I have a c++ class with name Class2 and I registered it with qmlRegisterType method and I use it as code behind of qml pages in my application.
I have qml file and in onCompleted I want to get an object of Class2 and set it to cl2 in the qml file .but I get an error
left-hand side of assignment operator is not an lvalue
Rectangle{
Component.onCompleted: {
cl2=ObjectAccessor.class2obj;//error is here
}
Class2 {
id: cl2
onMessageChanged: { }
}
}
In qml file I changed the defenition of Class2
property Class2 cl2: Class2 {}
but now i get another error
Cannot assign [undefined] to Class2*
How can i set object of `Class2' so i can all data on this object an all signals?
First of all class2obj being undefined means it is not property interfaced to be accessible from QML.
Second, you cannot assign a qml object to another qml object. It depends on what you want to do.
If you want to assign identity, you will have to put it into a target reference, implemented as a property:
property Class2 cl2: null
...
cl2=ObjectAccessor.class2obj
If you want to assign value, as in to make the property values of the one object to the other, you will have to that member by member.
And if all you need is to connect to a signal, then all you need is a connection element, provided that ObjectAccessor.class2obj actually works:
Connections {
target: ObjectAccessor.class2obj
onMessageChanged: doStuff()
}
For a more complete answer, you will have to post the code related to Class2 and ObjectAccessor. And also clarify that bit:
so i can all data on this object an all signals
Related
Right up-front, I'll apologize: This is a monster question, but I wanted to provide what I hope is all of the pertinent details.
I've got a QML-based GUI that I was tasked with taking over and developing from proof-of-concept to release. I believe the GUI is based on an example provided by QT (Automotive, maybe?). The GUI is being compiled for web-assembly (emscripten) and features a "back-end data-client" which communicates with our hardware controller via a socket and communicates with the GUI via signals. The GUI is accessed via web browser and communicates with the Data_Client via QWebSocket.
The GUI proof was initially created with a very "flat" hierarchy where every element is created and managed within a single ApplicationWindow object in a single "main" QML file. A Data_Client object is instantiated there and all the other visual elements are children (at various levels) of the ApplicationWindow:
ApplicationWindow {
id: appWindow
//various properties and stuff
Item {
id: clientHolder
property Data_Client client
}
ColumnLayout {
id: mainLayout
anchors.fill: parent
layoutDirection: Qt.LeftToRight
//And so on...
The Data_Client C++ currently emits various signals in response to various things that happen in the controller application. In the main .QML the signals are handled as follows:
Connections {
target: client
onNew_status_port_data:
{
textStatusPort.text = qdata;
}
onNew_status_data_act_on:
{
imageStatusData.source = "../imagine-assets/ledGoodRim.png";
}
//and so on...
What I'm trying to do is create a ChannelStatusPanel object that holds the various status fields and handles the updates to those fields (text, images, etc.) when it receives information from the Data_Client backend. There are multiple instances of this ChannelStatusPanel contained in a MainStatusPanel which is made visible or not from the main ApplicationWindow:
Having said all of that (Phew!), I come finally to my question(s). What is the correct way to signal a specific instance of the ChannelStatusPanel object from the Data_Client with the various data items needed to drive changes to the visual elements of the ChannelStatusPanel?
I thought I was being clever by defining a ChannelStatusObject to hold the values:
Item {
id: channelStatusObject
property int channel
property int enabled //Using the EnabledState enum
property string mode
property int bitrate
property int dataActivity //Using the LedState enum
//and more...
property int packetCount
}
In the ChannelStatusPanel.qml, I then created a ChannelStatusObject property and a slot to handle the property change:
property ChannelStatusObject statusObject
onStatusObjectChanged: {
//..do the stuff
From the Data_Client C++ I will get the information from the controller application and determine which "channel" I need to update. As I see it, I need to be able to do the following things:
I need to determine which instance of ChannelStatusPanel I need to update. How do I intelligently get a reference to the instance I want to signal? Is that just accomplished through QObject::findChild()? Is there a better, faster, or smarter way?
In the Data_Client C++, do I want to create an instance of ChannelStatusObject, set the various fields within it appropriately, and then set the ChannelStatusPanel instance's ChannelStatusObject property equal to the newly created ChannelStatusObject? Alternatively, is there a mechanism to get a reference to the Panel's ChannelStatusObject and set each of its properties (fields) to what I want? In C++, something like this:
QQmlComponent component(&engine, "ChannelStatusObject.qml");
QObject *statObj= component.create();
QQmlProperty::write(statObj, "channel", 1)
QQmlProperty::write(statObj, "bitrate", 5000);
QQmlProperty::write(statObj, "enabled", 0);
//Then something like using the pointer from #1, above, to set the Panel property
//QObject *channelPanel;
QQmlProperty::write(channelPanel, "statusObject", statObj)
Is there some other, more accepted or conventional paradigm for doing this? Is this too convoluted?
