Does Qt need a modern C++ GUI API?

cfreeloader

Below is the code that implements lambda-based Properties. After the definition of the Property<T> template, I have included an example "Rectangle" class that uses this Property<T> template. From this you can see the bind of the area is defined usinga lambda. Then, below that, I have a main program that uses an instance of the Rectangle and connects various signals so that you can see the effect of changing the width or height of the rectangle and that the area is recomputed whenever either width or height is changed.

I used gcc-4.7 to compile this:
g++ -std=c++11 -I$HOME/tools/boost/include -o dprop main.cpp
Since I transcribed this manually (don't ask...), I hope I didn't make too many typos.

@
#include <iostream>
#include <functional>
#include <boost/signals2.hpp>

using std::cout;
using std::endl;
using std::function;
using namespace boost::signals2;

template<typename T>
class Property
{
public:
// Signals are based on boost::signals2 library
// This signal only is emitted only when the functor is evaluated and
// the actual value of the property has changed.
signal<void ()> changed;

// This signal is emitted whenever the value of the property “might” be 
// changed.  It indicates that the property should be accessed again to
// get its updated value.
signal<void ()> invalidated;

// Basic constructor takes a functor as an argument to “bind” to the property
Property(function<T()> evalFn)
: m_prevValue()
, m_evalFn(evalFn)
, m_valid(false)
{ evaluate(); }

// A constant value is translated to an lambda that returns that constant value
// and then the above constructor is called.  Calling a different constructor is a
// C++11 feature.
Property(const T & value)
: Property([=](){ return value; })
{ }

// This takes an eval function and also an arbitrary list of Properties on which the
// eval function depends.  This is using the C++ variadic template feature.  (notice the …)
template<typename... Deps>
Property(function<T()> evalFn, Deps &...deps)
: Property(evalFn)
{
    addDependencies(deps...);
}

// If this Property is already invalidated don’t do anything.  Otherwise, set it to
// not valid and emit the invalidated signal to propagate to all dependent properties.
void invalidate() const {
    if (!m_valid)
        return;
    m_valid = false;
    invalidated();
}

// The variadic template base-case.
// Connect the invalidated signal in the antecedent property to this
// property’s invalidate function.
Property & addDependencies(Property<T> &p) {
    p.invalidated.connect([this](){this->invalidate();});
}

// The variadic template recursive case
template<typename P, typename... Deps>
Property & addDependencies(P &dep, Deps &... deps) {
    addDependencies(dep);
    addDependencies(deps...);
}

// So that you can write:  x = [](){ return foo; };
Property & operator=(function<T()> evalFn) {
    m_evalFn = evalFn;
    invalidate();
    return *this;
}

// So that you can write: x = 234;
Property & operator=(const T &value) { return *this = [=](){return value;}; }

// if necessary, evaluate the function object and if it is different than
// before, emit the changed signal and return true.  If the newly evaluated
// value is the same as before, do not emit invalidated signal and instead return false.
bool evaluate() {
    if (m_valid)
        return false;

    m_valid = true;
    T newValue = m_evalFn();
    if (!(newValue==m_prevValue)) {
        std::swap(newValue,m_prevValue);
        changed();
        return true;
    }
    return false;
}

// conversion function so that you can access the property like a value.
// e.g. :   cout << x << endl;
operator T() const {
    evaluate();
    return m_prevValue;
}

private:
mutable T m_prevValue;
function<T()> m_evalFn;
mutable bool m_valid;
};

// Example Rectangle defined using Property<T> where the area Property is
// bound using a lambda. Notice that the dependencies are width and height
// properties.
struct Rectangle
{
Property<int> width;
Property<int> height;
Property<int> area;

Rectangle(int w, int h)
: width(w)
, height(h)
, area([=](){return width*height;}, width, height )
{ }

};

// Example main program using the above Rectangle.
int main()
{
Rectangle r(100,200);

r.area.changed.connect([](){ cout<<"area changed"<<endl; });
r.width.changed.connect([](){ cout<<"width changed"<<endl; });
r.height.changed.connect([](){ cout<<"height changed"<<endl; });

r.area.invalidated.connect( [&r]() {
    cout << "width="&lt;&lt;r.width&lt;&lt;" height="&lt;&lt;r.height&lt;&lt;" area="&lt;&lt;r.area&lt;&lt;endl;
});

cout &lt;&lt; "width="&lt;&lt;r.width&lt;&lt;" height="&lt;&lt;r.height&lt;&lt;" area="&lt;&lt;r.area&lt;&lt;endl;

cout &lt;&lt; "changing width" <<endl;
r.width = 50;

cout << "changing height" <<endl;
r.height = 100;

}
@

Since it is getting late here, I will post the QML and equivalent C++ code tomorrow.

capisce

bq. As I went through this exercise a lot of questions came up as to when exactly the bound properties are read and how they exactly are updated under the hood.

