WebView.EvaluateJavaScriptAsync return null

[CodingOctocat]

I have implemented the WebView scrolling to the beginning of the paragraph of the specified string with the help of chatGPT (I need to use this cumbersome method because the target web page is not standardized).

I found that EvaluateJavaScriptAsync returns null using a callback function, Promise, but I changed it to the following (direct return value) and resultStr gets true/false.

If you use a callback function, Promise, then call EvalJsToScrollToAsync in the WebView.Navigated event, then the scrolling works fine, but the return value is null, then call EvalJsToScrollToAsync again, and the scrolling works fine as well as the return value.But if js is refactored to return the value directly, then the first call to EvalJsToScrollToAsync will get the return value normally.

This is the version that works properly:

private async Task<bool> EvalJsToScrollToAsync(string section)
{
    string encodedSection = HttpUtility.JavaScriptStringEncode(section);

    string js = $$"""
        function scrollToText(targetText) {
            // Get all text nodes in the document
            function getTextNodes(node) {
                let textNodes = [];
                if (node.nodeType === Node.TEXT_NODE) {
                    textNodes.push(node);
                } else {
                    for (let child of node.childNodes) {
                        textNodes.push(...getTextNodes(child));
                    }
                }
                return textNodes;
            }

            // Find the location of the target text in the text node
            let textNodes = getTextNodes(document.body);
            let foundNode = null;
            for (let node of textNodes) {
                let nodeText = node.textContent.replace(/\s+/g, '');
                if (nodeText.startsWith(targetText.replace(/\s+/g, ''))) {
                    foundNode = node;
                    console.log(foundNode);
                    break;
                }
            }

            if (foundNode) {
                // Creating a Temporary Marker
                let tempMarker = document.createElement('span');
                tempMarker.id = 'temp-scroll-marker';
                tempMarker.style.position = 'absolute';
                tempMarker.style.visibility = 'hidden';
                tempMarker.style.height = '0';
                tempMarker.style.width = '0';

                foundNode.parentNode.insertBefore(tempMarker, foundNode);

                // Temporarily disable manual scrolling
                document.body.style.overflow = 'hidden';

                // Scroll to the position of the temporary marker
                tempMarker.scrollIntoView({ behavior: 'smooth', block: 'start', inline: 'start' });

                // Wait for scrolling to complete and remove the temporary marker
                if (window.onscrollend !== undefined) {
                    let onScrollEnd = () => {
                        document.body.style.overflow = '';
                        window.removeEventListener('scrollend', onScrollEnd);
                        tempMarker.remove();
                        console.log('onscrollend');
                    };
                    window.addEventListener('scrollend', onScrollEnd);
                } else {
                    setTimeout(() => {
                        document.body.style.overflow = '';
                        tempMarker.remove();
                        console.log('setTimeout');
                    }, 1000);
                }
                return true;
            } else {
                return false;
            }
        }

        // Call the function to scroll to the target text
        scrollToText('{{encodedSection}}');
        """;

    string resultStr = await _webView.EvaluateJavaScriptAsync(js);

    bool result = resultStr == "true";

    if (!result)
    {
        await Toast.Make($"Failure to locate {section}").Show();
    }

    return result;
}