I would go about this using Qt's model-view-controller (delegate) paradigm. That is, your C++ code should expose some list-like Q_PROPERTY of channel status objects, which in turn expose their own data as properties. This can be done using a QQmlListProperty, as demonstrated here.
However, if the list itself is controlled from C++ code -- that is, the QML code does not need to directly edit the model, but only control which ones are shown in the view and possibly modify existing elements -- then it can be something simpler like a QList of QObject-derived pointers. As long as you do emit a signal when changing the list, this should be fine:
class ChannelStatus : public QObject
{
Q_OBJECT
public:
Q_PROPERTY(int channel READ channel CONSTANT)
Q_PROPERTY(int enabled READ enabled WRITE setEnabled NOTIFY enabledChanged)
// etc.
};
class Data_Client : public QObject
{
Q_OBJECT
public:
Q_PROPERTY(QList<ChannelStatus*> statusList READ statusList NOTIFY statusListChanged)
// ...
};
The ChannelStatus class itself must be registered with the QML type system, so that it can be imported in QML documents. Additionally, the list property type will need to be registered as a metatype, either in the main function or as a static variable. Otherwise, only lists of actual QObject pointers are recognised and you would have to provide yours as such.
qmlRegisterUncreatableType<ChannelStatus>("LibraryName", 1, 0,
"ChannelStatus", "Property access only.");
qRegisterMetaType<QList<ChannelStatus*>>();
You then use this property of the client on the QML side as the model property of a suitable QML component, such as a ListView or a Repeater inside a container like RowLayout. For example:
import LibraryName 1.0
ListView {
model: client.statusList
delegate: Column {
Label { text: modelData.channel }
Image { source: modelData.enabled ? "foo" : "bar" }
// ...
}
}
As you can see, the model data is implicitly attached to the delegate components. Any NOTIFYable properties will have their values automatically updated.
Below is how I've setup my application.
Front-end is in QML. Back-end is in Qt C++. H/W Controller application is in C.
In Qt C++ back-end, I have QObject derived database and databasePoint classes.
database object holds a QMap of databasePoint objects.
Each databasePoint object has a unique point name, which is used as an identifier.
database object is created in main.cpp and exposed to QML as a context property.
database class has a method to return a pointer to databasePoint object.
In QML, this method is used to create databasePoint objects.
When this method is called, a databasePoint object is created and added to QMap if it doesn't already exist.
databasePoint class has read-write value properties.
These properties are used for communication between UI, back-end and controller.
In a timer, latest value is polled from controller periodically, whenever there's a change, the value property is updated.
When the value property is updated from UI, the value is written to controller.
class database : public QObject
{
public slots:
databasePoint* getDbpointObject(QString pointName);
private:
QMap<QString, databasePoint*> dbPointMap;
};
class databasePoint : public QObject
{
Q_PROPERTY(QVariant value READ value WRITE setValue NOTIFY valueChanged)
public:
QVariant value(void);
void setValue(QVariant value);
QVariant my_value;
signals:
void valueChanged();
};
DatabasePointComponent.qml:
Item {
required property string pointName
property var pointObj: database.getDbpointObject(pointName)
}
MyScreen.qml:
DatabasePointComponent{
id: dbTEMP
pointName: "TEMP"
}
TextInput {
text: dbTEMP.pointObj.value
onAccepted: dbTEMP.pointObj.value = text
}
I have a publisher/subscriber key-value DB class in Qt/C++. The subscribers can connect by passing the key ( string ) , their QObject pointer and the property.
Whenever a value of the subscribed key changes, the properties of the subscribed QObject changes to the new value. Works in Qt/C++ fine.
Now I want to make a view in QML. Is it possible to pass from QML to C++ an object with 3 parameters:
QObject pointer of the QML object
property as string
DB-key as string
?
The preferable solution were, as if the property connects to another property:
Item{ myQmlProp: MyCppInst("myDBKey") }
EDIT
What currently works is this solution:
Item{
id:myqmlitem
myQmlProp: MyCppInst("myDBKey","myQmlProp",myqmlitem)
}
or like this:
Item{
id:myqmlitem
Component.onCompleted:{
MyCppPublisher.subscribe("myDBKey1","myQmlProp1",myqmlitem)
MyCppPublisher.subscribe("myDBKey2","myQmlProp2",myqmlitem)
}
}
Compared to the preferable solution, I have to pass the connected property name and the QML item instance explicitly. But it is ok, many thanks for the answers!
I've hoped to use QML's this-Keyword but have learned, that it is currently undefined in QML :-(
Just give the object an id and pass that id to the function, it will become a QObject * on the C++ side. Then you can use the meta system to access properties by name:
// qml
Item {
id: someitem
...
