Simple acces of C++ object data from QML - c++

Question

I'm making a board game in Qt/C++ using qml. All the important game data is represented in a single class. My intention is to have the qml sheets access this one object and draw the game board depending on the data.
What is the simplest approach to exposing the C++ data members to QML?
Now I know the question has been asked, I've seen the answers and the Qt Documentation. I am, however, not satisfied. What I've seen, the way to do this seems to be to make Q_PROPERTY out of every single variable I want to access from QML. This looks to me tedious and unnecessary, not to mention it will stretch the code to 3x it's original length, making it significantly worse to read. Also, in most cases I won't need write function to the data members, for example.
And why bother with Q_PROPERTY overhead when I could just write Q_INVOKABLE getters for just the situations I need?
Here's an example of how simple I hoped it would be when I read in the Qt Project documentation: "This enables C++ data and functions to be accessible directly from QML, often with little or no modification."
class game : public QObject
{
Q_OBJECT
public:
explicit game(QObject *parent = 0);
colors NPC[3]; // colors being an enum declared elsewhere
player players[4]; // player is a non-QObject class containing player stats
}
...
game gMain;
QDeclarativeContext *context = viewer.rootContext();
context->setContextProperty("Game",&gMain);
QML in my ideal world:
Image {
id : Image1
source: { if (Game.NPC[0] === 0) {
if (Game.players[1].LifeCount > 0) {
return "pics/FigG.png"
}
else {
return "pics/StoneG.png"
}
}
Now how close to that can I actually get with QML and how do I go about it?
I'm especially interested in handling simple C++ style arrays and enums (have a lot of those in the game) - would I need to write helper functions, e.g. int Game.GetNPCAt(int i) instead of using just Game.NPC[i] ?
I realize that the way I DON'T want to do it is the tried and trusted, and for good reason... however in my situation (small one-man project) it seems like using a cannon to kill a fly (although the GUI building part in qml is amazingly simple and quite a joy to use) - also having to wrap around every data member including the simplest like an int seems... ridiculously excessive.
Maybe I have missed something somewhere, In which case I humbly apologize. Thank you for any thoughts on the matter.

Answer 1

In order:
Q_PROPERTY: When you look at the page that you quoted, they discuss using the Q_PROPERTY method to expose properties to QML. If you don't use Q_PROPERTY, it is my understanding that your variables won't be registered by QMLViewer (or what have you). The Q_PROPERTY needs a Q_INVOKABLE to get/set your variables. If you don't use Q_PROPERTY, though, your class properties will not appear in QML.
Setting the image source: If you may remember, QML is a forge between CSS and JavaScript. If you're just looking to make the image's source change depending on a condition outside of your Image element, you can just create a JavaScript function to achieve what you have quoted:
Image {
id: Image1
function getImage()
{
if (Game.NPC[0] === 0)
{
if (Game.players[1].LifeCount > 0) {
Image1.source="pics/FigG.png";
}
else {
Image1.source="pics/StoneG.png";
}
}
}
However, the function won't run by itself: you'll have to associate it with a signal, which I would create in your C++ class (put the function under a label named signals: (NOT within public -- see here on how to write signals)). Based on your example, I'm guessing that your C++ object is called Game.
Game {
id: gameKeeper //or whatever you want to name it
onUpdate: Image1.getImage() //replace onUpdate with your signal
}
Image {
id: Image1
function getImage()
{
if (gameKeeper.NPC[0] === 0)
{
if (gameKeeper.players[1].LifeCount > 0) {
Image1.source="pics/FigG.png";
}
else {
Image1.source="pics/StoneG.png";
}
}
}
In theory, you should be able to reference arrays this way with JavaScript (I'm not all that familiar with JS myself).
Array handling: On the C++ side, it looks like the best way to do it is through a QList. Fortunately, a QList iterates very similar to a normal array. I found this, which should help -- just ignore the second dimension.
Hope this helps.