[Bug] WebView.EvaluateJavascriptAsync is encoding input and output

[rowillis]

Description

The implementation of WebView.EvaluateJavascriptAsync is non-standard and appears to be encoding the input and output in unexpected ways. Other 3rd party plugins and other toolsets/languages (such as flutter) are behaving as expected.

Steps to Reproduce

1. Create a new XF project
2. Add a webview to the XAML with any webpage as the source, and a "Navigated" event handler
3. In the "Navigated" eventhandler, add the following:
   var webView = sender as WebView; var html = webView.EvaluateJavascriptAsync("document.documentElement.outerHTML");

See https://stackoverflow.com/questions/63228670/get-html-document-from-xamarin-forms-webview and attached sample for more info.

Expected Behavior

The html should a standard HTML string with no encoding.

Actual Behavior

The html is encoded instead the straight HTML. Even after decoding, you will notice that string literals are double-escaped.

NOTE: If this behavior is deemed correct for some reason, then I would convert this into a feature request for an additional method that does not munge the input/output.

Basic Information

• Version with issue: XF 4.7.0.1239
• Last known good version: unknown
• IDE: VS Mac
• Platform Target Frameworks:
  • iOS: 11.6
  • Android: 10.0, API level 29
  • UWP: n/a
• Android Support Library Version:
• Nuget Packages: Default (Xamarin.Essentials 1.5.3.2, Xamarin.Forms 4.7.0.1239)
• Affected Devices:

Reproduction Link

Workaround

Lucas Zhang suggested using WebUtility.HtmlDecode(html) and regex to decode the string, but other well-formed JS scripts still fail (whereas they do not with other 3rd party implementations and other tools such as flutter), leading me to believe that the implementation is targeted at some very specific use case and is doing more than it should.
TestWebView.zip

[jgold6]

It seems that the 3rd party Plugin.HybridWebView strips out the extraneous stuff:
https://github.com/devsmadeofsteel/Plugin.HybridWebView/blob/master/HybridWebView/Droid/HybridWebViewRenderer.cs#L359

                // Unescape that damn Unicode Java bull.
                response = Regex.Replace(_callback.Value.ToString(), @"\\[Uu]([0-9A-Fa-f]{4})", m => char.ToString((char)ushort.Parse(m.Groups[1].Value, NumberStyles.AllowHexSpecifier)));
                response = Regex.Unescape(response);

Xam.Forms just returns whatever the platform returns from the native web view EvaluateJavascript methods.

[B2alex]

any update on this? the solution that jgold6 has mentioned doesn't fully work for me.

[KyNam]

Why JSON.stringify the result?

Xamarin.Forms/Xamarin.Forms.Core/WebView.cs

Line 104 in 4b59b6f

script = "try{JSON.stringify(eval('" + script + "'))}catch(e){'null'};";

			//make all the platforms mimic Android's implementation, which is by far the most complete.
			if (Xamarin.Forms.Device.RuntimePlatform != "Android")
			{
				script = EscapeJsString(script);
				script = "try{JSON.stringify(eval('" + script + "'))}catch(e){'null'};";
			}

[dzebas]

Is this going to be fixed?

[n-ate]

Stumbled on this bug. I noticed that I can send
"alert(99); true;"
and it works fine, but if I send
"alert(99);\ntrue;"
or
"alert(99);\r\ntrue;"
that it fails silently.
Are multiline scripts not supported? Is this related to this bug?

Update: if I escape \ with a second \ then it seems to work.

[trivalik]

Stumbled on this bug. I noticed that I can send "alert(99); true;" and it works fine, but if I send "alert(99);\ntrue;" or "alert(99);\r\ntrue;" that it fails silently. Are multiline scripts not supported? Is this related to this bug?

Update: if I escape \ with a second \ then it seems to work.

This is no bug. I created a bug for MAUI WebView and it seems it is wanted, for whatever reason.

[bricefriha]

Stumbled on this bug. I noticed that I can send "alert(99); true;" and it works fine, but if I send "alert(99);\ntrue;" or "alert(99);\r\ntrue;" that it fails silently. Are multiline scripts not supported? Is this related to this bug?

Update: if I escape \ with a second \ then it seems to work.

Thanks @n-ate!

So basically, the workaround is:

// Your js
string script = @"
                           alert(99);
                           true;";

await webView.EvaluateJavaScriptAsync(script.Trim().Replace("\r", "\\r").Replace("\n", "\\n"); 

At least it works on my side

[trivalik]

@bricefriha it is enought to replace with an empty string. This will save you a few nanoseconds on the browser side ignoring newlines. The only exception is if you have text which require the newline.

[bricefriha]

@bricefriha it is enought to replace with an empty string. This will save you a few nanoseconds on the browser side ignoring newlines. The only exception is if you have text which require the newline.

It's what I was thinking about at first as well, but when I tested it didn't work.

But feel free to test it on your side, I might have done something wrong 😉

[trivalik]

It looks like this bug will be fixed for MAUI: dotnet/maui#11905