The bound properties must be QObject properties, declared with the Q_PROPERTY macro in C++, which means they typically have a getter, and usually also a setter and a getter if they're mutable. The Q_PROPERTY declaration makes Qt's meta object system aware of the properties, so that they among other can be found with a string lookup. Here's an example Q_PROPERTY statement from QQuickRectangle:

Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged)

So when the QML binding like "x: parent.width / 2 - width / 2" is parsed, connections are automatically made to the properties' changed signals. The source properties for the binding are thus read first at the time the binding is first evaluated, and then whenever the change signal for one of the properties changes. One awesome feature of Qt signal / slot connections is that they work across threads, so you could have a C++ worker object in a separate thread exposed to QML, doing heavy computations in an asynchronous manner without interrupting the UI.

The connections are also automatically disconnected when either the receiving or sending object is destroyed. Whenever a property change leads to a visual change in a quick item it calls update() to signal to the quick canvas that it is dirty and thus the corresponding scene graph node / nodes might need updating before the next frame is rendered. The call to update() happens automatically in the backend (though you might need to call it manually if you implement a custom quick item).

Properties declared in QML automatically get the corresponding changed signal. Here's some more information on QML properties: http://doc.qt.nokia.com/qt5/qml-properties.html

I have to say your property binding system for C++ looks quite promising, gotta love C++11 lambdas. For the best interoperability with Qt I'd love to see it support Qt properties and signals though :)

lgeyer

Resources are usually finite and more often than not there's more than one solution to a single problem. It is therefore all the more important to select that solution, which provides the most benefit (usability, maintainability, future-proofness and performance) to the majority of people. Such a selection has to be the result of an objective discussion, and everyone involved in this decision process is heavily interested in all the different opinions - because two heads are better than one.

But there are three fundamental requirements for such a discussion:

a common basic knowledge (what),
a reasonable argument (why) and
a feasible alternative (how).

And since the beginning of the public discussion about QtQuick and more specifically QML vs C++ I see a serious lack of all of the three, mostly at the end of the native proponents. An expectation needs not to be a fact, an opinion is not an argument, the loudest voice needs not to be the most important one and a "No" is absolutely worthless, if it isn't followed by "because".

So I compiled this list of popular misunderstandings so that we have a common basic knowledge we can base our discussion on. And the best things is - you do not even have to believe what's written there; Qt is open source, the Qt Labs explain almost everything, the mailing lists are ready for providing information and in the end you could just try it on your own, dissect, benchmark and profile it and gain your own experience.

And again, everyone is heavily interested in an objective discussion, your opinion and your worries on this topic, even me, and even if the result is that the current solution selected for QtQuick was not the most benefitial. But the discussion is rather pointless as long as we talk about two different things.

I know that many native proponents feel unheared, but as long as there is denial and prejudice, as long as statements start with "We all know" and "I think" instead of "We have proof, that", as long as people who use and take their advantage of QML (and there are actually a lot of it) are defamed of beeing "fanboys", conspirators or victims of conspirators, who have no clue about C++ and fall for fallacies and the only reason they actually use QML is because they must be some kind of dumb and they are "not right in their mind", as long as people cry for facts and numbers, just to devalue them as "gibberish" while at the same time providing none, as long as there is aggression instead of creativity and as long as people keep rehashing the same disproven resentments over and over again they will stay mostly unheared because time is precious - too precious for a pointless skirmish.

We have evaluated (and we are activly using) QtQuick and we have found out that when done properly it scales better than our native interface implementations because we can focus on describing the interface and we can delegate the implementation to the QtQuick runtime, whose additional one-time cost turned out to be minuscule and irrelevant for our applications. The same way it is irrelevant for the SceneGraph if it was built from the QtQuick runtime or from native code.

