How do I integrate payments in a mobile app?

<!-- AndroidManifest.xml -->
<manifest ...>
    <queries>
        <!-- KakaoTalk -->
        <package android:name="com.kakao.talk" />
        <!-- ISP/PayBook -->
        <package android:name="kvp.jjy.MispAndroid320" />
        <!-- Add package names for the card issuers / banks your integration uses -->
    </queries>
</manifest>

import android.content.ActivityNotFoundException;
import android.content.Intent;
import android.net.Uri;
import android.os.Build;
import android.text.TextUtils;
import android.webkit.URLUtil;
import android.webkit.WebResourceRequest;
import android.webkit.WebView;
import android.webkit.WebViewClient;

import java.net.URISyntaxException;

// Add external app launch URL handling to your WebViewClient.
webView.setWebViewClient(new WebViewClient() {

    // URL handling method called on Android N (API 24) and above.
    @Override
    public boolean shouldOverrideUrlLoading(WebView view, WebResourceRequest request) {
        String url = request.getUrl().toString();
        return handlePaymentUrl(view, url);
    }

    // Implement this as well if you need to support devices below Android N (API 24).
    @SuppressWarnings("deprecation")
    @Override
    public boolean shouldOverrideUrlLoading(WebView view, String url) {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.N) {
            return handlePaymentUrl(view, url);
        }

        return super.shouldOverrideUrlLoading(view, url);
    }

    private boolean handlePaymentUrl(WebView view, String url) {
        if (TextUtils.isEmpty(url)) {
            return false;
        }

        // Return false for http, https, and javascript URLs so WebView handles them normally.
        if (URLUtil.isNetworkUrl(url) || URLUtil.isJavaScriptUrl(url)) {
            return false;
        }

        final Uri uri;

        try {
            uri = Uri.parse(url);
        } catch (Exception e) {
            return false;
        }

        // Parse and launch intent:// URLs according to the Android Intent URI spec.
        if ("intent".equals(uri.getScheme())) {
            final Intent intent;

            try {
                intent = Intent.parseUri(url, Intent.URI_INTENT_SCHEME);
            } catch (URISyntaxException e) {
                return false;
            }

            try {
                view.getContext().startActivity(intent);
                return true;
            } catch (ActivityNotFoundException e) {
                // If the intent URL includes a package and the app isn't installed, redirect to the Play Store.
                String packageName = intent.getPackage();

                if (!TextUtils.isEmpty(packageName)) {
                    try {
                        Intent marketIntent = new Intent(
                            Intent.ACTION_VIEW,
                            Uri.parse("market://details?id=" + packageName)
                        );
                        view.getContext().startActivity(marketIntent);
                        return true;
                    } catch (Exception marketException) {
                        return false;
                    }
                }

                return false;
            } catch (Exception e) {
                return false;
            }
        }

        // Forward standard App Scheme URLs (ispmobile://, kftc-bankpay://, etc.) to the appropriate app.
        try {
            Intent schemeIntent = new Intent(Intent.ACTION_VIEW, uri);
            view.getContext().startActivity(schemeIntent);
            return true;
        } catch (Exception e) {
            // The app is not installed or no Activity can handle this URL.
            // If you need to redirect to the Play Store, use the Package Name from the App Scheme list below.
            return false;
        }
    }
});

import android.os.Build;
import android.webkit.CookieManager;
import android.webkit.WebSettings;

WebSettings settings = webView.getSettings();

// Allow JavaScript so payment window scripts can execute.
settings.setJavaScriptEnabled(true);

// Enable DOM Storage so the payment window can use Local Storage.
settings.setDomStorageEnabled(true);

// Use the default cache policy.
settings.setCacheMode(WebSettings.LOAD_DEFAULT);

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
    // Allow mixed content in case the payment/authentication flow loads http resources.
    settings.setMixedContentMode(WebSettings.MIXED_CONTENT_ALWAYS_ALLOW);

    // Allow cookies needed during the payment authentication flow.
    CookieManager cookieManager = CookieManager.getInstance();
    cookieManager.setAcceptCookie(true);
    cookieManager.setAcceptThirdPartyCookies(webView, true);
}

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.webkit.JsResult;
import android.webkit.WebChromeClient;
import android.webkit.WebView;

webView.setWebChromeClient(new WebChromeClient() {

    // Display JavaScript alert() calls as a native dialog.
    @Override
    public boolean onJsAlert(WebView view, String url, String message, final JsResult result) {
        new AlertDialog.Builder(view.getContext())
            .setMessage(message)
            .setPositiveButton(android.R.string.ok, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    result.confirm();
                }
            })
            .setCancelable(false)
            .create()
            .show();
        return true;
    }

    // Pass the user's confirm/cancel choice back to the payment window for JavaScript confirm() calls.
    @Override
    public boolean onJsConfirm(WebView view, String url, String message, final JsResult result) {
        new AlertDialog.Builder(view.getContext())
            .setMessage(message)
            .setPositiveButton(android.R.string.ok, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    result.confirm();
                }
            })
            .setNegativeButton(android.R.string.cancel, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    result.cancel();
                }
            })
            .create()
            .show();
        return true;
    }
});

<!-- Info.plist -->
<key>LSApplicationQueriesSchemes</key>
<array>
    <string>kakaotalk</string> <!-- KakaoTalk -->
    <string>ispmobile</string> <!-- ISP -->
    <string>kb-acp</string>    <!-- KB Kookmin Card -->
    <!-- Add other card issuer schemes -->
</array>

// ViewController.swift
func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, decisionHandler: @escaping (WKNavigationActionPolicy) -> Void) {
    if let url = navigationAction.request.url {
        // Non-http/https URLs are app schemes
        if url.scheme != "http" && url.scheme != "https" {
            // Check if the app is installed and open it
            if UIApplication.shared.canOpenURL(url) {
                UIApplication.shared.open(url)
                decisionHandler(.cancel) // Cancel WebView navigation
                return
            }
        }
    }
    decisionHandler(.allow) // Allow normal web page navigation
}

- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [[NSHTTPCookieStorage sharedHTTPCookieStorage]
        setCookieAcceptPolicy:NSHTTPCookieAcceptPolicyAlways];

    return YES;
}