Tracking of update time in DownloadState

If download info becomes stale (due to slow responses from aria)
the download speed is shown as ---.

Known issues:
  - A stale download is refreshed only due to a GUI event (such
    as scrolling or mouse hover).

src/contentmanagerdelegate.cpp

@@ -129,7 +129,7 @@ void showDownloadProgress(QPainter *painter, QRect box, const DownloadState& dow
     double progress  = (double) (downloadInfo.progress) / 100;
     progress = -progress;
     auto completedLength = downloadInfo.completedLength;
-    auto downloadSpeed = downloadInfo.downloadSpeed;
+    auto downloadSpeed = downloadInfo.getDownloadSpeed();
 
     if (downloadInfo.status == DownloadState::PAUSED) {
         createResumeSymbol(painter, dcl.pauseResumeButtonRect);

src/downloadmanagement.cpp

@@ -37,15 +37,29 @@ DownloadState::Status getDownloadStatus(QString status)
 
 } // unnamed namespace
 
-void DownloadState::update(const DownloadInfo& downloadInfos)
+void DownloadState::update(const DownloadInfo& info)
 {
-    double percent = downloadInfos["completedLength"].toDouble() / downloadInfos["totalLength"].toDouble();
-    percent *= 100;
-    percent = QString::number(percent, 'g', 3).toDouble();
-    auto completedLength = convertToUnits(downloadInfos["completedLength"].toDouble());
-    auto downloadSpeed = convertToUnits(downloadInfos["downloadSpeed"].toDouble()) + "/s";
-    const auto status = getDownloadStatus(downloadInfos["status"].toString());
-    *this = {percent, completedLength, downloadSpeed, status};
+    const auto completedBytes = info["completedLength"].toDouble();
+    const double percentage = completedBytes / info["totalLength"].toDouble();
+
+    progress = QString::number(100 * percentage, 'g', 3).toDouble();
+    completedLength = convertToUnits(completedBytes);
+    downloadSpeed = convertToUnits(info["downloadSpeed"].toDouble()) + "/s";
+    status = getDownloadStatus(info["status"].toString());
+    lastUpdated = std::chrono::steady_clock::now();
+}
+
+double DownloadState::timeSinceLastUpdate() const
+{
+    typedef std::chrono::duration<double> Seconds;
+
+    const auto dt = std::chrono::steady_clock::now() - lastUpdated;
+    return std::chrono::duration_cast<Seconds>(dt).count();
+}
+
+QString DownloadState::getDownloadSpeed() const
+{
+    return timeSinceLastUpdate() > 2.0 ? "---" : downloadSpeed;
 }
 
 ////////////////////////////////////////////////////////////////////////////////

src/downloadmanagement.h

@@ -8,6 +8,7 @@
 #include <QString>
 #include <QVariant>
 
+#include <chrono>
 #include <memory>
 
 #include <kiwix/downloader.h>
@@ -18,7 +19,7 @@ typedef QMap<QString, QVariant> DownloadInfo;
 
 class DownloadState
 {
-public:
+public: // types
     enum Status {
         UNKNOWN,
         WAITING,
@@ -27,13 +28,23 @@ public:
         PAUSED
     };
 
+
+public: // data
+
     double progress = 0;
     QString completedLength;
-    QString downloadSpeed;
     Status status = UNKNOWN;
 
-public:
+public: // functions
     void update(const DownloadInfo& info);
+    QString getDownloadSpeed() const;
+
+    // time in seconds since last update
+    double timeSinceLastUpdate() const;
+
+private: // data
+    QString downloadSpeed;
+    std::chrono::steady_clock::time_point lastUpdated;
 };
 
 class DownloadManager : public QObject