Integrating QML and C++

tomy

Hi all,

As the first step for being familiar with the technique integrating QML and C++ I went for this example.
I ran it and I have some questions:

Why do we need a Q_PROPERTY macro there? Yeah, I know that it's said that it declares a property that could be accessed from QML. But we have two functions for both reading and writing to the data member. (userName) We could have declared a data member named, say, userName, and haven't declared a a Q_PROPERTY macro seemingly.
The program does almost nothing and just gets texts! Emitting the signal won't work either there.

jsulm

@tomy said in Integrating QML and C++:

We could have declared a data member named

You mean a public member? Well, it is called encapsulation in object oriented languages: you make your data fields private and provide read/write methods where you can control the access (for example refuse changes which should not be possible).

J.Hilk

@tomy said in Integrating QML and C++:

I ran it and I have some questions:

Why do we need a Q_PROPERTY macro there? Yeah, I know that it's said that it declares a property that could be accessed from QML. But we have two functions for both reading and writing to the data member. (userName) We could have declared a data member named, say, userName, and haven't declared a a Q_PROPERTY macro seemingly.

The program does almost nothing and just gets texts! Emitting the signal won't work either there.

hi tomy

let's see if I can makes a bit more clear for you, with my (limited) knowledge.

first of lets change the example q_propery a bit so one does not get confused with the nomenclature

//original
Q_PROPERTY(QString userName READ userName WRITE setUserName NOTIFY userNameChanged)

//new
Q_PROPERTY(QString userName READ getUserName WRITE setUserName NOTIFY userNameChanged)

On the QML-Side the property is named userName
you can do property bindings with it:

userName : "FixesText"

//or
userName: otherStringProperty

if a bound property is changed the signal userNameChanged is emitted

on QML-Side you can attach to that via

onUserNameChanged: console.log("Username was changed", userName)

on cpp side via QObject:::connect()

connect(this, myClass::userNameChanged, [=]{qDebug() << "Username was changed" << getUserName();});

the signal is always emitted on both sides, QML and cpp

assigning a new string to the property on qml side

userName = "New Username"

actually envokes the cpp function setUserName

If you envoke setUserName from the cpp class, userNameChanged is emitted, if the string actually is different from before:

If a QML property is bound to userName, QML will call getUserName to get the new string and update all bound properties.

emitting userNameChanged() from cpp without actually chaning m_userName will have no visible change in your QML code, but it will update all bound properties.

chaning m_userName without emitting the signal userNameChanged on cpp side will not update the QML part

tomy

@jsulm
No, I exactly meant a private date member, say, next to m_userName.

mrjj

@tomy
But private members are private and
cannot be accessed by any one outside the class.
:)

tomy

@J.Hilk

Hi J.Hilk, thank you very much.

//original
Q_PROPERTY(QString userName READ userName WRITE setUserName NOTIFY userNameChanged)

//new
Q_PROPERTY(QString userName READ getUserName WRITE setUserName NOTIFY userNameChanged)

On the QML-Side the property is named userName

So declaring a Q_PROPERTY is the only way to have a property/variable inside the C++ code and also have access to it from QML side and declaring such a property inside the private or even public area (on the C++ side) doesn't provide us with accessing it from the QML side, yes?

if a bound property is changed the signal userNameChanged is emitted

Like here:
onTextChanged: backend.userName = text
Yeah?

on QML-Side you can attach to that via
onUserNameChanged: console.log("Username was changed", userName)

I used it inside the BackEnd:

BackEnd {
        id: backend
        onUserNameChanged: console.log("Username was changed", userName)
    }

If a QML property is bound to userName, QML will call getUserName to get the new string and update all bound properties.

Like here?
text: backend.userName

J.Hilk

Hi @tomy

So declaring a Q_PROPERTY is the only way to have a property/variable inside the C++ code and also have access to it from QML side and declaring such a property inside the private or even public area (on the C++ side) doesn't provide us with accessing it from the QML side, yes?

Q_Property is a Macro so I would asume it does not matter if its in the public, or private section of your class. But your read &write functions defenitly have to be public.

It is not the only way, but the most convenient. I know of one more way to get a value from a cpp class:

//in your header
public:
    Q_INVOKABLE int getIntValueFromCpp(){return qrand();}

//In qml
val intValue = myCppObject.getIntValueFromCpp()

if a bound property is changed the signal userNameChanged is emitted

Like here:
onTextChanged: backend.userName = text
Yeah?

Yes as soon as text changes the in your backend setUserName is invoked, thanks to the Q_PROPERTY makro, and because of the body of setUserName userNameChanged is emitted if text != userName

If a QML property is bound to userName, QML will call getUserName to get the new string and update all bound properties.

Like here?
text: backend.userName

yes:
: = Propertybinding
= = Asigning a Value

keep in mind, asigning a value (=) will destroy a previous binding.