Added handler for correct JSON deserialization, addresses #195 (#287)

* Added handler for correct JSON deserialization (even with multiple body data writes), addresses #195

* Fixed typo

* Added JSON handler example to readme

* Moved Json Handler uri parameter to constructor

* Use request->_tempObject for JSON char data buffer

* Moved onRequest to constructor in JSON async handler

README.md

@@ -30,6 +30,7 @@ To use this library you might need to have the latest git versions of [ESP32](ht
 		- [GET, POST and FILE parameters](#get-post-and-file-parameters)
 		- [FILE Upload handling](#file-upload-handling)
 		- [Body data handling](#body-data-handling)
+		- [JSON body handling with ArduinoJson](#json-body-handling-with-arduinojson)
 	- [Responses](#responses)
 		- [Redirect to another URL](#redirect-to-another-url)
 		- [Basic response with HTTP Code](#basic-response-with-http-code)
@@ -313,6 +314,20 @@ void handleBody(AsyncWebServerRequest *request, uint8_t *data, size_t len, size_
   }
 }
 ```
+If needed, the `_tempObject` field on the request can be used to store a pointer to temporary data (e.g. from the body) associated with the request. If assigned, the pointer will automatically be freed along with the request.
+
+### JSON body handling with ArduinoJson
+Endpoints which consume JSON can use a special handler to get ready to use JSON data in the request callback:
+```cpp
+#include "AsyncJson.h"
+#include "ArduinoJson.h"
+
+AsyncCallbackJsonWebHandler* handler = new AsyncCallbackJsonWebHandler("/rest/endpoint", [](AsyncWebServerRequest *request, JsonVariant &json) {
+  JsonObject& jsonObj = json.as<JsonObject>();
+  // ...
+});
+server.addHandler(handler);
+```
 
 ## Responses
 ### Redirect to another URL

src/AsyncJson.h

@@ -1,9 +1,9 @@
-// ESPasyncJson.h
+// AsyncJson.h
 /*
-  Async Response to use with arduinoJson and asyncwebserver
+  Async Response to use with ArduinoJson and AsyncWebServer
   Written by Andrew Melvin (SticilFace) with help from me-no-dev and BBlanchon.
 
-  example of callback in use
+  Example of callback in use
 
    server.on("/json", HTTP_ANY, [](AsyncWebServerRequest * request) {
 
@@ -17,11 +17,27 @@
     request->send(response);
   });
 
+  --------------------
+
+  Async Request to use with ArduinoJson and AsyncWebServer
+  Written by Arsène von Wyss (avonwyss)
+
+  Example
+
+  AsyncCallbackJsonWebHandler* handler = new AsyncCallbackJsonWebHandler("/rest/endpoint");
+  handler->onRequest([](AsyncWebServerRequest *request, JsonVariant &json) {
+    JsonObject& jsonObj = json.as<JsonObject>();
+    // ...
+  });
+  server.addHandler(handler);
+  
 */
 #ifndef ASYNC_JSON_H_
 #define ASYNC_JSON_H_
 #include <ArduinoJson.h>
 
+const char* JSON_MIMETYPE = "application/json";
+
 /*
  * Json Response
  * */
@@ -57,7 +73,7 @@ class AsyncJsonResponse: public AsyncAbstractResponse {
   public:
     AsyncJsonResponse(bool isArray=false): _isValid{false} {
       _code = 200;
-      _contentType = "application/json";
+      _contentType = JSON_MIMETYPE;
       if(isArray)
         _root = _jsonBuffer.createArray();
       else
@@ -80,4 +96,68 @@ class AsyncJsonResponse: public AsyncAbstractResponse {
       return len;
     }
 };
+
+typedef std::function<void(AsyncWebServerRequest *request, JsonVariant &json)> ArJsonRequestHandlerFunction;
+
+class AsyncCallbackJsonWebHandler: public AsyncWebHandler {
+private:
+protected:
+  const String _uri;
+  WebRequestMethodComposite _method;
+  ArJsonRequestHandlerFunction _onRequest;
+  int _contentLength;
+  int _maxContentLength;
+public:
+  AsyncCallbackJsonWebHandler(const String& uri, ArJsonRequestHandlerFunction onRequest) : _uri(uri), _method(HTTP_POST|HTTP_PUT|HTTP_PATCH), _onRequest(onRequest), _maxContentLength(16384) {}
+  void setMethod(WebRequestMethodComposite method){ _method = method; }
+  void setMaxContentLength(int maxContentLength){ _maxContentLength = maxContentLength; }
+  void onRequest(ArJsonRequestHandlerFunction fn){ _onRequest = fn; }
+
+  virtual bool canHandle(AsyncWebServerRequest *request) override final{
+    if(!_onRequest)
+      return false;
+
+    if(!(_method & request->method()))
+      return false;
+
+    if(_uri.length() && (_uri != request->url() && !request->url().startsWith(_uri+"/")))
+      return false;
+
+    if (!request->contentType().equalsIgnoreCase(JSON_MIMETYPE))
+      return false;
+
+    request->addInterestingHeader("ANY");
+    return true;
+  }
+
+  virtual void handleRequest(AsyncWebServerRequest *request) override final {
+    if(_onRequest) {
+      if (request->_tempObject != NULL) {
+        DynamicJsonBuffer jsonBuffer;
+        JsonVariant json = jsonBuffer.parse((uint8_t*)(request->_tempObject));
+        if (json.success()) {
+          _onRequest(request, json);
+          return;
+        }
+      }
+      request->send(_contentLength > _maxContentLength ? 413 : 400);
+    } else {
+      request->send(500);
+    }
+  }
+  virtual void handleUpload(AsyncWebServerRequest *request, const String& filename, size_t index, uint8_t *data, size_t len, bool final) override final {
+  }
+  virtual void handleBody(AsyncWebServerRequest *request, uint8_t *data, size_t len, size_t index, size_t total) override final {
+    if (_onRequest) {
+      _contentLength = total;
+      if (total > 0 && request->_tempObject == NULL && total < _maxContentLength) {
+        request->_tempObject = malloc(total);
+      }
+      if (request->_tempObject != NULL) {
+        memcpy((uint8_t*)(request->_tempObject) + index, data, len);
+      }
+    }
+  }
+  virtual bool isRequestHandlerTrivial() override final {return _onRequest ? false : true;}
+};
 #endif