webview: Add a 'watch' option to the build process

Use `yarn build-webview -w` or `yarn build-webview --watch`

The input files are watched for changes. If asset files change,
they are copied again to the asset folders. If the JavaScript file
changes, it is recompiled.

.eslintrc.yaml

@@ -119,4 +119,4 @@ rules:
       const, wildcard, reactotron, camelcase, hydrator, perf, stacktrace, Dir, fs,
       avoider, octicons, centerer, ldap, gravatar, identicon, blueimp, filename, wildcards,
       jpeg, jpg, tif, mov, notif, diag, bmp, viewport, scalable, polyfill, rect, touchstart,
-      touchend, touchmove, remoteuser, sso, submessages, nbsp, args, readdir]
+      touchend, touchmove, remoteuser, sso, submessages, nbsp, args, readdir, argv, stdout]

tools/build-webview.js

@@ -31,5 +31,35 @@ const compileJS = () => {
   fs.writeFileSync(`${outputIosFolder}zulip.js`, compiledCode);
 };
 
-copyResourceFiles();
-compileJS();
+const build = () => {
+  copyResourceFiles();
+  compileJS();
+};
+
+const watch = () => {
+  process.stdout.write('Watching...\n');
+
+  fs.watch(inputFolder, {}, (eventType, filename) => {
+    if (filename) {
+      process.stdout.write('Asset changed. Copying... ');
+      copyResourceFiles();
+      process.stdout.write('Done.\n');
+    }
+  });
+
+  fs.watch(sourceFilename, {}, (eventType, filename) => {
+    if (filename) {
+      process.stdout.write('Code changed. Compiling... ');
+      compileJS();
+      process.stdout.write('Done.\n');
+    }
+  });
+};
+
+const args = process.argv.slice(2);
+
+if (args.length === 0) {
+  build();
+} else if (args.indexOf('--watch') !== -1 || args.indexOf('-w') !== -1) {
+  watch();
+}