Reputation: 14632
I have a Qt GUI class preferencesWindow
that, obviously, is responsible for handling the user preferences. I have some fields that manage the connection to a database server. When a field is left, dbsChanged()
method is called. Below is some code I managed to write:
void preferencesWindow::dbsChanged() {
QFuture<QStringList> loader = run(this, &preferencesWindow::get_databases);
QStringList databases = loader.result();
if (databases.length()) {
this->ui.database->show();
this->ui.nodb_label->hide();
this->ui.database->clear();
this->ui.database->addItems(databases);
this->ui.okButton->setDisabled(false);
this->ui.validationStatus->setPixmap(QPixmap(":/icon/tick.png"));
} else {
this->ui.database->hide();
this->ui.nodb_label->show();
this->ui.okButton->setDisabled(true);
this->ui.validationStatus->setPixmap(QPixmap(":/icon/error.png"));
}
}
QStringList preferencesWindow::get_databases() {
QSqlDatabase test_connection;
if (QSqlDatabase::contains("PREFEREMCES_LIVE_TEST_CONNECTION"))
test_connection = QSqlDatabase::database("PREFEREMCES_LIVE_TEST_CONNECTION");
else test_connection = QSqlDatabase::addDatabase("QMYSQL", "PREFEREMCES_LIVE_TEST_CONNECTION");
test_connection.setHostName(this->ui.serverAddress->text());
test_connection.setUserName(this->ui.username->text());
test_connection.setPassword(this->ui.password->text());
test_connection.setDatabaseName(this->ui.database->currentText());
test_connection.setPort(this->ui.serverPort->value());
test_connection.open();
qDebug() << "Error: " << test_connection.lastError();
QSqlQuery show_databases = test_connection.exec("show databases");
QStringList databases;
while (show_databases.next()) {
databases.append(show_databases.value(0).toString());
}
QSqlDatabase::removeDatabase("PREFERENCES_LIVE_TEST_CONNECTION");
return databases;
}
Since get_databases
can take a long time, I thought that putting in on a separate thread as you can see in these two lines:
QFuture<QStringList> loader = run(this, &preferencesWindow::get_databases);
QStringList databases = loader.result();
could solve the problem. It runs on a separate thread, but it still freezes the GUI (while working).
How should I rewrite this entire process? I though of some solutions, but I am not really sure about their performance, and I don't want to work uselessly...
Upvotes: 1
Views: 770
Reputation: 14632
You can use QFutureWatcher
to monitor that status of the QFuture
object, like written in the documentation:
// Instantiate the objects and connect to the finished signal.
MyClass myObject;
QFutureWatcher<int> watcher;
connect(&watcher, SIGNAL(finished()), &myObject, SLOT(handleFinished()));
// Start the computation.
QFuture<int> future = QtConcurrent::run(...);
watcher.setFuture(future);
Upvotes: 1
Reputation: 2292
The QFuture will wait until the thread sets the result when your call loader.result()
. You have to wait for that value later.
I guess you could store the future object as member of preferencesWindow
and send yourself a signal
, when finishing get_databases
. So you give your application time to process other events during this wait time.
Upvotes: 2
Reputation: 409482
It freezes the GUI because even though the get_databases
call is in a separate thread, you still wait for the results which causes the freeze.
I don't know how to do it in Qt, but the normal thing would be to open a dialog saying "please wait" or something with a cancel button, and have the worker thread send a signal to the parent (GUI) thread when done.
Upvotes: 3