In spite of that QML is far from dominating our applications. Quite contrary to, most of the code is still C++ - although we wish we could have done more code in QML, because it allowed us to achieve the same goal using less code and less time. QML is "real value" (and real money) for us - and from what I've heared not only for us. There are thousands of threads just here at the QtDN in the QtQuick forums - this doesn't seem like "no one in there right mind is using it".

I'm quite sure you can elaborate on that -we know- you think that a native implementation would be monotonically more performant.

And even at the cost of repeating myself - we could have done the same thing in pure C++ using the already available native SceneGraph API, the Animation Framework and Statemachine Framework. The ramp-up time for QML was quite short as almost any developer was already familiar with JSON and JavaScript, two of the most used data-interchange and scripting languages.

If you don't want to evaluate QtQuick on your own you might take a look at the large-scale projects already available, like the ones mentioned by capisce or KDE, the QtMediaHub and the N9 (which is by the way Nokias best-selling, best-rated smartphone, altough beeing deliberatily held back for political reasons; it has sold millions of units, is still selling more units than all Lumia phones combined and has a projected sales level of 25 million units a year; its operating system is still beeing developed, even though with a different name, and is expected to drive Nokias next billion devices - provided Nokia still exists then). QtQuick applications can also be found in the App Store, the Android Market and the Nokia Store - and their number is increasing rapidly.

lgeyer

QtQuick has got many advantages, non-trivial bindings are just one of them, and they don't vanish just because they are of no use to you. The majority of people is using graphical tools to design their user interfaces and this is a hard requirement for any graphical toolkit. The prerequisite is a meta-language as native-language tools are way to complex to be created. This is the reason QtDesigner is using .ui files, VisualStudio is using resources and XAML, GTK+ is using XML. No, you can't take advantage of QML if you don't use QML (which is, as I already said, easily extensible using C++ and QML) and no, you can't take advantage of it if you use a third-party meta-programming library like boost, but this true for any advanced Qt, including a hypothetical native QtQuick interface.

It's actually a bit weird that the declarative approach of QtQuick is reputed to have no advantages, yet everyone is trying to re-create the declarative API in C++ as an example of how a modern, native API should look like, including non-trivial bindings and lazy evaluation.

The bad news is, there are things you cannot express in a declarative language and there are things you cannot express in an imperative language. I am eager to see - honestly, I'm not beeing a drag here, I'm really curious - how one expresses <code>for(int i = 0; i < 10; i++) { ... }</code> in a declarative language (preferably QML) and <code>Item { value: innerItem.value + anotherValue; Item { id: innerItem; value: anotherValue; } }</code> in an imperative language (preferably C++).

I am also eager to see how source-based network transparency can be easily achieved in C++; do we integrate a complete build environment in our applications and compile the source on-the-fly; or are we using some kind of meta-language which is interpreted (like QML); or do we precompile the source and transport binaries (which isn't source-based anymore but at least it works) and disqualify our application from almost any controlled platform and application store, as downloading and executing binary data is prohibited on all.

How do we all this without introdcuing any binary contracts - which is the severe issue with native frameworks, especially with emerging technology.

I've seen that you are already working on a C++ API and honestly, I like it, I really do, but unfortunately I have more bad news for you. Although your implementation is possible, it is unsuitable for what it tries to achieve. The public API of Qt5 is C++98, which means no boost, no variadic templates and lambdas as requirement, and a tight connection to meta-object model of Qt.

I do think Qt will benefit from a native QtQuick API. But I also think (after activly working with QML) that it is (just) the second best solution to solve the given problem. If there are resources left (or if there is a strong momentum from the community) there should be native QtQuick API, but by then all effort should be put it the most benifitial (usability, maintainability, future-proofness and performance) solution.

If all you have is a hammer, everything looks like a nail.

? Offline

bq. It’s actually a bit weird that the declarative approach of QtQuick is reputed to have no advantages, yet everyone is trying to re-create the declarative API in C++ as an example of how a modern, native API should look like...

I think it is more like the declarative approach of QtQuick mimics the data structure of a program, written in an imperative language, just in a slightly different way, where structuring is not logical but visual. I don't think those theoretical C++ code examples aim at exploiting that, C++ programmers are bound to know data structure, there is no other way around, what the example code tries to exploit is the modularity of QML, which is present in QtGui as well, but is relying on a sub-optimal model when it comes to modern UI. It is probably due to the fact QtQuick arrived with very little components and literally no functional components, just some basic building blocks with which people can build functional components. In this regard I find the paradigm behind QtQuick more flexible and attractive, and while you can still do the same with QtGui it is more cumbersome, truly more cumbersome than it needs to be.

