HTTP 302 and 304 Support

Add support for http redirection (302) and http not modified (304) to
reduce the load the server.
server.redirect(“url”, “location”, exclude-ip) will respond with 302 to
redirect the browser to a different url, this is useful for backward
compatibility and to redirect call to CDN when not no AP mode.
server.serveStatic has a new optional parameter to get the
Last-Modified date for all files serve for this location, when the
browser request have the same If-Modified-Since header value, the
server respond with 304 code instead of serving the file.

src/ESPAsyncWebServer.h

@@ -297,7 +297,9 @@ class AsyncWebServer {
     void on(const char* uri, WebRequestMethod method, ArRequestHandlerFunction onRequest, ArUploadHandlerFunction onUpload);
     void on(const char* uri, WebRequestMethod method, ArRequestHandlerFunction onRequest, ArUploadHandlerFunction onUpload, ArBodyHandlerFunction onBody);
 
-    void serveStatic(const char* uri, fs::FS& fs, const char* path, const char* cache_header = NULL);
+    void redirect(const char* url, const char* location, const uint32_t exclude_ip = 0);
+
+    void serveStatic(const char* uri, fs::FS& fs, const char* path, const char* cache_header = NULL, const char* modified_header = NULL);
 
     void onNotFound(ArRequestHandlerFunction fn);  //called when handler is not assigned
     void onFileUpload(ArUploadHandlerFunction fn); //handle file uploads

src/WebHandlerImpl.h

@@ -24,6 +24,18 @@
 
 #include "stddef.h"
 
+class RedirectWebHandler: public AsyncWebHandler {
+  protected:
+    String _url;
+    String _location;
+    uint32_t _exclude_ip;
+  public:
+    RedirectWebHandler(const char* url, const char* location, uint32_t exclude_ip)
+      : _url(url), _location(location) { _exclude_ip = exclude_ip; }
+    bool canHandle(AsyncWebServerRequest *request);
+    void handleRequest(AsyncWebServerRequest *request);
+};
+
 class AsyncStaticWebHandler: public AsyncWebHandler {
   private:
     String _getPath(AsyncWebServerRequest *request); 
@@ -32,10 +44,11 @@ class AsyncStaticWebHandler: public AsyncWebHandler {
     String _uri;
     String _path;
     String _cache_header;
+    String _modified_header;
     bool _isFile;
   public:
-    AsyncStaticWebHandler(FS& fs, const char* path, const char* uri, const char* cache_header)
-      : _fs(fs), _uri(uri), _path(path), _cache_header(cache_header){
+    AsyncStaticWebHandler(FS& fs, const char* path, const char* uri, const char* cache_header, const char* modified_header)
+      : _fs(fs), _uri(uri), _path(path), _cache_header(cache_header), _modified_header(modified_header) {
 
       _isFile = _fs.exists(path) || _fs.exists((String(path)+".gz").c_str());
       if (_uri != "/" && _uri.endsWith("/")) {

src/WebHandlers.cpp

@@ -21,6 +21,25 @@
 #include "ESPAsyncWebServer.h"
 #include "WebHandlerImpl.h"
 
+
+bool RedirectWebHandler::canHandle(AsyncWebServerRequest *request)
+{
+  // We can redirect when the request url match and ip doesn't match
+  if (request->url() == _url && _exclude_ip != request->client()->localIP()) {
+    DEBUGF("[RedirectWebHandler::canHandle] TRUE\n");
+    return true;
+  }
+  return false;
+}
+
+void RedirectWebHandler::handleRequest(AsyncWebServerRequest *request)
+{
+  AsyncWebServerResponse *response = request->beginResponse(302);
+  response->addHeader("Location", _location);
+  request->send(response);
+}
+
+
 bool AsyncStaticWebHandler::canHandle(AsyncWebServerRequest *request)
 {
   if (request->method() != HTTP_GET) {
@@ -33,7 +52,10 @@ bool AsyncStaticWebHandler::canHandle(AsyncWebServerRequest *request)
   if (request->url().startsWith(_uri)) {
     String path = _getPath(request);
     if (_fs.exists(path) || _fs.exists(path + ".gz")) {
-       DEBUGF("[AsyncStaticWebHandler::canHandle] TRUE\n");
+      if (_modified_header.length() != 0) {
+        request->addInterestingHeader("If-Modified-Since");
+      }
+      DEBUGF("[AsyncStaticWebHandler::canHandle] TRUE\n");
       return true;
     }
   }
@@ -82,14 +104,19 @@ void AsyncStaticWebHandler::handleRequest(AsyncWebServerRequest *request)
   String path = _getPath(request);
 
   if (_fs.exists(path) || _fs.exists(path + ".gz")) {
-    AsyncWebServerResponse * response = request->beginResponse(_fs, path);
-    if (_cache_header.length() != 0)
-      response->addHeader("Cache-Control", _cache_header);
-    request->send(response);
+    if (_modified_header.length() != 0 && _modified_header == request->header("If-Modified-Since")) {
+      request->send(304); // Sed not modified
+    } else {
+      AsyncWebServerResponse * response = request->beginResponse(_fs, path);
+      if (_modified_header.length() !=0)
+        response->addHeader("Last-Modified", _modified_header);
+      if (_cache_header.length() != 0)
+        response->addHeader("Cache-Control", _cache_header);
+      request->send(response);
+    }
   } else {
     request->send(404);
   }
   path = String();
 
-
 }

src/WebServer.cpp

@@ -104,8 +104,12 @@ void AsyncWebServer::on(const char* uri, ArRequestHandlerFunction onRequest){
   addHandler(handler);
 }
 
-void AsyncWebServer::serveStatic(const char* uri, fs::FS& fs, const char* path, const char* cache_header){
-  addHandler(new AsyncStaticWebHandler(fs, path, uri, cache_header));
+void AsyncWebServer::redirect(const char* url, const char* location, const uint32_t exclude_ip){
+  addHandler(new RedirectWebHandler(url, location, exclude_ip));
+}
+
+void AsyncWebServer::serveStatic(const char* uri, fs::FS& fs, const char* path, const char* cache_header, const char* modified_header){
+  addHandler(new AsyncStaticWebHandler(fs, path, uri, cache_header, modified_header));
 }
 
 void AsyncWebServer::onNotFound(ArRequestHandlerFunction fn){