Proxy requests and round trip JavaScript invocations (#43)

* Add support for proxying requests sent by HybridWebView: Proxying web requests from the browser through native code to allow for modifying the request, and creating custom responses.
* Add round trip invoke JS-.NET-JS: Add round trip invoke JS-.NET-JS by leveraging the proxy framework that was created.

HybridWebView/HybridWebView.cs

@@ -1,9 +1,12 @@
-using System.Text.Json;
+using System.Diagnostics;
+using System.Text.Json;
 
 namespace HybridWebView
 {
     public partial class HybridWebView : WebView
     {
+        internal const string ProxyRequestPath = "proxy";
+
         /// <summary>
         /// Specifies the file within the <see cref="HybridAssetRoot"/> that should be served as the main file. The
         /// default value is <c>index.html</c>.
@@ -34,6 +37,12 @@ public partial class HybridWebView : WebView
 
         public event EventHandler<HybridWebViewRawMessageReceivedEventArgs>? RawMessageReceived;
 
+        /// <summary>
+        /// Async event handler that is called when a proxy request is received from the webview.
+        /// </summary>
+
+        public event Func<HybridWebViewProxyEventArgs, Task>? ProxyRequestReceived;
+
         public void Navigate(string url)
         {
             NavigateCore(url);
@@ -117,7 +126,66 @@ public virtual void OnMessageReceived(string message)
 
         }
 
-        private void InvokeDotNetMethod(JSInvokeMethodData invokeData)
+        /// <summary>
+        /// Handle the proxy request message.
+        /// </summary>
+        /// <param name="args"></param>
+        /// <returns>A Task</returns>
+        public virtual async Task OnProxyRequestMessage(HybridWebViewProxyEventArgs args)
+        {
+            // Don't let failed proxy requests crash the app.
+            try
+            {
+                // When no query parameters are passed, the SendRoundTripMessageToDotNet JavaScript method is expected to have been called.
+                if (args.QueryParams != null && args.QueryParams.TryGetValue("__ajax", out string? jsonQueryString))
+                {
+                    if (jsonQueryString != null)
+                    {
+                        var invokeData = JsonSerializer.Deserialize<JSInvokeMethodData>(jsonQueryString);
+
+                        if (invokeData != null && invokeData.MethodName != null)
+                        {
+                            object? result = InvokeDotNetMethod(invokeData);
+
+                            if (result != null)
+                            {
+                                args.ResponseContentType = "application/json";
+
+                                DotNetInvokeResult dotNetInvokeResult;
+
+                                var resultType = result.GetType();
+                                if (resultType.IsArray || resultType.IsClass)
+                                {
+                                    dotNetInvokeResult = new DotNetInvokeResult()
+                                    {
+                                        Result = JsonSerializer.Serialize(result),
+                                        IsJson = true,
+                                    };
+                                }
+                                else
+                                {
+                                    dotNetInvokeResult = new DotNetInvokeResult()
+                                    {
+                                        Result = result,
+                                    };
+                                }
+                                args.ResponseStream = new MemoryStream(System.Text.Encoding.UTF8.GetBytes(JsonSerializer.Serialize(dotNetInvokeResult)));
+                            }
+                        }
+                    }
+                }
+                else if (ProxyRequestReceived != null) //Check to see if user has subscribed to the event.
+                {
+                    await ProxyRequestReceived(args);
+                }
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine($"An exception occurred while handling the proxy request: {ex.Message}");
+            }
+        }
+
+        private object? InvokeDotNetMethod(JSInvokeMethodData invokeData)
         {
             if (JSInvokeTarget is null)
             {
@@ -140,7 +208,7 @@ private void InvokeDotNetMethod(JSInvokeMethodData invokeData)
                     .Zip(invokeMethod.GetParameters(), (s, p) => JsonSerializer.Deserialize(s, p.ParameterType))
                     .ToArray();
 
-            var returnValue = invokeMethod.Invoke(JSInvokeTarget, paramObjectValues);
+            return invokeMethod.Invoke(JSInvokeTarget, paramObjectValues);
         }
 
         private sealed class JSInvokeMethodData
@@ -155,6 +223,15 @@ private sealed class WebMessageData
             public string? MessageContent { get; set; }
         }
 
+        /// <summary>
+        /// A simple internal class to hold the result of a .NET method invocation, and whether it should be treated as JSON.
+        /// </summary>
+        private sealed class DotNetInvokeResult
+        {
+            public object? Result { get; set; }
+            public bool IsJson { get; set; }
+        }
+
         internal static async Task<string?> GetAssetContentAsync(string assetPath)
         {
             using var stream = await GetAssetStreamAsync(assetPath);

HybridWebView/HybridWebViewProxyEventArgs.cs

@@ -0,0 +1,39 @@
+namespace HybridWebView
+{
+    /// <summary>
+    /// Event arg object for a proxy request from the <see cref="HybridWebView"/>.
+    /// </summary>
+    public class HybridWebViewProxyEventArgs
+    {
+        /// <summary>
+        /// Creates a new instance of <see cref="HybridWebViewProxyEventArgs"/>.
+        /// </summary>
+        /// <param name="fullUrl">The full request URL.</param>
+        public HybridWebViewProxyEventArgs(string fullUrl)
+        {
+            Url = fullUrl;
+            QueryParams = QueryStringHelper.GetKeyValuePairs(fullUrl);
+        }
+
+        /// <summary>
+        /// The full request URL.
+        /// </summary>
+        public string Url { get; }
+
+        /// <summary>
+        /// Query string values extracted from the request URL.
+        /// </summary>
+        public IDictionary<string, string> QueryParams { get; }
+
+        /// <summary>
+        /// The response content type.
+        /// </summary>
+
+        public string? ResponseContentType { get; set; } = "text/plain";
+
+        /// <summary>
+        /// The response stream to be used to respond to the request.
+        /// </summary>
+        public Stream? ResponseStream { get; set; } = null;
+    }
+}