I honestly don't get it why so many people assume C++ supporters are AGAINST QML. QML has its benefits, I've said that far too many times, and all this time this initiative was not pushing against QML in any way, just insisting on providing cool new functionality to the ageing QtGui. And please, don't step forward with the "we have limited resources" arguement, especially while suggesting we do all that work ourselves. If Nokia didn't intend to offer excessive functionality to QML in order to give it an artificial advantage by not paying attention to solving the problems of QtGui for a new C++ API, if the backend was done more flexible and with a public native API, that extra effort woudn't be needed now. It would have been much easier designing QtQuick with native use in mind and adding the QML functionality on top of that than having to go back and do it for a QML centered backend. It is not OUR fault the Qt development team decided to take the wrong direction, there is a saying in my native language "measure twice, cut once" because if you cut without carefully measuring, chances are you will come short and have to cut another piece, waste the old one, just because you didn't take the right considerations.

It is most certainly not the first time a big company buys a smaller company with a nice product, and channels the further development of this product through some proprietary language. It is not the first time cool new C++ backends with cool new functionality get little to no native access, so that developers are forced to adopt a new language. And don't get me wrong, QML is not hard, it is trivial, it is far from a programming language, you DON'T DO QML PROGRAMMING, you do data structure description, and since we all know about data structure, is it that much of a stretch to want to do it ourselves and skip on the entire QML chain of not-really-necessary-but-made-necessary tools?

A nice, public QtQuick API would not only please native code supporters, but will make QtQuick more extensible, which will in turn bring benefits to the QML front as well.

As trivial as it may be to interface C++ to QML, doing it for a few objects is nothing compared to extensively doing it, over and over again. QML doesn't take the needs of professional software with custom functionality in consideration. For trivial apps with small number of custom elements, QML is an ease, but this changes as the number of custom elements increases, to the point QML becomes a burden.

Let's face it, creative developers need a little more than rectangles, creative developers need custom graphics. And so far, all QML examples I've seen that go beyond built in primitives are relying on raster images, which are static and resolution dependent. And when you reach for custom and dynamic graphics, you are met with all the tedious interfacing, working around QML limitations, and QML gets unattractive proportionality to the number of custom elements which you need to implement.

I think the absence of a QML canvas deserves most of the blame, a QML canvas, supported by the scenegraph, and native, OpenGL accelerated painter would address the majority of drawbacks of using QML for non-trivial apps. And since the extensive amount glue between QML and C++ is the biggest annoyance (at least for me), I think that could use some work, both on the C++ API front as well as the design limitations of QML, which produce a lot of the necessity for glue code.

The lack of innovation on the C++ front is just one of the aspects in which I think Nokia exerts negative influence on Qt, and while Nokia employees continue denying the obvious, I think the negative influence of Nokia is indisputable. For one, if Trolltech was never purchased, I have the feeling Qt would have had official support of iOS and Android today, which is where money is, and not some half-backed, stuck community ports. Now let's see someone denying the lack of support for the most widespread platforms is not Nokia's fault, but some dedication of Qt developers, like we are supposed to consider QML.

So, if not a C++ public API, then at least focus on a good CANVAS for QML, that would remove a lot of the burden of using QML that is associated with custom components. That would allow to keep C++ and glue to a minimum, only for extensive computation. That request to cut C++ to a minimum might sound strange coming from someone, who is viewed as a C++ supporter, but again, the issue is not with having to use more than C++, the issue is with interfacing and gluing lots of stuff together. Currently, QML is limited to require tedious C++ extenssion, and making QtQuick work with a native public C++ API is by far not the only solution to this issue. Reducing the need to interface C++ to QML works equally well.

Uwe Kindler

bq. For trivial apps with small number of custom elements, QML is an ease, but this changes as the number of custom elements increases, to the point QML becomes a burden.

Is this just an assertion or can you prove this? We use QML here in a desktop application with a number of plugins and we have not the impression that it scales bad. So it would be great if you could bring some facts to prove your statement.

