WIP: extract loading logic from model

Loading logic should be extracted from the model, to make
loading the data from other sources, such as a local database for
synchronised items, possible.

core/include/JellyfinQt/support/loader.h

@@ -0,0 +1,90 @@
+#ifndef JELLYFIN_SUPPORT_LOADER_H
+#define JELLYFIN_SUPPORT_LOADER_H
+
+#include <exception>
+#include <stdexcept>
+
+#include <QJsonDocument>
+#include <QUrlQuery>
+#include <QString>
+
+#include <JellyfinQt/apiclient.h>
+
+namespace Jellfyin {
+namespace Support {
+
+
+class LoadException : public std::runtime_error {
+public:
+    explicit LoadException(const char *message)
+        : std::runtime_error(message) {}
+};
+
+static const int HTTP_TIMEOUT = 30000; // 30 seconds;
+
+/**
+ * Interface describing a way to load items. Used to abstract away
+ * the difference between loading from a cache or loading over the network.
+ *
+ * @tparam R the type of data that should be fetched, R for result.
+ * @tparam P the type of paramaters given, to determine which resource should
+ *           be loaded.
+ */
+template <typename R, typename P>
+class Loader {
+    using ApiClient = Jellyfin::ApiClient;
+public:
+    explicit Loader(ApiClient *apiClient) : m_apiClient(apiClient) {}
+    /**
+     * @brief load Loads the given resource.
+     * @param parameters Parameters to determine which resource should be loaded.
+     * @return The resource if successfull.
+     */
+    virtual R load(const P &parameters) const;
+    /**
+     * @brief Heuristic to determine if this resource can be loaded via this loaded.
+     *
+     * For example, a Loader that requires the network to be available should return false
+     * if the network is not available.
+     * @return True if this loader is available, false otherwise.
+     */
+    virtual bool isAvailable() const;
+protected:
+    ApiClient *m_apiClient;
+};
+
+template <typename R, typename P>
+class HttpLoader : public Loader<R, P> {
+public:
+    R load(const P &parameters) const override {
+        QNetworkReply *reply = m_apiClient->get(url(parameters), query(parameters));
+        QByteArray array;
+        while (!reply->atEnd()) {
+            if (!reply->waitForReadyRead(HTTP_TIMEOUT)) {
+                if (reply->error() == QNetworkReply::NoError) {
+                    reply->deleteLater();
+                    throw LoadException("HTTP timeout");
+                }
+                reply->deleteLater();
+                throw LoadException("HTTP error");
+            }
+            array.append(reply->readAll());
+        }
+        reply->deleteLater();
+        QJsonParseError error;
+        QJsonDocument document = QJsonDocument::fromJson(array, &error);
+        if (error.error != QJsonParseError::NoError) {
+            throw LoadException(error.errorString().toLocal8Bit().constData());
+        }
+        return parse(document);
+    }
+protected:
+    virtual const QString url(const P &parameters) const;
+    virtual const QUrlQuery query(const P &parameters) const;
+    virtual R parse(const QJsonObject &object) const;
+};
+
+} // NS Support
+} // NS Jellyfin
+
+#endif // JELLYFIN_SUPPORT_LOADER_H