)

webapp/src/nativeApp.ts

@@ -5,6 +5,7 @@ import {exportUserSettingsBlob, importUserSettingsBlob} from './userSettings'
 
 declare interface INativeApp {
     settingsBlob: string | null;
+    receiveMessage: (msg: any) => void | null
 }
 
 declare const NativeApp: INativeApp
@@ -15,14 +16,17 @@ export function importNativeAppSettings(): void {
     }
     const importedKeys = importUserSettingsBlob(NativeApp.settingsBlob)
     const messageType = importedKeys.length ? 'didImportUserSettings' : 'didNotImportUserSettings'
-    postWebKitMessage({type: messageType, settingsBlob: exportUserSettingsBlob(), keys: importedKeys})
+    postMessage({type: messageType, settingsBlob: exportUserSettingsBlob(), keys: importedKeys})
     NativeApp.settingsBlob = null
 }
 
 export function notifySettingsChanged(key: string): void {
-    postWebKitMessage({type: 'didChangeUserSettings', settingsBlob: exportUserSettingsBlob(), key})
+    postMessage({type: 'didChangeUserSettings', settingsBlob: exportUserSettingsBlob(), key})
 }
 
-function postWebKitMessage(message: any) {
-    (window as any).webkit?.messageHandlers.nativeApp?.postMessage(message)
+function postMessage(message: any) {
+    if (typeof NativeApp === 'undefined' || !NativeApp.receiveMessage) {
+        return
+    }
+    NativeApp.receiveMessage(message)
 }