do 304 requests and cache them in sqlite

src/storage/base_request.cpp

@@ -0,0 +1,66 @@
+#include <mbgl/storage/base_request.hpp>
+#include <mbgl/storage/response.hpp>
+#include <mbgl/storage/request.hpp>
+
+#include <uv.h>
+
+#include <cassert>
+
+namespace mbgl {
+
+BaseRequest::BaseRequest() : thread_id(uv_thread_self()) {
+}
+
+BaseRequest::~BaseRequest() {
+    assert(thread_id == uv_thread_self());
+}
+
+void BaseRequest::notify() {
+    assert(thread_id == uv_thread_self());
+
+    // The parameter exists solely so that any calls to ->remove()
+    // are not going to cause deallocation of this object while this call is in progress.
+    std::shared_ptr<BaseRequest> retain = self;
+
+    // Swap the lists so that it's safe for callbacks to call ->cancel()
+    // on the request object, which would modify the list.
+    std::forward_list<std::unique_ptr<Callback>> list;
+    list.swap(callbacks);
+
+    // Only notify when we actually have a response. When the response code is 0, the request was
+    // canceled and we don't need to notify.
+    if (response) {
+        // Now that we have our private copy, notify all observers.
+        for (std::unique_ptr<Callback> &callback : list) {
+            (*callback)(*response);
+        }
+    }
+
+    self.reset();
+}
+
+Callback *BaseRequest::add(Callback &&callback, const std::shared_ptr<BaseRequest> &request) {
+    assert(thread_id == uv_thread_self());
+    assert(this == request.get());
+    if (response) {
+        // We already have a response. Notify right away.
+        callback(*response);
+        return nullptr;
+    } else {
+        self = request;
+        callbacks.push_front(std::unique_ptr<Callback>(new Callback(std::move(callback))));
+        return callbacks.front().get();
+    }
+}
+
+void BaseRequest::remove(Callback *callback) {
+    assert(thread_id == uv_thread_self());
+    callbacks.remove_if([=](const std::unique_ptr<Callback> &cb) {
+        return cb.get() == callback;
+    });
+    if (callbacks.empty()) {
+        self.reset();
+    }
+}
+
+}