I've been trying to start with a simple app that retrieves data from a simple HTML page upon clicking a button and stumbled upon a rather helpful tutorial on QT-Project and have been trying to implement it for my own project.
Everything manages to compile until I try to actually try to implement the loadImage function (as found in the tutorial). (I actually had to initialize m_pImgCtrl as Filedownloader * m_pImgCtrl = new FileDownloader(imageUrl, this); and I'm not exactly sure how it's suppose to work without prior object declaration?)
From what I get, m_pImgCtrl isn't actually defined in the loadImage() function since it is initialized outside of the function? Or does the connect() function do something that I'm not too aware of?
Thanks for the help!
The tutorial doesn't tell you the whole story.
The code in section Usage is supposed to be part of a class MainWindow – the controller of your main window (see line 1 of tutorial's last snippet). This class contains a slot loadImaged() called when the NetworkReply has finished. It also has a member FileDownloader * m_pImgCtrl.
For instance, the second Usage snippet could be part of a slot MainWindow::buttonClicked() like
void MainWindow::buttonClicked()
{
QUrl imageUrl("http://qt.digia.com/Documents/1/QtLogo.png");
m_pImgCtrl = new FileDownloader(imageUrl, this);
connect(m_pImgCtrl, SIGNAL(downloaded()), SLOT(loadImage()));
}
Related
I am currently working on a Qt project where dynamic plugin loading is central. I am loading DLLs by using Qt's QPluginLoader. The different plugins are accessed through CameraPluginStructs, defined as follows:
struct CameraPluginStruct
{
QString name;
QString filePath;
CameraPluginInterface* plugin;
QPluginLoader* pluginLoader;
CameraPluginStruct(QString filePath=nullptr, QString name=nullptr):
filePath(filepath), name(name), plugin(nullptr), pluginLoader(nullptr){}
};
To load a plugin, the following function is called:
void loadPlugin(CameraPluginStruct& p)
{
p.pluginLoader = new QPluginLoader(p.filePath);
p.pluginLoader->load();
QObject* possiblePlugin = p.pluginLoader->instance(); //QPluginLoader.instance();
if(possiblePlugin)
{
// cast from QObject to correct type:
p.plugin = qobjectcast<CameraPluginInterface>(possiblePlugin);
}
}
And to unload a plugin, I use this function:
void unloadPlugin(CameraPluginStruct& p)
{
p.pluginLoader->unload(); // QPluginLoader.unload();
p.pluginLoader->~QPluginLoader();
p.pluginLoader = nullptr;
p.plugin = nullptr;
}
I have made a few, simple test plugins that write messages to the console when the constructor is called. For simplicity, let's say that I have two test plugins, DLL A and DLL B. When I load A by using the loadPlugin() function, the constructor in the plugin is called, and the corresponding message is written in the console. I can do the same thing with B, and everything seems to work – B's constructor message is written to the console, and the other functions seem to work as well.
Now, the problem occurs when I try to create another CameraPluginStruct connected to either A or B. No message is written to the console, which leads me to think that the constructor is not called. Even though, I am able to call the other test functions in the plugin with success (DoSomething(), see below). If I unload all CameraPlugins connected to A or B, and then load the DLL again, the constructor is called again on the first load.
The QPluginLoader.instance() call is described as follows in the documentation:
"Returns the root component object of the plugin. (...) The component object is a QObject. Use qobject_cast() to access interfaces you are
interested in." http://doc.qt.io/qt-5/qpluginloader.html#instance
Wouldn't it then be natural that the constructor in the DLL would be called each time, and not only the first time?
As I've understood, a DLL is only loaded once for any program. Therefore I have also tried to use only one QPluginLoader per DLL file, with the same result. Qt also says that:
"Multiple instances of QPluginLoader can be used to access the same physical plugin." http://doc.qt.io/qt-5/qpluginloader.html#details
Therefore I can't see how this can be a source to the problem anyway.
I would really appreciate if anyone could clarify how the QPluginLoader.instance() really works. Why is the constructor – at least how it seems – only called the first time I use the instance() call?
