added async http request and removed old sync impl (#6)

# Description

In order to make http requests non-blocking the previous synchronous
calls are replaced with asynchronous ones. Additionally, the new
implementation in the http.js script utilizes async-await syntax, which
is easier to understand than the old XMLHttpRequest approach.

What does this PR do?
Fixes LimeChain#506

How were these changes implemented and what do they affect?
Changes have been made to the HttpRequest class and http.js script,
which are called by the JsonUtil class. These modifications will not be
visible to all users of the JsonUtil class.

---------

Co-authored-by: Georgi Grigorov <[email protected]>

src/main/java/com/limechain/teavm/HttpRequest.java

@@ -1,8 +1,36 @@
 package com.limechain.teavm;
 
+import lombok.extern.java.Log;
+import org.teavm.interop.Async;
+import org.teavm.interop.AsyncCallback;
 import org.teavm.jso.JSBody;
+import org.teavm.jso.JSFunctor;
+import org.teavm.jso.JSObject;
+import org.teavm.jso.core.JSError;
 
+import java.util.logging.Level;
+
+@Log
 public class HttpRequest {
-    @JSBody(params = {"method", "url", "body"}, script = "return httpRequestSync(method, url, body);")
-    public static native String httpRequestSync(String method, String url, String body);
+
+    @Async
+    public static native String asyncHttpRequest(String method, String url, String body);
+
+    private static void asyncHttpRequest(String method, String url, String body, AsyncCallback<String> callback) {
+        createAsyncHttpRequest(method, url, body, (error, response) -> {
+            if (error != null) {
+                log.log(Level.WARNING, error.getMessage());
+            } else {
+                callback.complete(response);
+            }
+        });
+    }
+
+    @JSBody(params = {"method", "url", "body", "callback"}, script = "return asyncHttpRequest(method, url, body, callback);")
+    public static native void createAsyncHttpRequest(String method, String url, String body, HttpRequestCallback callback);
+
+    @JSFunctor
+    private interface HttpRequestCallback extends JSObject {
+        void apply(JSError error, String response);
+    }
 }

src/main/java/com/limechain/utils/json/JsonUtil.java

@@ -4,6 +4,9 @@
 
 public class JsonUtil {
 
+    // Prevents instantiation
+    private JsonUtil() {}
+
     public static Object parseJson(String jsonString) {
         return new JsonParser(jsonString).parse();
     }
@@ -13,6 +16,6 @@ public static String stringify(Object object) {
     }
 
     public static String readJsonFromFile(String filePath) {
-        return HttpRequest.httpRequestSync("GET", filePath, null);
+        return HttpRequest.asyncHttpRequest("GET", filePath, null);
     }
 }

src/main/webapp/js/http.js

@@ -1,15 +1,22 @@
-function httpRequestSync(method, url, body) {
-    var xhr = new XMLHttpRequest();
-    xhr.open(method, url, false); // false for synchronous request
-    xhr.setRequestHeader('Content-Type', 'application/json');
-    if (method === 'POST' && body) {
-        xhr.send(body);
-    } else {
-        xhr.send();
-    }
-    if (xhr.status === 200) {
-        return xhr.responseText;
-    } else {
-        throw new Error('Request failed with status ' + xhr.status);
+async function asyncHttpRequest(method = 'GET', url, body = null, callback) {
+    try {
+        const response = await fetch(url, {
+            method: method,
+            headers: {
+                'Content-Type': 'application/json'
+            },
+            body: method === 'POST' ? body : undefined
+        });
+
+        if (!response.ok) {
+            callback(new Error(`Request failed with status: ${response?.status}`), null);
+            return;
+        }
+
+        const result = await response.text();
+        callback(null, result);
+
+    } catch (error) {
+        callback(new Error(`Error during sending request: ${error?.message}`), null);
     }
 }