use a file source object to abstract loading of resources

src/util/filesource.cpp

@@ -1,6 +1,9 @@
 #include <mbgl/util/filesource.hpp>
 #include <mbgl/platform/platform.hpp>
 
+#include <fstream>
+#include <sstream>
+
 namespace mbgl {
 
 FileSource::FileSource(const std::shared_ptr<uv::loop> &loop)
@@ -18,10 +21,39 @@ const std::string &FileSource::getBase() const {
 void FileSource::load(ResourceType type, const std::string &url, std::function<void(platform::Response *)> callback) {
     // convert relative URLs to absolute URLs
 
-    // try to load from disk (not yet)
+    const std::string absoluteURL = [&]() -> std::string {
+        const size_t separator = url.find("://");
+        if (separator == std::string::npos) {
+            // Relative URL.
+            return base + url;
+        } else {
+            return url;
+        }
+    }();
 
-    // load from the internet
-    platform::request_http(url, callback, loop);
+    const size_t separator = absoluteURL.find("://");
+    const std::string protocol = separator != std::string::npos ? absoluteURL.substr(0, separator) : "";
+
+    if (protocol == "file") {
+        // load from disk
+        const std::string path = absoluteURL.substr(separator + 3);
+        std::ifstream file(path);
+
+        platform::Response response(callback);
+        if (!file.good()) {
+            response.error_message = "file not found (" + path + ")";
+        } else {
+            std::stringstream data;
+            data << file.rdbuf();
+            response.code = 200;
+            response.body = data.str();
+        }
+
+        callback(&response);
+    } else {
+        // load from the internet
+        platform::request_http(absoluteURL, callback, loop);
+    }
 }
 
 }