Thank you!
Here is the code found in the DLLs (the output texts differ in A and B):
TestDLL::TestDLL()
{
std::cout << "This is written from the constructor in A \n";
}
QString TestDLL::Name() const
{
return "Hello, writing from Name() \n";
}
void TestDLL::DoSomething() const
{
qDebug() << "Hello, this text comes from DoSomething()"\n;
}
When your plugin is loaded (i.e. the first time QPluginLoader::instance() is called), then a single instance of it is created - this is your root instance. The root instance is the only instance QPluginLoader will ever create for you.
If you want more, then you create a createInstance() or clone() method for your plugin class, so that new instances can be created from the root instance. Or more conventionally, make your plugin class a factory for the class type you wish to expose.
I added a new method to the CustVendPaym class called sendersBankCompanyStatementName of type BankCompanyStatementName.
This is the code of said method:
public BankCompanyStatementName sendersBankCompanyStatementName(BankCompanyStatementName _sendersBankCompanyStatementName = sendersBankCompanyStatementName)
{
sendersBankCompanyStatementName = _sendersBankCompanyStatementName;
return sendersBankCompanyStatementName;
}
I added the definition in the classDeclaration method:
BankCompanyStatementName sendersBankCompanyStatementName;
Then in the method vendPaym in the VendOutPaym class, a new instance of VendPaym (which extends CustVendPaym) is created:
vendPaym = new VendPaym();
//A bunch of properties are set then one I created:
vendPaym.sendersBankCompanyStatementName (bankAccountTable.BankCompanyStatementName);
If I break there, I see the assignment with the value I'm expecting working correctly, but then the debugger (watch) never actually shows the new property I added with the value that's supposed to be in it.
Then if I just continue code execution, the AOS server in which I'm developing just crashes :|
Any ideas, am I doing something obviously wrong ?
Thanks.
EDIT: If I rollback my changes (that is deleting the newly added method and removing any references to it) everything works as it was before.
Have you compiled forward the CustVendPaym class?
I have 2 classes: one maintains some loop (at leas for 2-3 minutes; and is inherited from QObject) and another shows up a progress dialog (inherited from QDialog).
I want to start the loop as soon as the dialog is shown. My first solution was:
int DialogClass::exec()
{
QTimer::singleShot(0, LoopClassPointer, SLOT(start()));
return __super::exec();
}
There is a problem with throwing exceptions from slots. so I considered a possibility to make public slot start() just a public function. But now I don't know how to make it works well. Things like this:
int DialogClass::exec()
{
LoopClassPointer->start();
QApplication::processEvents();
return __super::exec();
}
don't help. The dialog doesn't appears.
Is there a common approach to this kind of situations?
some details, according to questions:
I have to work with system with its own styles, so we have a common approach in creating any dialogs: to inherit them from stytle class, which is inherited from QDialog.
my 'LoopClassPointer' is an exported class from separate dll (there is no UI support in it).
I have a 'start' button in main app, which connected with a slot, which creates progress dialog and 'LoopClassPointer'. at the moment I send 'LoopClassPointer' instance in the dialog and don't whant to make significant changes in the architecture.
Take a look at QtDemo->Concurrent Programming->Run function
e.g. in Qt 4.8: http://qt-project.org/doc/qt-4.8/qtconcurrent-runfunction.html
In this situation, I recommend you separate the logic of the loop from the dialog. Gui elements should always be kept separate.
It's great that your worker class is derived from QObject because that means you can start it running on a separate thread: -
QThread* m_pWorkerThread = new QThread;
Worker* m_pWorkerObject = new Worker; // assuming this object runs the loop mentioned
// Qt 5 connect syntax
connect(m_pWorkerThread, &QThread::started, m_pWorkerObject, &WorkerObject::start);
connect(m_pWorkerThread, &QThread::finished, m_pWorkerThread, &QThread::deleteThis);
m_pWorkerObject->moveToThread(m_pWorkerThread);
m_pWorkerThread->start();
If you're not familiar with using QThread, then start by reading this.