? Offline

What do you mean by "scales bad"? I am not talking about performance here, I am talking about functional code to glue code ratio, and that after a certain point the ease of QML is totally displaced by the tediousness of interfacing and gluing stuff together. How many custom elements that do custom painting do you have in your application? How much native functionality are you interfacing to QML? What do you mean by facts? Prove my statement? Is this an investigation or it is just you lacking common sense? There is a full hour dev talk on C++/QML integration, and it is just covering the basics, now imagine doing this over and over and over again, imagine projects where custom elements are 90% and more of your code, you still don't see how the limitations of QML and the workarounds around those can become a burden?

Uwe Kindler

bq. QML is totally displaced by the tediousness of interfacing and gluing stuff together

Can you prove this or do you think repeating something often can substitute a proof. Ok, if proofes are overvalued here, then I simply claim you are wrong :O).

bq. How many custom elements that do custom painting do you have in your application

Because desktop components are not available yet, all components are custom.

bq. now imagine doing this over and over and over again, imagine projects where custom elements are 90% and more of your code, you still don’t see how the limitations of QML and the workarounds around those can become a burden

I don't need to imagine something because we already use QML here - and obviously you don't use QML because you simply state plain wrong things and from all you say here it becomes clear that you never used QML and therefore you have absolutey now idea what you are talking about - that's quite funny.

bq. Is this an investigation or it is just you lacking common sense

I simply asked you for a prove of your statement and you answered with an offense - nothing more to say :O)

? Offline

@Uwe - "all" may sound like a lot, unless the total number of elements in your app is like 5, which could very well be the case, since you avoided sharing a concrete number in favor of "all". And then again, stuff like desktop components i.e. push buttons, scroll bars, check boxes and so on - you can entirely do in QML, there is no complexity, no advanced painting, no extensive logic involved in such components and there is no necessity for those to be C++ components. You don't even say what kind of application you are developing, how complex is it, the amount of business logic it contains relative to UI code. It sounds like it is you who fails to substantiate his claim, for it doesn't sound like your application can even fit in the profile of apps, the development of which renders QML a burned rather than an advantage.. And in case you failed to extract a PROOF from my well known facts, from my last post and from the dozens that preceded, then chances are do you lack common sense, it's just like a religious person asking for evidence god doesn't exist ;)

Uwe Kindler

@temp

bq. It sounds like it is you who fails to substantiate his claim

I claimed nothing, i wrote:

bq. ...and we have not the impression that it scales bad

that is simply a subjective impression we got from using QML in our application.

bq. And in case you failed to extract a PROOF from my well known facts..

I did not found any facts in your posts - only assumptions or repetition of something you read somewhere.

bq. For trivial apps with small number of custom elements, QML is an ease, but this changes as the number of custom elements increases, to the point QML becomes a burden.

I simply do not understand how you can claim something like this.

How big is your application?
How many QML elements do you use?
Which concrete problems arised when you used a great number of custom QML elements?

If you make a desktop application, you need to choose the right tool and need to do the right desicions. For some things in an application QWidgets are much better suited than QML Components and for other parts of the application QML components are better suited. We mixed both in our application - like QtCreator does it.

So simply claiming something does not mean that it is the truth.

? Offline

bq. So simply claiming something does not mean that it is the truth.

To me this sounds like it applies to your earlier statement:

bq. We use QML here in a desktop application with a number of plugins and we have not the impression that it scales bad.

Where is the substantiation of that claim? Oh, but it isn't a claim, just a subjective impression, well in that case lets call my "claim" simply an objective impression.

You didn't provide any the specifics I noted you'd need to provide to actually be able to tell whether you have a point or not. Cuz right now you sound like you haven't done a lot of C++ components for QML. In short, like someone, who is clueless to the issue at hand.

Watch this talk, and try to imagine what would be if you need to make those considerations and add that glue throughout 90% of the development process.
http://qt-project.org/videos/watch/programming-with-qt-quick-6-6-c-integration

It is no brainer really, QML is good when you already have the components, to just quickly and easily stitch them together into a working app. I have yet to see a QML application that actually does something beside being a front end for web services or tech demonstration. QML is perfect for such case scenarios, however a productivity oriented application with scores of custom components and performance critical logic is a completely different matter. And believe it or not, some people are after that kind of applications, so I assume you will not really get the point until you try and do it for yourself. Until then, I stand by my previous claim that you just happen to be clueless on that subject, not because you are stupid or anything, simply because you haven't been in that situation, you haven't experienced it, so you couldn't possibly know.

Now either prove me wrong by tossing an amazingly powerful and productive QML application in my face, or finally realize some people have higher requirements and standards, and that just because QML happens to work fine for your "mysterious application" doesn't mean it works for serious projects.

