[webview_flutter] Add listener for scroll position change in Android

[TheVinhLuong]

Description

Adding scroll listener and ability to scroll programmatically in Android.

Related Issues

flutter/flutter#42442
flutter/flutter#31027

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[amirh]

Thanks for the contribution!

I'm following the initial PR review policy, this PR isn't trivial to review so I'm labeling it with "backlog" and we will prioritize according to the issue's priority.

Relevant issue: flutter/flutter#42442

[andzejsw]

@amirh any progress on this?

[TheVinhLuong]

@andzejsw In the meantime, you can try out this plugin, it has an implemented scroll listener

[stuartmorgan]

I took an initial look at this; at a high level it looks good, but per this is a case where we should land it in parallel with an iOS implementation per the policy described at https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages#platform-support

If anyone is interested in moving this forward, I encourage you to open a PR based on this one that adds an iOS implementation; once we have both we can review this one and the iOS portion of that one.

This will also need to be updated for the federation changes (which is likely currently blocked on flutter/flutter#94051 for the moment; we hope to resolve that at the beginning of 2022).

[stuartmorgan]

@TheVinhLuong Are you planning on updating this for the structural changes in the plugin (federation, Android conversion to a more Dart-based implementation), so that it would be ready as the foundation for a cross-platform PR?

[TheVinhLuong]

@stuartmorgan Sorry for my late reply. I will update the PR for the Android side, also for iOS if I can make it.

[Hixie]

@TheVinhLuong Just a friendly check-in to see if this is still something you have on your radar?

[TheVinhLuong]

@TheVinhLuong Just a friendly check-in to see if this is still something you have on your radar?

Yes, I have implemented it both for android and ios. I am writing test now.

[TheVinhLuong]

Hi @stuartmorgan, can you review my PR? I have updated the PR according to the new plugin federation format.

Since this PR is just an implementation for the Android side the automated test for changelogs is failing for other platforms. Please add an 'override: no versioning needed' label for this PR if there is nothing wrong. Thank you!

[stuartmorgan]

Since this PR is just an implementation for the Android side the automated test for changelogs is failing for other platforms. Please add an 'override: no versioning needed' label for this PR if there is nothing wrong.

You just need to revert the tool-generated pubspec changes in those other packages.

I'll review when I get a chance, but in the meantime you have Dart unit test and analysis failures in CI that will need to be addressed.

[bparrishMines]

Left some early feedback. Are you able to do the iOS portion as well or would you like help with that?

[bparrishMines]

Instead of adding a new method to WebView, you should add a nonScrollChanged callback to the Dart WebView. The same way WebViewClient adds a callback: https://github.com/flutter/plugins/blob/main/packages/webview_flutter/webview_flutter_android/lib/src/android_webview.dart#L759

I would also make the callback include the WebView for convenience:

final void Function(WebView webView, int left, int top, int oldLeft, int oldTop)? onScrollChanged;

[TheVinhLuong]

Do you mean WebViewExtendedApi is not necessarily needed? I did read how the WebViewClient and attempted to do the same but couldn't find another way to set a scroll listener to a WebView without defining the WebViewExtendedApi .

The difference between setting a ScrollListener and a WebViewClient is that the webkit.WebView expose the method setWebViewClient for public use regardless of the Android required API level, this is unlike setOnScrollChangeListener since this is the method required min API level 23. The workaround here is to override the methodonScrollChanged of the WebView and get the offset changed event from there.

Do you have another approach for this?

[bparrishMines]

Yea you can start by adding the onScrollChanged parameter to WebView:

class WebView {
  WebView({this.useHybridComposition = false, this.onScrollChanged}) : super.detached() {
    api.createFromInstance(this);
  }

  final void Function(WebView webView, int left, int top, int oldLeft, int oldTop)? onScrollChanged;
}

Then will need to establish a Flutter API for WebView in the pigeons/android_webview.dart file:

@FlutterApi()
abstract class WebViewFlutterApi {
  void onScrollChanged(int identifier, ...);
}

Then you can run the generator and implement the Flutter API that is generated. The implementation goes in lib/src/android_webview_api_impl.dart.

Then in Java you'll need to override the onScrollChanged method in the WebViewPlatformView class found in WebViewHostApiImpl.java class. You can then instantiate a WebViewFlutterApi and call the WebViewFlutterApi.onScrollChanged method in this overridden method.

I may have missed a few things in this explanation, so definitely feel free to ask me for more clarification. The plugin is still suffering from tech debt, so adding new features still requires more work than they will in the future. I can also contribute code to this PR if needed.

[TheVinhLuong]

Thank you for the feedback. I did as you said but even though it helped to make things simple I still end up needing to retain WebViewExtendedApi .

[bparrishMines]

Ios can also support listening to scroll events, so we should add the method to the platform interface and implement for both platforms. This will probably change to something like "Implements support for PlatformWebViewController.setOnScrollChange".

[bparrishMines]

To follow the current convention of the interface, this should probably be setOnScrollChanged(...) or setOnContentOffsetChanged(...). I'm impartial to either of these.

[ditman]

Sadly, I don't think this feature is currently implementable in the web, since there's no communication mechanism between the iframe that is being injected and the flutter web app.

[TheVinhLuong]

Left some early feedback. Are you able to do the iOS portion as well or would you like help with that?

I think I can do it. Can I open a new PR for the iOS implementation?

[bparrishMines]

This PR should follow https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages#changing-federated-plugins and include the iOS implementation in this PR. Then once this has been approval, you can split the code from each package into their own PRs.

[TheVinhLuong]

@bparrishMines Thanks for your feedback. Could you please take a look again at Android implementations for this? If no problems arise I will apply the same logic for the iOS side.

[stuartmorgan]

We've just completed the migration of the plugin code to the flutter/packages repository, as described in https://flutter.dev/go/flutter-plugins-repo-migration, and this repository is now being archived. Unfortunately that means that all in-progress PRs here must be moved to flutter/packages.

Please see our instructions for an explanation of how to move your PR, and if you have any issues moving your PR please don't hesitate to reach out in the #hackers-ecosystem channel in Discord.

Our apologies that your PR was caught in this one-time transition. We're aware that it's disruptive in the short term, and appreciate your help in getting us to a better long-term state!