Allow "default" headers to be defined for all responses. (me-no-dev#230)

* Add DefaultHeaders singleton.

* Add documentation for DefaultHeaders.

Author: tetious

README.md

@@ -77,7 +77,7 @@ To use this library you might need to have the latest git versions of [ESP32](ht
 	- [Setting up the server](#setting-up-the-server)
 		- [Setup global and class functions as request handlers](#setup-global-and-class-functions-as-request-handlers)
 		- [Methods for controlling websocket connections](#methods-for-controlling-websocket-connections)
-
+	- [Adding default headers to all responses](#adding-default-headers)
 ## Why should you care
 - Using asynchronous network means that you can handle more than one connection at the same time
 - You are called once the request is ready and parsed
@@ -1339,3 +1339,30 @@ Example of OTA code
   });
 
 ```
+
+### Adding Default Headers
+
+In some cases, such as when working with CORS, or with some sort of custom authentication system, 
+you might need to define a header that should get added to all responses (including static, websocket and EventSource).
+The DefaultHeaders singleton allows you to do this.
+
+Example:
+
+```arduino
+DefaultHeaders::Instance().addHeader("Access-Control-Allow-Origin", "*");
+webServer.begin();
+```
+
+*NOTE*: You will still need to respond to the OPTIONS method for CORS pre-flight in most cases. (unless you are only using GET)
+
+This is one option:
+
+```arduino
+webServer.onNotFound([](AsyncWebServerRequest *request) {
+	if (request->method() == HTTP_OPTIONS) {
+		request->send(200);
+	} else {
+		request->send(404);
+	}
+});
+```

src/ESPAsyncWebServer.h

@@ -414,6 +414,31 @@ class AsyncWebServer {
     void _rewriteRequest(AsyncWebServerRequest *request);
 };
 
+class DefaultHeaders {
+  using headers_t = LinkedList<AsyncWebHeader *>;
+  headers_t _headers;
+  
+  DefaultHeaders()
+  :_headers(headers_t([](AsyncWebHeader *h){ delete h; }))
+  {}
+public:
+  using ConstIterator = headers_t::ConstIterator;
+
+  void addHeader(const String& name, const String& value){
+    _headers.add(new AsyncWebHeader(name, value));
+  }  
+  
+  ConstIterator begin() const { return _headers.begin(); }
+  ConstIterator end() const { return _headers.end(); }
+
+  DefaultHeaders(DefaultHeaders const &) = delete;
+  DefaultHeaders &operator=(DefaultHeaders const &) = delete;
+  static DefaultHeaders &Instance() {
+    static DefaultHeaders instance;
+    return instance;
+  }
+};
+
 #include "WebResponseImpl.h"
 #include "WebHandlerImpl.h"
 #include "AsyncWebSocket.h"

src/WebResponses.cpp

@@ -94,7 +94,11 @@ AsyncWebServerResponse::AsyncWebServerResponse()
   , _ackedLength(0)
   , _writtenLength(0)
   , _state(RESPONSE_SETUP)
-{}
+{
+  for(auto header: DefaultHeaders::Instance()) {
+    _headers.add(new AsyncWebHeader(header->name(), header->value()));
+  }
+}
 
 AsyncWebServerResponse::~AsyncWebServerResponse(){
   _headers.free();