commit ac644776a28b52e0952794629828f5e8304a6ea0
author Amir Hardon <amirh@users.noreply.github.com> Mon Feb 04 12:57:33 2019 -0800
committer GitHub <noreply@github.com> Mon Feb 04 12:57:33 2019 -0800
tree 48426ef9f182a764cded366112d8712a4811d785
parent 7d494b433f7aab6d702e5a412a6cce3be1e77956

Add WebView JavaScript channels (Dart side). (#1116)

Allows JavaScript code running in the WebView to send messages that will be received by the Flutter app's Dart code.

To keep the PRs smaller this does not include the platform side implementations which will be sent in following PRs.

Right now there are no return values on the JavaScript side, we could think of returning a JavaScript promise (or a promise equivalent) in the future, but as a first step I start without it (`evaluateJavascript` can be used to pass messages in the other direction so this shouldn't be a blocking limitation).
I believe that if we end up adding a return value on the JavaScript side this could be done without making a breaking change.

### Sample usage
Dart:
```dart
WebView(
  javascriptChannels: <JavascriptChannel>[
    JavascriptChannel(name: 'Print', onMessageReceived: (String msg) {print(msg); }),
  ].toSet(),
),
```

JavaScript (running inside the WebView):
```javascript
Print.postMessage('Hello!');
```


Fixes: https://github.com/flutter/flutter/issues/24837