afreerunner · January 9, 2019 02:20 · Jan 7, 2017
diff --git a/main.cpp b/main.cpp
@@ -0,0 +1,100 @@
+#include <QtWidgets>
+#include <QtSql>
+
+//to ensure the database connection is closed and removed at the end
+//of the write/read func call
+class SQLConnectionRAIIWrapper{
+public:
+    SQLConnectionRAIIWrapper(QString fileName){
+        //use a unique connection name, to ensure thread safety
+        connectionName = QString("SQLSETTINGS%1").arg((int)QThread::currentThreadId());
+        QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE", connectionName);
+        db.setDatabaseName(fileName);
+        db.open();
+    }
+    ~SQLConnectionRAIIWrapper(){
+        getDb().close();
+        QSqlDatabase::removeDatabase(connectionName);
+    }
+    QSqlDatabase getDb(){ return QSqlDatabase::database(connectionName); }
+private:
+    QString connectionName;
+};
+
+//write function creates a temporary file, stores
+//map in it (using sqlite), writes data from the temp file into the device
+bool writeSqliteFunc(QIODevice& device, const QSettings::SettingsMap& map){
+    QTemporaryFile tempFile;
+    if(!tempFile.open()) return false;
+    QString tempFileName = tempFile.fileName();
+    tempFile.close();
+    {
+        SQLConnectionRAIIWrapper connectionWrapper(tempFileName);
+        QSqlDatabase db = connectionWrapper.getDb();
+        if(!db.isOpen()) return false;
+        QSqlQuery query(db);
+        if(!query.exec("CREATE TABLE IF NOT EXISTS settings(key TEXT PRIMARY KEY, value TEXT);"))
+            return false;
+        query.prepare("REPLACE INTO settings(key, value) VALUES(?, ?)");
+        QVariantList keys;
+        for(QVariant key: map.keys()) keys.append(key);
+        query.addBindValue(keys);
+        query.addBindValue(map.values());
+        if(!query.execBatch()) return false;
+    }
+    if(!tempFile.open()) return false;
+    auto bytesWritten = device.write(tempFile.readAll());
+    tempFile.close();
+    return bytesWritten > 0;
+}
+
+//uses a similar strategy to read the data from a temporary file into the map
+bool readSqliteFunc(QIODevice& device, QSettings::SettingsMap& map){
+    QTemporaryFile tempFile;
+    if(!tempFile.open()) return false;
+    QString tempFileName = tempFile.fileName();
+    tempFile.write(device.readAll());
+    tempFile.close();
+
+    SQLConnectionRAIIWrapper connectionWrapper(tempFileName);
+    QSqlDatabase db = connectionWrapper.getDb();
+    if(!db.isOpen()) return false;
+    QSqlQuery query(db);
+    if(!query.exec("SELECT key, value FROM settings;"))
+        return false;
+    while(query.next()){
+        QString key = query.record().value("key").toString();
+        QVariant value = query.record().value("value");
+        map[key]= value;
+    }
+    return true;
+}
+
+int main(int argc, char* argv[]){
+    QApplication a(argc, argv);
+
+    const QSettings::Format sqliteFormat = QSettings::registerFormat("sqlite", &readSqliteFunc, &writeSqliteFunc);
+
+    QWidget w;
+    QFormLayout layout(&w);
+    QLineEdit lineEditA;
+    QLineEdit lineEditB;
+    QPushButton buttonSave("Save");
+    layout.addRow("KeyA:", &lineEditA);
+    layout.addRow("KeyB:", &lineEditB);
+    layout.addRow(&buttonSave);
+
+    //read settings
+    QSettings settings("settings.sqlite", sqliteFormat);
+    lineEditA.setText(settings.value("KeyA").toString());
+    lineEditB.setText(settings.value("KeyB").toString());
+    QObject::connect(&buttonSave, &QPushButton::clicked, [&]{
+        //save settings
+        QSettings settings("settings.sqlite", sqliteFormat);
+        settings.setValue("KeyA", lineEditA.text());
+        settings.setValue("KeyB", lineEditB.text());
+    });
+    w.show();
+
+    return a.exec();
+}