HybridWebView/KnownStaticFiles/HybridWebView.js

@@ -1,10 +1,19 @@
 // Standard methods for HybridWebView
 
 window.HybridWebView = {
+    /**
+     * Sends a message to .NET using the built in
+     * @param {string} message Message to send.
+     */
     "SendRawMessageToDotNet": function SendRawMessageToDotNet(message) {
         window.HybridWebView.SendMessageToDotNet(0, message);
     },
 
+    /**
+     * Invoke a .NET method. No result is expected.
+     * @param {string} methodName Name of .NET method to invoke.
+     * @param {any[]} paramValues Parameters to pass to the method.
+     */
     "SendInvokeMessageToDotNet": function SendInvokeMessageToDotNet(methodName, paramValues) {
         if (typeof paramValues !== 'undefined') {
             if (!Array.isArray(paramValues)) {
@@ -18,6 +27,64 @@ window.HybridWebView = {
         window.HybridWebView.SendMessageToDotNet(1, JSON.stringify({ "MethodName": methodName, "ParamValues": paramValues }));
     },
 
+    /**
+     * Asynchronously invoke .NET method and get a result. 
+     * Leverages the proxy to send the message to .NET.
+     * @param {string} methodName Name of .NET method to invoke.
+     * @param {any[]} paramValues Parameters to pass to the method.
+     * @returns {Promise<any>} Result of the .NET method.
+     */
+    "SendInvokeMessageToDotNetAsync": async function SendInvokeMessageToDotNetAsync(methodName, paramValues) {
+        const body = {
+            MethodName: methodName
+        };
+
+        if (typeof paramValues !== 'undefined') {
+            if (!Array.isArray(paramValues)) {
+                paramValues = [paramValues];
+            }
+
+            for (var i = 0; i < paramValues.length; i++) {
+                paramValues[i] = JSON.stringify(paramValues[i]);
+            }
+
+            if (paramValues.length > 0) {
+                body.ParamValues = paramValues;
+            }
+        }
+
+        const message = JSON.stringify(body);
+
+        try {
+            // Android web view doesn't support getting the body of a POST request, so we use a GET request instead and pass the body as a query string parameter.
+            var requestUrl = `${window.location.origin}/proxy?__ajax=${encodeURIComponent(message)}`;
+
+            const rawResponse = await fetch(requestUrl, {
+                method: 'GET',
+                headers: {
+                    'Accept': 'application/json'
+                }
+            });
+            const response = await rawResponse.json();
+
+            if (response) {
+                if (response.IsJson) {
+                    return JSON.parse(response.Result);
+                }
+
+                return response.Result;
+            }
+        } catch (e) { }
+
+        return null;
+    },
+
+    /**
+     * Sends a message to .NET using the built in 
+     * @private
+     * @param {number} messageType The type of message to send.
+     * @param {string} messageContent The message content.
+     */
     "SendMessageToDotNet": function SendMessageToDotNet(messageType, messageContent) {
         var message = JSON.stringify({ "MessageType": messageType, "MessageContent": messageContent });
 

HybridWebView/Platforms/Android/AndroidHybridWebViewClient.cs

@@ -1,4 +1,5 @@
 using Android.Webkit;
+using Java.Time;
 using Microsoft.Maui.Platform;
 using System.Text;
 using AWebView = Android.Webkit.WebView;
@@ -15,8 +16,10 @@ public AndroidHybridWebViewClient(HybridWebViewHandler handler) : base(handler)
         }
         public override WebResourceResponse? ShouldInterceptRequest(AWebView? view, IWebResourceRequest? request)
         {
-            var requestUri = request?.Url?.ToString();
-            requestUri = QueryStringHelper.RemovePossibleQueryString(requestUri);
+            var fullUrl = request?.Url?.ToString();
+            var requestUri = QueryStringHelper.RemovePossibleQueryString(fullUrl);
+
+            var webView = (HybridWebView)_handler.VirtualView;
 
             if (new Uri(requestUri) is Uri uri && HybridWebView.AppOriginUri.IsBaseOf(uri))
             {
@@ -25,7 +28,7 @@ public AndroidHybridWebViewClient(HybridWebViewHandler handler) : base(handler)
                 string contentType;
                 if (string.IsNullOrEmpty(relativePath))
                 {
-                    relativePath = ((HybridWebView)_handler.VirtualView).MainFile;
+                    relativePath = webView.MainFile;
                     contentType = "text/html";
                 }
                 else
@@ -40,7 +43,27 @@ public AndroidHybridWebViewClient(HybridWebViewHandler handler) : base(handler)
                     };
                 }
 
-                var contentStream = KnownStaticFileProvider.GetKnownResourceStream(relativePath!);
+                Stream? contentStream = null;
+
+                // Check to see if the request is a proxy request.
+                if (relativePath == HybridWebView.ProxyRequestPath)
+                {
+                    var args = new HybridWebViewProxyEventArgs(fullUrl);
+
+                    // TODO: Don't block async. Consider making this an async call, and then calling DidFinish when done
+                    webView.OnProxyRequestMessage(args).Wait();
+
+                    if (args.ResponseStream != null)
+                    {
+                        contentType = args.ResponseContentType ?? "text/plain";
+                        contentStream = args.ResponseStream;
+                    }
+                }
+
+                if (contentStream == null)
+                {
+                    contentStream = KnownStaticFileProvider.GetKnownResourceStream(relativePath!);
+                }
 
                 if (contentStream is null)
                 {