CppObj.cppFoo(someitem)
}
// c++
void cppFoo(QObject * obj) {
...obj->property("myDBKey")...
}
A reference would do as well, for example children[index].
What you could do is a function taking just your dbkey as a parameter, and return a QObject* exposing a Q_PROPERTY with a READ function and NOTIFY signal.
This way, you just have to tell with the notify signal the value has changed, and the QML will call the read function automatically.
It could be implemented like that Item{ myQmlProp: MyCppInst("myDBKey").value }.
If you know the db keys at compile time you could just add a property for each of them in your MyCppInst directly, or if you know them at the creation of your cpp class you could put them in a QQmlPropertyMap.
Usage would be like that : Item { myQmlProp: MyCppInst.myDbKey } (or MyCppInst["myDbKey"] if you need to be dynamic in the QML side).
I try to understand the documentation section Providing Attached Objects for Data Annotations, or specifically it's example.
If I get the steps right it works like this:
Create a Type (MessageBoardAttatchedType) which holds the signals and properties of that shall be accessible through the attached property
Create a static method that returns an instance of the affore mentioned type in the type, that shall be used as the attached property (MessageBoard)
Provide the information, that this attached type exists by invoking the macro QML_DECLARE_TYPEINFO(..., QML_HAS_ATTACHED_PROPERTIES)
The following questions arrise for me:
(Minor one): In the documentation we have this code:
static MessageBoard *qmlAttachedProperties(QObject *object)
{
return new MessageBoardAttachedType(object);
}
The return type is MessageBoard, but we return a MessageBoardAttachedType. How is that made possible? MessageBoardAttachedType does not inherrit MessageBoard - is that some magic of the macro in 3.?
And the main question:
Here we learn, that attached properties may(?) be available just in very specific Items - e.g. just in the delegate root for the XView attached properties.
In the example it is not mentioned, that I can access the attatched property from a Message-object. How is that determined? By returning null from the
*qmlAttachedProperties(...)
if the passed *object does not please me (is not a Message e.g.)?
Returning a null pointer from qmlAttachedProperties will crash the QML engine.
You should always return an instance of an object in qmlAttachedProperties.
You could however display a warning when the object being attached doesn't fit your requirements, either in the qmlAttachedProperties function or in the constructor of the attached type.
For example that's how it's done for the StackView attached type and multiple others.
The qmlAttachedProperties of StackView returns an object inconditionnaly :
QQuickStackViewAttached *QQuickStackView::qmlAttachedProperties(QObject *object)
{
return new QQuickStackViewAttached(object);
}
The constructor of the StackView attached type outputs a warning if the attached object is not an QQuickItem (checking with qobject_cast), and does its magic only if it is.
QQuickStackViewAttached::QQuickStackViewAttached(QObject *parent)
: QObject(*(new QQuickStackViewAttachedPrivate), parent)
{
Q_D(QQuickStackViewAttached);
QQuickItem *item = qobject_cast<QQuickItem *>(parent);
if (item) {
connect(item, &QQuickItem::visibleChanged, this, &QQuickStackViewAttached::visibleChanged);
QQuickItemPrivate::get(item)->addItemChangeListener(d, QQuickItemPrivate::Parent);
d->itemParentChanged(item, item->parentItem());
} else if (parent) {
qmlWarning(parent) << "StackView must be attached to an Item";
}
}
Alright. I have searched a lot but haven't got a good solution yet. I am new to Qt. I have a class which is a QQuickItem like so,
class MyQuickItemClass : public QQuickItem
{
Q_OBJECT
SetInfo(SomeCppClass object)
};
I do a qmlRegisterType in my main.cpp to register it on the qml side like this,
qmlRegisterType< MyQuickItemClass >("MyQuickItemClass", 1, 0, "MyQuickItemClass");
All fine till here. But -> I want to set an object instance & some properties in MyQuickItemClass which some C++ logic in it as well & then pass the MyQuickItemClass object to qml. Or, get a valid instance of MyQuickItemClass from Qml. How can I get a vlid instance MyQuickItemClass object instance from QML on C++ side in main.cpp ?
I tried doing the following learning from the link here. But this technique creates two separate objects of MyQuickItemClass. One from QML, & one from c++ side. Hence does not work for me.
