Description of Change

WebView's Navigating event does not allow for async code to be executed to determine whether to cancel the navigation. The implementation creates a race condition and makes the cancellation functionality confusing, the following code will not cancel the WebView navigation event:

public MainPage()
{
    WebView.Navigating += async (s, e) =>
    {
        await Task.Delay(2000);

        e.Cancel = true;
    };
}

This makes it impossible to delay navigation in a WebView whilst doing a check such as seeing if the user can navigate to a url, without writing a hacky workaround ontop of this behaviour.

I make a first attempt at this PR in November and it was suggested by @PureWeen to refactor the code to use a DeferralToken, used by Shell to achieve the functionality (old pr: #12756). I've made a new PR since I made a completely new branch.

Issues Resolved

• fixes [Bug] WebView Navigating Race Condition #12720

API Changes

Added:

• Added DeferralToken, DeferralTokenEventArgs

Changed:

• Renamed ShellDeferralToken => DeferralToken
• Removed deferral token functionality from ShellNavigatingArgs into DeferrableEventArgs
• WebViewNavigatingArgs now inherit from DeferralTokenEventArgs
• WKWebViewRenderer now uses the deferral token to handle navigation decisions

New API Usage:

WebView.Navigating += async (s, e) =>
{
    var token = e.GetDeferral();

    await Task.Delay(2000);

    var shouldCancel = CanBrowseUrl(e.Url);

    if (shouldCancel)
        e.Cancel();

    token.Complete();
};

Platforms Affected

• Core / All
• iOS Currently

Behavioral/Visual Changes

This needs to be discussed but it would break and app using WebViewNavigatingArgs.Cancel to cancel navigation. I did slap an Obsolete attribute onto the property (and rename to OldCancel whilst the PR is work in progress, but it causes the control gallery to fail building. Maybe it can be added back when the PR is good to go & i go back through the control gallery and change all references to deferral.

Before/After Screenshots

Not applicable

Testing Procedure

• Open the file Xamarin.Forms.Controls.Issues.Shared.Issue12720.xaml.cs
• You will find 3 event handlers in the constructor, for each event (only ever comment 1 in)
• Comment in the constructor, comment out the others
• Run the Control Gallery app
• Go to Test Cases
• Find issue G12720
• Use the web browser:

OnNavigatingWithoutDeferral
No impact to browser

OnNavigatingWithDeferral
User can't browse to any sites containing bbc.co.uk such as:

• https://www.bbc.co.uk/news/

OnNavigatingWithDeferralAsync
All navigation occurs normally unless the user attempts to browse to any sites containing bbc.co.uk such as:

• https://www.bbc.co.uk/news/

Requests to this site will be evaluated with a task that increases in duration, and always get cancelled

Todo Before Merge

I've put this PR up to discuss the refactors I've made with the team before committing more time, this PR should be marked as do not merge until all work has been completed.

• Ensure Shell functioning correctly (affected by deferral refactor)
• Update Android WebViewRenderer
• Update GTK WebViewRenderer
• Update macOS WebViewRenderer
• Update Tizen WebViewRenderer
• Update WPF WebViewRenderer
• Move code from deferral folder to appropriate place (I'm using that folder to keep organised in my work, I'm not very used to working in a codebase with so little folders 😖 )
• Remove mfractor from git ignore (otherwise git tried to commit about 50 files)
• Cleanup WebNavigationArgs (base class or interface):
  To get my code working I ended up changing the base class to DeferrableEventArgs, I added an interface for the WebNavigationArgs, we need to discuss what you would like to see about this. I'd personally prefer the interface.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)