Uwe Kindler

@temp

bq. well in that case lets call my “claim” simply an objective impression.

Finally you got it - hurray. That is the thing I try to say the whole time. The people that read your posts should know, that this is all your impression and that all you say about QML is not based on facts but that it is only your personal opinion.

bq. and that just because QML happens to work fine for your “mysterious application” doesn’t mean it works for serious projects.

And just because you don't get along with QML does not mean it does not work for serious projects ;-P

lgeyer

So, we are basically right back at denial, prejudice and unproven personal opinions straight in the follow-up to the post that was intended to actually help you and to make your voice more heard and valuable.

So much for that.

? Offline

[quote author="Uwe Kindler" date="1336116905"]The people that read your posts should know, that this is all your impression and that all you say about QML is not based on facts but that it is only your personal opinion.[/quote]

And all this time I assumed people realized every opinion in this thread is a personal one. Perhaps you assumed I speak the words of god or something like that? What part of my statements did ever hint at my posts not expressing my opinions? And sorry to break your awesome theory, but my personal opinion is based on facts. And FYI I get along with QML just fine, just in my line of work, the extra effort to get along with QML is arguably worth it. So please, at least have the courtesy of not speaking in my name stuff that is not really true. Just because QML doesn't address the needs of some developers doesn't mean they are too stupid to get it, you know... But apparently that is your fixed idea or people, having a problem with the limitations and overheads of using QML.

@Lukas Geyer - what the H are you talking about? Denial? Prejudice? Where???
Pardon me but but voice your personal opinions just as loudly, and accusing people of what you do yourself is called hypocrisy. What kind of proof do you mean? I tried developing serious applications with QML, I experienced the tediousness, associated with writing extensive amount of C++ components for QML, and that's all the proof I need to voice an opinion based on my experience. Or do you imply I am a liar or something? Why do QML supporters keep asking me to prove the obvious, and kind of proof exactly do you mean? Can you even answer, or is it just some desperate effort to make me appear as unable to support my arguments by not providing that "evidence" whatever that may be? And just to get you a taste of your own medicine, where is the proof of me being wrong??? Because you, a guy whose avatar is a suit with a Qt emblem on it says so?

I never implied my opinion has more weight than that of anyone else, but the same thing cannot be said for some other individuals around here. What has weight is the collective opinion, and as this, as well as the other poll indicates, it seems the people, completely happy with QML are a minority. So if you are that much frustrated by my edgy individuality, at least show some respect to all those other people, who appear to be more or less on the same page as me!

Uwe Kindler

@temp

bq. and as this, as well as the other poll indicates, it seems the people, completely happy with QML are a minority

It is absolutely clear that nobody is perfectly happy with QML at the moment because it simply misses some important things like desktop components, theming or QtQuick 2.0. So normally nobody would vote for the first item. So it is really astonishing for me that such a lot of people voted for the first item in this poll.

bq. ...but my personal opinion is based on facts

Ok, then you might puplish a link to these facts? I ask for facts because I'm seriously interested in these facts that prove that QML is not suited for normal "serious" applications and not because I want to argue.

It is god if people raise their concerns about QML here and it is good to discuss about these concerns. But in my opinion it is not OK to claim things like

bq. For trivial apps with small number of custom elements, QML is an ease, but this changes as the number of custom elements increases, to the point QML becomes a burden.

without proving anything. The next one claims that you are going blind and deaf and that you get crippled fingers if you use QML often ;O)

tw55413

One example of something that is trivial with C++ Qt, but impossible with QML (It might be possible, but I have not found how to do it) is to use a variable number of widgets.

For example, suppose which fields to display is something the program decides at runtime, base on the user login etc. So the program comes up with a list of fields to put on the form. Using the C++ API one would do something like this (pseudocode in python) :

@for field_name in field_names_to_display:
layout.addRow( field_name, QLineEdit() )@

And the layout would contain all the needed fields, be it 2 or 20 fields.

Now these kind of things are huge time savers in larger applications. I'd like to know if these kind
of things are possible in QML, and if possible without JavaScript.

capisce

@Erik

In QML you would probably use a Repeater for that, see http://qt-project.org/doc/qt-4.8/qml-repeater.html

tw55413

@capisce

Ahah, thank you for the link. I'm trying to see the parallels between the QML data binding and the traditional model-view-delegate.

lgeyer

I've seen you've already taken the easy way out, but there are still some things that are worth beeing mentioned.

