[FIXED] "net::ERR_UNKNOWN_URL_SCHEME" error when url is"intent://kakaopay..."

Issue

I have build a hybrid-app using flutter's web_view_plugin(webview). One of our paying methods require to open a 3rd party app(in this case kakaotalk). But the flutter webview plugin does not provide this function and returned net::ERR_UNKNOWN_URL_SCHEME. I did some research and I understand that the problem lies in the url. If the url does not start with http or https, it will cause this error.

So, to solve this problem I had to change native java code. Now I have no experience at all with java and android, so fixing the native code was very difficult. I understand that I have to modify shouldOverrideUrlLoading part, in order to allow the url that starts with intent:// and also I have to put in some validation to check if the app is installed or not.(If not installed the user should be redirected to playstore)

The code which I added is in shouldOverrideUrlLoading. I also did three imports. The rest is code, generated by flutter

package com.flutter_webview_plugin;

import android.annotation.TargetApi;
import android.graphics.Bitmap;
import android.os.Build;
import android.webkit.WebResourceRequest;
import android.webkit.WebResourceResponse;
import android.webkit.WebView;
import android.webkit.WebViewClient;

import java.util.HashMap;
import java.util.Map;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import android.content.Intent; //added import
import android.net.Uri; //added import
import android.content.ActivityNotFoundException; //added import
/**
 * Created by lejard_h on 20/12/2017.
 */

public class BrowserClient extends WebViewClient {
    private Pattern invalidUrlPattern = null;

    public BrowserClient() {
        this(null);
    }

    public BrowserClient(String invalidUrlRegex) {
        super();
        if (invalidUrlRegex != null) {
            invalidUrlPattern = Pattern.compile(invalidUrlRegex);
        }
    }

    public void updateInvalidUrlRegex(String invalidUrlRegex) {
        if (invalidUrlRegex != null) {
            invalidUrlPattern = Pattern.compile(invalidUrlRegex);
        } else {
            invalidUrlPattern = null;
        }
    }

    @Override
    public void onPageStarted(WebView view, String url, Bitmap favicon) {
        super.onPageStarted(view, url, favicon);
        Map<String, Object> data = new HashMap<>();
        data.put("url", url);
        data.put("type", "startLoad");
        FlutterWebviewPlugin.channel.invokeMethod("onState", data);
    }

    @Override
    public void onPageFinished(WebView view, String url) {
        super.onPageFinished(view, url);
        Map<String, Object> data = new HashMap<>();
        data.put("url", url);

        FlutterWebviewPlugin.channel.invokeMethod("onUrlChanged", data);

        data.put("type", "finishLoad");
        FlutterWebviewPlugin.channel.invokeMethod("onState", data);

    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    @Override
    public boolean shouldOverrideUrlLoading(WebView view, WebResourceRequest request) {
        // returning true causes the current WebView to abort loading the URL,
        // while returning false causes the WebView to continue loading the URL as usual.
        String url = request.getUrl().toString();
        boolean isInvalid = checkInvalidUrl(url);
        Map<String, Object> data = new HashMap<>();
        data.put("url", url);
        data.put("type", isInvalid ? "abortLoad" : "shouldStart");

        FlutterWebviewPlugin.channel.invokeMethod("onState", data);
        return isInvalid;
    }

    @Override
    public boolean shouldOverrideUrlLoading(WebView view, String url) {
        // returning true causes the current WebView to abort loading the URL,
        // while returning false causes the WebView to continue loading the URL as usual.
        if (url.startsWith(INTENT_PROTOCOL_START)) {
                final int customUrlStartIndex = INTENT_PROTOCOL_START.length();
                final int customUrlEndIndex = url.indexOf(INTENT_PROTOCOL_INTENT);
                if (customUrlEndIndex < 0) {
                    return false;
                } else {
                    final String customUrl = url.substring(customUrlStartIndex, customUrlEndIndex);
                    try {
                        view.getContext().startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse(customUrl)));
                    } catch (ActivityNotFoundException e) {
                        final int packageStartIndex = customUrlEndIndex + INTENT_PROTOCOL_INTENT.length();
                        final int packageEndIndex = url.indexOf(INTENT_PROTOCOL_END);

                        final String packageName = url.substring(packageStartIndex, packageEndIndex < 0 ? url.length() : packageEndIndex);
                        view.getContext().startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse(GOOGLE_PLAY_STORE_PREFIX + packageName)));
                    }
                    return true;
                }
            } else {
                return false;
            }
        // boolean isInvalid = checkInvalidUrl(url);
        // Map<String, Object> data = new HashMap<>();
        // data.put("url", url);
        // data.put("type", isInvalid ? "abortLoad" : "shouldStart");

        // FlutterWebviewPlugin.channel.invokeMethod("onState", data);
        // return isInvalid;
    }


    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    @Override
    public void onReceivedHttpError(WebView view, WebResourceRequest request, WebResourceResponse errorResponse) {
        super.onReceivedHttpError(view, request, errorResponse);
        Map<String, Object> data = new HashMap<>();
        data.put("url", request.getUrl().toString());
        data.put("code", Integer.toString(errorResponse.getStatusCode()));
        FlutterWebviewPlugin.channel.invokeMethod("onHttpError", data);
    }

    @Override
    public void onReceivedError(WebView view, int errorCode, String description, String failingUrl) {
        super.onReceivedError(view, errorCode, description, failingUrl);
        Map<String, Object> data = new HashMap<>();
        data.put("url", failingUrl);
        data.put("code", errorCode);
        FlutterWebviewPlugin.channel.invokeMethod("onHttpError", data);
    }

    private boolean checkInvalidUrl(String url) {
        if (invalidUrlPattern == null) {
            return false;
        } else {
            Matcher matcher = invalidUrlPattern.matcher(url);
            return matcher.lookingAt();
        }
    }
}

The code compiles, but it still returns the same error net::ERR_UNKNOWN_URL_SCHEME when I try to pay with the "3rd party app(kakaotalk)"

Solution

I encountered a similar error on Android before when Firebase Dynamic Links are being forced to be loaded in a WebView. In my case, FDL is expected to be handled by Google Play Services in Android. But since the WebView doesn't know what to do with the link it's forced to display, the WebView returns "net::ERR_UNKNOWN_URL_SCHEME" error. I'm unsure if this is the same case as yours since I'm unable to verify the link that you're trying to load apart from "intent://kakaopay..."

You can try opening the link externally by using url_launcher. Use RegEx to filter intent URLs and check if the URL can be launched and be handled externally (outside the app).

var yourURL = "URL goes here";
// Check if URL contains "intent"
yourURL.contains(RegExp('^intent://.*\$')){
  // Check if the URL can be launched
  if (await canLaunch(yourURL)) {
    await launch(yourURL);
  } else {
    print('Could not launch $yourURL');
  }
}

Also, the plugin (web_view_plugin) that you're using seems to be outdated, and I can't find it here https://pub.dev/packages?q=web_view_plugin. Flutter has its official WebView plugin (webview_flutter) that has been released and I suggest checking it out if it fits your use case.

Answered By - Omatt
Answer Checked By - Gilberto Lyons (JavaFixing Admin)