From 9167398309e916b7be01d652fef3684fd28951de Mon Sep 17 00:00:00 2001
From: Vishesh Handa <me@vhanda.in>
Date: Tue, 19 May 2020 19:30:32 +0200
Subject: [PATCH] KatexWidget: Allow multiple widgets to exist

Since there can be only one instance of the WebView, we need to lock in
between render calls to the webview.

Related to #125
---
 lib/widgets/katex_widget.dart | 57 +++++++++++++++++++++++++++++------
 pubspec.lock                  |  7 +++++
 pubspec.yaml                  |  1 +
 3 files changed, 55 insertions(+), 10 deletions(-)

diff --git a/lib/widgets/katex_widget.dart b/lib/widgets/katex_widget.dart
index 0ee8d478..3c4cf0c9 100644
--- a/lib/widgets/katex_widget.dart
+++ b/lib/widgets/katex_widget.dart
@@ -1,9 +1,11 @@
+import 'dart:async';
 import 'dart:io';
 
 import 'package:flutter/material.dart';
 
 import 'package:flutter_webview_plugin/flutter_webview_plugin.dart';
 import 'package:path/path.dart' as p;
+import 'package:mutex/mutex.dart';
 
 class KatexWidget extends StatefulWidget {
   final String input;
@@ -15,10 +17,13 @@ class KatexWidget extends StatefulWidget {
 }
 
 class _KatexWidgetState extends State<KatexWidget> {
+  static final _globalMutex = Mutex();
+  static var flutterWebViewPlugin = FlutterWebviewPlugin();
+
   String imagePath;
   JavascriptChannel jsChannel;
+  StreamSubscription<WebViewStateChanged> _onStateChanged;
 
-  final flutterWebViewPlugin = FlutterWebviewPlugin();
   final selectedUrl = 'https://gitjournal.io/test_katex.html';
 
   @override
@@ -32,17 +37,46 @@ class _KatexWidgetState extends State<KatexWidget> {
         print(message.message);
 
         var uri = UriData.parse(message.message);
-        var tmpFile = p.join(Directory.systemTemp.path, "katex.png");
+
+        String tmpFile;
+        var num = 0;
+        while (true) {
+          tmpFile = p.join(Directory.systemTemp.path, "katex_$num.png");
+          if (!File(tmpFile).existsSync()) {
+            break;
+          }
+          num += 1;
+        }
         File(tmpFile).writeAsBytesSync(uri.contentAsBytes());
 
-        setState(() {
-          print("State has been set");
-          imagePath = tmpFile;
-        });
+        if (mounted) {
+          setState(() {
+            print("State has been set $tmpFile");
+            imagePath = tmpFile;
+          });
+        }
+
+        flutterWebViewPlugin.close();
+        _onStateChanged.cancel();
+        _onStateChanged = null;
+
+        print("Releasing Katex mutex lock ${widget.input}");
+        _globalMutex.release();
       },
     );
 
-    flutterWebViewPlugin.onStateChanged.listen((WebViewStateChanged state) {
+    _initAsync();
+  }
+
+  void _initAsync() async {
+    print("Trying to acquire Katex mutex lock ${widget.input}");
+    await _globalMutex.acquire();
+    print("Acquired to Katex mutex lock ${widget.input}");
+
+    flutterWebViewPlugin.close();
+
+    _onStateChanged =
+        flutterWebViewPlugin.onStateChanged.listen((WebViewStateChanged state) {
       if (!mounted) return;
 
       if (state.type == WebViewState.finishLoad) {
@@ -50,7 +84,6 @@ class _KatexWidgetState extends State<KatexWidget> {
       }
     });
 
-    flutterWebViewPlugin.close();
     flutterWebViewPlugin.launch(
       selectedUrl,
       hidden: true,
@@ -61,13 +94,17 @@ class _KatexWidgetState extends State<KatexWidget> {
 
   @override
   void dispose() {
-    flutterWebViewPlugin.dispose();
+    // flutterWebViewPlugin.dispose();
 
+    if (_onStateChanged != null) {
+      _onStateChanged.cancel();
+    }
     super.dispose();
   }
 
   void _renderKatex() {
     var katex = widget.input;
+    print("Trying to render $katex");
     var js = """katex.render("$katex", document.body, {
     throwOnError: false
 });
@@ -86,7 +123,7 @@ html2canvas(document.body, {backgroundColor: 'rgba(0, 0, 0, 0)', removeContainer
       return Container();
     }
 
-    print("Building Network Image");
+    print("Building Network Image $imagePath");
     return Image.file(File(imagePath));
   }
 }
diff --git a/pubspec.lock b/pubspec.lock
index 48233d5d..db78ee03 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -509,6 +509,13 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "1.0.2"
+  mutex:
+    dependency: "direct main"
+    description:
+      name: mutex
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.0.3"
   node_interop:
     dependency: transitive
     description:
diff --git a/pubspec.yaml b/pubspec.yaml
index 1ccce37d..4bf8b681 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -41,6 +41,7 @@ dependencies:
   flutter_emoji: ">= 2.0.0"
   git_url_parse2: ^0.0.1
   synchronized: ^2.2.0
+  mutex: ^1.0.3
   steel_crypt: ^1.7.1+1
   font_awesome_flutter: ^8.7.0
   sentry: ">=3.0.0 <4.0.0"