The SceneGraph is not an exclusive feature of QML, see QSG* classes. There is a hardware-accelerated painter interface, see QGL* classes. There is also a traditional QPainter interface to QML, see QQuickPaintedItem. QML can be easily extended with C++ code, see QQuickItem.

QML has got a canvas interface, which allows drawing of straight and curved lines, simple and complex shapes, graphs, and referenced graphic images. It can also add text, colors, shadows, gradients, and patterns, and do low level pixel operations, see Canvas item.

Images in QML can be supplied in any of the standard image formats supported by Qt, including bitmap formats such as PNG and JPEG, and vector graphics formats such as SVG. If you need to display animated images, use the AnimatedImage element. BorderImage provides automatically scaled raster images.

QML is at an advantage due to beeing an declarative, interpreted language (id est not beeing C++). An interpreted API has to maintain source code compatibility, whereas an native API has to additionally maintain binary compatibility, thus is usually less extensible, as there are binary contracts to be abode.

QObject, QObjectList, QStringList, QVariant, QVariantList, QVariantMap and QAbstractItemModel can be directly exported to QML without any additional glue code. qmlRegisterType() allows for registering custom types, which are then available in QML immediately. QML supports C++ inheritance hierarchies and can freely coerce between known, valid object types. The QtQuick engine takes advantage of the meta-object information already present for each object.

There already is an extensive list of professional, productivity-oriented applications created with QtQuick, see KDE, QtMediaHub, N9, mixd.tv, httvBOX, Ocean Blue, IP4.tv et al.

QML is a replacement to QtWidgets in the same way the Graphics View Framework is a replacement to QtWidgets. QtQuick stands for Qt User Interface Creation Kit, not Qt Application Creation Kit.

Nokia spent 150 million dollars to acquire Trolltech just to release the framework under the terms of the LGPL, so that everyone can use Qt for closed-source non-commercial and commercial applications for free. Since then, Qt has grown exponentially in number of features, users, applications, SDK downloads, full time developers and specialized companies. Although Qt beeing an Open Governance community project now, Nokia is still (by far) number one code contributor, main contributor of legal resources to define and protect the Qt trademark, licensing and Contributor License Agreement, main funder of Qt training courses and materials, the Qt Certification and Qt Ambassadors program and the Qt Project infrastructure, the Qt Contributors Summit, the Qt Developer Days and experimental activities around Qt5 such as the QtonPi, Qt5 for N9, Qt5 for PhoneGap or INdT.

Yes, we've still got a long way to go with QtQuick and QML and there is definitely room for improvement. Therefore we need a vivid discussion about the future development. But such a discussion is quite impossible as long as we have such a diverging perception and as long as most subsequent conclusions are based on it.

I do not presume to know what the majority of people thinks about QtQuick and QML, and you shouldn't do that either. Especially as based on a biased poll with insignificant participation [the Qt SDK is downloaded by millions of people, 130 have participated in "your" poll, 85 of them have voted in favor of an (additional) native API], which doesn't even allow for differentiation between a native API beeing a replacement or an additional, secondary option to QML. You must have missed that in the second poll you are referring to (which suffers from the same insignificant participation) two-thirds of the people have voted in favor of QML.

But what I do know is that a lot of the argumentation against QML is based on denial, prejudice and personal opinions. It would help me a lot if I would actually see all the excessive amount of glue code that is obviously somewhere, if I would see the huge performance impact of QML on our applications, if I would see all those, or at least one, applications that have been made quite impossible due the way QtQuick works and if I would see that perfect native C++98 API, which has all the advantages of QML without its disadvantages. Where is all this?

d3fault

Lukas Geyer, you keep stating that we need to keep to fact-based arguing... so I'll keep this short:

Hardware Accelerated C++ GUI API with bindings does not require a JavaScript Interpreter, a Virtual Machine, or a brand new declarative language.

There, that's it. There is no fact-based argument to defeat that. Please try as it will be amusing to read. Now please stop with the same old tired push of "use the QtQuick back-end directly" (QSG* classes) HURRRR you troll. We can both agree that there's a reason QML exists: to serve as a front-end to drive the back-end that you're proposing we manually drive. If it's so easy, why was QML invented to drive it? We want a purely C++ front-end to drive the QtQuick back-end, get it?