[webview_flutter_web] Add support for JavaScript channels #1

Frank3K · 2023-04-03T10:13:21Z

This pull request implements JavaScript channels for webview_flutter_web, allowing postMessage communication from a web application to Flutter.

The regular webview_flutter package already supports JavaScript channels for iOS and Android, making is possible to send messages from a web application to Flutter.

Related issues

Flutter issue asking for support: flutter/flutter#101758.

How it works

On iOS and Android it is possible to send messages on a JavaScript channel using the postMessage API. This API is very simple: it only allows a single string argument (docs). On Web, there's the postMessage API on window, on which arbitrary data can be sent. Since the postMessage API for Android and iOS is a subset (string vs object), the web postMessage API can be used to implement the JavaScript channel.

Demo-videos

The following demo show a small application which sends a postMessage to the Flutter application.

Screen.Recording.2023-04-06.at.12.42.57.mov

Screen.Recording.2023-04-06.at.20.33.57.mov

PoC code

Flutter side

    ...
    webViewController = WebViewController();
    webViewController.addJavaScriptChannel(
      'flutterApp',
      onMessageReceived: (message) {
        ScaffoldMessenger.of(context).showSnackBar(
          SnackBar(
            content: Text(message.message),
          ),
        );
      },
    );
   ...

Web side

HTML

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8" />
    <title>HTML5 Example Page</title>
    <meta name="viewport" content="width=device-width, initial-scale=1" />
    <meta name="description" content="HTML5 Example Page" />
    <style type="text/css">
      body {
        font-family: sans-serif;
      }
    </style>
    <script type="text/javascript" src="demo.js"></script>
  </head>
  <body>
    <header>
      <h1>HTML5 Example Page</h1>
    </header>
    <button onclick="helloWorld()">Hello world</button>
    <button onclick="userAgent()">Show userAgent</button>
  </body>
</html>

JavaScript

function sendMessage(message) {
  const channel = window["flutterApp"] || window.parent;
  if (!channel) {
    return;
  }

  channel.postMessage(message, '*');
}

function helloWorld() {
  sendMessage("Hello world");
}

function userAgent() {
  sendMessage(window.navigator.userAgent);
}

Web implementation notes and limitations

Security

In the above PoC the postMessage is sent with * as target origin. This is considered bad practice. The web application should use the origin of the flutter web application as the target origin.

Named JavaScript channels

On iOS and Android it is possible to name a JavaScript channel. On web, this is not possible since that would require injecting JavaScript code into the iframe (which is not possible cross-origin). Therefore the web application uses window.parent (using window["flutterApp"] || window.parent) to get to the flutter application. Therefore the web implementation only allows a single channel.

Pre-launch Checklist

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Frank3K · 2023-04-03T10:15:00Z

packages/webview_flutter/webview_flutter_web/lib/src/web_webview_controller.dart

+    // TODO: what to return?
+    return Future.wait([]);


@omartinma I'm not sure what to return here (and below).

Hey @Frank3K, When using Future<void> there is no need to explicitly return anything.

That's what I thought too, but when I remove the return Future.wait([]); I get the following error:

The body might complete normally, causing 'null' to be returned, but the return type, Future<void>, is a potentially non-nullable type.
Try adding either a return or a throw statement at the end. dart(body_might_complete_normally)

@Frank3K It could be because you are missing the async modifier in the function.

It should look like this:

@override Future<void> addJavaScriptChannel( JavaScriptChannelParams javaScriptChannelParams, ) async { ... }

Thanks, I tried something similar earlier, but could not get it to compile properly back then.

Frank3K · 2023-04-03T11:34:34Z

Please note that the current implementation contains a prefer_function_declarations_over_variables warning. Let's first see if we think this solution is a good solution in the first place.

Frank3K · 2023-04-04T12:25:41Z

packages/webview_flutter/webview_flutter_web/lib/src/web_webview_controller.dart

+    final Null Function(html.Event) handler = (html.Event event) {
+      if (event is html.MessageEvent) {
+        javaScriptChannelParams.onMessageReceived(
+            JavaScriptMessage(message: event.data.toString()));
+      }
+    };


@omartinma This triggers a prefer_function_declarations_over_variables warning. Should we fix that warning before submitting the PR? I tried refactoring it myself, but could not get the typings correct.

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Frank3K commented Apr 3, 2023

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Frank3K commented Apr 4, 2023

View reviewed changes

Frank3K changed the title ~~feat: add support for JavaScript channels~~ [webview_flutter_web] Add support for JavaScript channels Apr 6, 2023

feat: add support for JavaScript channels

5b5b6e3

Frank3K force-pushed the feat/javascript-channels branch from a7799e5 to 5b5b6e3 Compare April 6, 2023 18:59

omartinma and others added 6 commits April 10, 2023 10:04

chore: add void

adf825a

refactor: private handler to avoid analyzer error

1be348e

chore: fix no_leading_underscores_for_local_identifiers linter warning

01c905f

Merge remote-tracking branch 'origin/main' into feat/javascript-channels

01cb968

docs: update README

b313f90

chore: bump version number

bd2f600

Frank3K mentioned this pull request Jun 21, 2023

[webview_flutter_web] Improve support for JavaScript channels #2

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[webview_flutter_web] Add support for JavaScript channels #1

[webview_flutter_web] Add support for JavaScript channels #1

Frank3K commented Apr 3, 2023 •

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Frank3K commented Apr 3, 2023

Frank3K Apr 4, 2023

[webview_flutter_web] Add support for JavaScript channels #1

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[webview_flutter_web] Add support for JavaScript channels #1

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Frank3K commented Apr 3, 2023 • edited Loading

Related issues

How it works

Demo-videos

PoC code

Flutter side

Web side

HTML

JavaScript

Web implementation notes and limitations

Security

Named JavaScript channels

Pre-launch Checklist

Frank3K Apr 3, 2023

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