The only other thing you require is to periodically send signals from your worker object with progress of its work and connect that to a slot in the dialog, which updates its display of the progress.
I have a question about accessing QML components on the C++ side, sorry it might be little beginner.
I am currently working with the sample map view, the app alone works great, but after I try to put this sample together with a navigation panel, it stopped working.
I have added my own main.qml, another page called menu.qml, and renamed the app's original main.qml to map.qml. So now user would require to navigate the panel like this: main.qml -> menu.qml -> map.qml
My problem is:
Originally, from the sample, app constructor saves a mapView pointer when it creates main.qml so later it can add markers/change location to the mapview.
It gets mapView pointer by calling:
QmlDocument *qml = QmlDocument::create("asset:///main.qml").parent(this);
qml->setContextProperty("_mapViewTest", this);
AbstractPane *root = qml->createRootObject<AbstractPane>();
QObject* mapViewAsQObject = root->findChild<QObject*>(QString("mapViewObj"));
mapView = qobject_cast<bb::cascades::maps::MapView*>(mapViewAsQObject);
However, I have changed the name to map.qml, findChild returned as null. Since my app is unable to find mapView, the whole app crushes when I try to set a marker/locate myself.
I tried to QmlDocument::create("asset:///map.qml"), and use the same method as above to find the mapview, and I was able to get it, but nothing happens when I try to put down a marker/locate.
I have also tried to add a new function that calls findChild and get the mapView pointer. The function gets called in onCreationCompleted inside the map.qml page, but mapViewAsQObject still returned null and app crushed.
Any hints would be really appreciated! Thanks
I have found the solution to my problem, when the next button is clicked, I have to nav push the page using C++, this way it can find the right mapView pointer.
QmlDocument *qml = QmlDocument::create("asset:///map.qml").parent(this);
qml->setContextProperty("_mapViewTest", this);
Page *root = qml->createRootObject<Page>();
QObject* mapViewAsQObject = root->findChild<QObject*>(QString("mapViewObj"));
... etc
nav->push(root);
I'm trying to get the information of several of a class' member variables on the receiving end of a slot/signal setup, so I'd like to pass the entire class through. Unfortunately, after the class has been passed, the member variables seem to be empty. Here's some code snippets:
This sets up the signal to pass the class
signals:
void selected(const ControlIcon *controlIcon);
this is the slot/signal connection
connect(controllerList->serialController, SIGNAL(selected(const ControlIcon*)),
infoView, SLOT(serialControllerSelected(const ControlIcon*)));
I emit the signal from within the class to be passed
emit selected(this);
Here's the code to call on the class' member data
QLabel *ASCIIStringHolder = new QLabel;
ASCIIStringHolder->setText(controlIcon->m_ASCIIString);
Nothing shows up in the label, and when I set a breakpoint, I can see that there's nothing inside m_ASCIIString.
I looked to make sure that it was being assigned some text in the first place, and that's not the problem. I also tried the signal/slot setup with and without const.
Any help would be appreciated.
Qt signal/slot mechanism needs metainformation about your custom types, to be able to send them in emitted signals.
To achieve that, register your type with qRegisterMetaType<MyDataType>("MyDataType");
Consult official QMetaType documentation for more information about this.
First, since you are using an auto connection, do both sender and receiver live in the same thread? If not, it could happen that the call is queued and when it arrives, the data in the sender was already modified. You could try to use a direct connection just to make sure this isn't the problem.
Second, just for the fun of it, did you try to access the sender by using qobject_cast<ControlIcon*>(sender()) within the slot? This is how it is usually done if the signal doesn't pass this as an argument. Like this:
QLabel *ASCIIStringHolder = new QLabel;
// this is instead of the argument to the slot:
ControlIcon *controlIcon = qobject_cast<ControlIcon*>(sender());
ASCIIStringHolder->setText(controlIcon->m_ASCIIString);
The signal can't be declared to be passing a class and then actually pass the child of that class. I changed the signal, slot, and connect() to be SerialController (the child of ControllerIcon), and everything worked fine.