Following is how I am trying to do this after lot of searching.
int main(int argc, char *argv[])
{
qmlRegisterType< MyQuickItemClass >("MyQuickItemClass", 1, 0, "MyQuickItemClass");
QQmlApplicationEngine engine;
SomeCppClass someCppClassObject;
someCppClassObject.updateSomething();
MyQuickItemClass myquickItemObject;
myquickItemObject.SetInfo(someCppClassObject);
engine.rootContext()->setContextProperty("myquickItemObject", &myquickItemObject);
engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
return app.exec();
}
But, doing the above gets the constructor of MyQuickItemClass called twice. Once from cpp side when I created an object, and once from qml side. Verified this by placing a breakpoint in the constructor of MyQuickItemClass as well. As a result, someCppClassObject that I had set is null inside MyQuickItemClass when program runs. Because qml has made the final call to MyQuickItemClass to instantiate, thusly ignoring the MyQuickItemClass object that I created in main.cpp.
Here is my qml code for MyQuickItemClass:
import QtQuick 2.5
import MyQuickItemClass 1.0
ParentContainerItem {
id: parentItem
color: "black"
MyQuickItemClass {
id: myQuickItemID
visible: true
objectName: "myQuickItem"
property bool someProperty1: false
property bool someProperty2: true
anchors.top: parent.top
anchors.horizontalCenter: parent.horizontalCenter
}
//Other qml components
}
And this is the C++ class whose object needs to be set into MyQuickItemClass.
SomeCppClass {
//Pure C++ class. No Qt
}
Please note that I need to keep MyQuickItemClass derived from QQuickItem. Please suggest...
Generally it is a good idea to avoid accessing QML instantiated objects from outside as most of the access methods generated a dependency from C++ toward QML, restricting the way the QML tree is done.
E.g. requiring certain objects to exist at certain point in times, having specific objectName values, etc.
It is better to either "register" the object from QML side by calling a method on an exposed C++ object/API or to make the QML instantiate object register itself from within its own C++ code.
The latter is obviously inherently automatic, i.e. each instance of such a class would do that, while the former puts it at the discretion of the QML code which of the created instances it wants to make known.
Doing the following from a suggestion in discussion here solves the issue & gets a valid object to the QuickItem qml file
QQuickItem *myItem = engine.rootObjects()[0]->findChild<QQuickItem *>("myQuickItem");
I am new to QML and have a problem in accessing a property.property of a C++ object:
C++, frequency and station both are Qt metatype registered objects:
CStation *station = new CStation(...); // QObject
CFrequency *frequency = new CFrequency(..); // QObject
QQmlContext *qmlContext = viewer.rootContext();
qmlContext->setContextProperty("myatcstation", station);
qmlContext->setContextProperty("myfrequency", frequency);
QML:
RowLayout { ....
TextField {
text: myatcstation.toQString(true)
}
}
.... text: myfrequency.toQString(true)
This works, but when I write: text: myatcstation.frequency.toQString(true) I do get TypeError: Object [object Object] has no method 'toQString'
frequency is a property of class CStation set as Q_PROPERTY(CFrequency frequency READ getFrequency)
Crosscheck in C++ works:
CFrequency test = station->property("frequency").value<CFrequency>();
-- Edit: Frank's answer --
Both classes are derived from QObject, and it is not as per textbook as they are made copyable. I am aware of the Identity vs value situation.
Basically frequency is a value object, but I have made it QObject based so I am able to use properties with it (see Any chance to use non QObject classes with QML ). In the example, toString is Q_INVOKABLE, frequency in the non-working case returns a copy of a QObject derived CFrequency object.
-- Edit: Further findings --
When I change the frequency property to return CFrequency* instead of CFrequency it does not work either. As I get TypeError: Cannot call method 'toQString' of undefined the situation seems to be the same. CFrequency alone works, but QML does not understand that myatcstation.frequency is a frequency object which has toString.
CFrequency isn't a QObject I assume, otherwise you wouldn't return it by value but by pointer. To make `toQString() callable from QML, it must be either Q_INVOKABLE or a slot, which means that CFrequency must be a QObject as well.
If a station has only one frequency, consider moving the relevant information into the station object, i.e. add the frequency information you need as properties to CStation.
To get updates when the frequency changes, consider to use a property such as Q_PROPERTY(QString frequencyAsString READ frequencyAsString NOTIFY frequencyAsStringChanged) instead of toQString(). Properties have the update mechanism "built-in" via property bindings, while there's no good way to tell QML that it should call toQString again because the frequency changed.
I solved a similar problem:
class TopObject : public QObject
{
Q_OBJECT
Q_PROPERTY(ValueObject* theValue ...
...
}
class ValueObject : public QObject
{
Q_OBJECT
Q_PROPERTY(QString value ...
...
}
In the main application:
qRegisterMetaType<ValueObject>("ValueObject");
qmlRegisterType<ValueObject>("com.name.comp", 1, 0, "ValueObject");
...->setContextProperty("topObject", new TopObject());
And in the qml code:
import com.name.comp 1.0
... {
text: topObject.theValue.value
...
It needed both, returning the property (ValueObject) as pointer and registering it with qmlRegisterType.
See also Exchange Data and Objects between C++ and QML and vice versa